Transcript Physics Unit 2 Revision

P2 revision
Motion
Forces and their effects
Kinetic energy
Momentum
Electrical circuits
Mains electricity
Nuclear physics
P2 1.1 Distance-time graphs
How can we tell from a distance-time graph if an object is
stationary or moving at constant speed?
How do we calculate speed of a body?
The slope on a distance-time graph represents speed.
Speed (metre/second, m/s) = distance travelled (m)
time taken (s)
D
S
t
LO: understand how to draw and interpret graphs of motion
Using distance-time graphs
•
•
•
How steep the line is (the gradient) on a distance-time
graph tells you the speed that an object is moving
The steeper the line, the faster something is moving
Speed is measured in m/s
LO: understand how to draw and interpret graphs of motion
Calculating the gradient
Gradient =
Change in y
Change in x
Gradient =
∆y
∆x
The gradient of a distance-time graph represent the
speed of an object.
P2 1.2/3 Acceleration and
Velocity time graphs
What is the difference between speed and velocity?
What is acceleration and what are its units?
How can we tell from a velocity-time graph if an object is accelerating or
decelerating?
What does the area under a velocity-time graph represent?
Velocity is speed in a given direction (units – m/s).
Two objects may travel at the same speed but may have different
velocities.
Acceleration is the change in an object’s velocity per second (m/s2)
Acceleration =
change in velocity (m/s)
.
Time taken for the change (seconds)
The slope of the line on a velocity-time graph represents acceleration.
The area under the line on a velocity-time graph represents distance
travelled.
LO: understand how to draw and interpret graphs of motion
Acceleration
Acceleration can be calculated using the following equation:
Change in velocity
Acceleration =
Time taken
Final velocity – initial velocity
Acceleration =
a=
v-u
t
Time taken
•
•
•
•
a = acceleration (m/s2)
v = final velocity (m/s)
u = initial velocity (m/s)
t = time (s)
LO: understand how to draw and interpret graphs of motion
Using velocity-time graphs
•
•
How steep the line is (the gradient) on a velocity-time
graph tells you the acceleration of that object
The steeper the line, the greater the acceleration
•
acceleration is measured in m/s2
LO: understand how to draw and interpret graphs of motion
Measuring the acceleration
Gradient =
Change in y
Change in x
Gradient =
∆y
∆x
The gradient of a velocity time graph represent the
acceleration of an object.
. A car is driven along a straight, snow covered, road. The graph shows how the velocity of the car
changes from the moment the driver sees a very slow moving queue of traffic ahead.
(a)
Use the graph to calculate the distance the car travels while it is slowing down.
Show clearly how you work out your answer.
........................................................................................................................
........................................................................................................................
........................................................................................................................
Distance = ....................................... m
(3)
(a)
35 (m)
allow 1 mark for indicating the correct area
allow 1 mark for obtaining correct figures from the graph
allow 1 mark for calculating area of triangle (25) but
omitting the rectangle underneath (2 x 5)
3
The space shuttle takes 9 minutes to reach its orbital velocity of 8100 m/s.
(i)
Write down the equation that links acceleration, change in velocity and time taken.
...........................................................................................................................
(1)
(ii)
Calculate, in m/s2, the average acceleration of the space shuttle during the first
9 minutes of its flight. Show clearly how you work out your answer.
...........................................................................................................................
...........................................................................................................................
average acceleration = .............................................. m/s2
(2)
(iii)
How is the velocity of an object different from the speed of an object?
...........................................................................................................................
...........................................................................................................................
(1)
(b)
(i)
acceleration =
accept a =
or a =
do not accept velocity for change in velocity
do not accept change in speed
do not accept a =
1
(ii)
15 allow 1 mark for an answer of 900 or for correct use of 540 seconds
2
(iii)
velocity includes direction
accept velocity is a vector (quantity)
accept converse answer
1
P2 2.1 Forces between objects
What is the unit of force?
What can we say about the forces acting on two interacting
objects?
When two objects interact, they always exert equal and
opposite forces on each other.
The unit of force is Newtons (N)
P2 2.2 Resultant force
What is a resultant force?
What happens if the resultant force on an object is zero?
What happens if the resultant force on an object is not zero?
We can work out the effect of the forces on an object by
replacing them with a single force called the resultant force.
When the resultant force is zero, the object:
-remains stationary OR
- Moves at constant speed in the same direction
LO: calculate the forces acting on an object
Resultant force
If you have multiple forces acting on an object, you can
replace them with one single force that has the effect of
all the other forces combined together. This single force
is called the resultant force
LO: calculate the forces acting on an object
Rules for calculating the resultant
1. Forces that act in the same direction can be
added together
2. Forces that act opposite to each other must be
taken away
3. Forces that act vertically and horizontally CAN
NOT be added and taken away from each other
and MUST be considered separately.
LO: calculate the forces acting on an object
Effects of forces - acceleration
• The resultant force on a stationary (not moving)
object is zero!
• The resultant force on an object travelling at a
constant velocity is zero!
• If a resultant force is applied to an object, either
moving or stationary, it will accelerate in the
direction of the force
P2 2.3 Force and acceleration
How is resultant force, acceleration and mass related to each
other?
Resultant force (N) = mass (kg) x acceleration (m/s2)
F = ma
LO: calculate the forces acting on an object
Calculating forces
F=mxa
• F = force (N)
• m = mass (kg)
• a = acceleration (m/s2)
F
mxa
LO: calculate the forces acting on an object
Weight is a force
W=mxg
• W = weight (N)
• m = mass (kg)
• g = strength of gravity (m/s2)
W
mxg
.
(a)
The diagram shows the horizontal forces acting on a car travelling along a straight road.
(i)
Calculate the size of the resultant force acting on the car.
Show clearly how you work out your answer.
...............................................................................................................
...............................................................................................................
Resultant force = ......................................... N
(2)
(ii)
Describe the motion of the car when the forces shown in the diagram act on it.
...............................................................................................................
...............................................................................................................
...............................................................................................................
...............................................................................................................
(2)
(a)
(i)
1500
allow 1 mark for subtraction shown ie 2000 – 500
2
(ii)
it accelerates
1
in a forward direction
accept gains speed/velocity
1
A car driver sees a dog on the road ahead and has to make an emergency stop. The graph shows how the
speed of the car changes with time after the driver first sees the dog.
(a)
Which part of the graph represents the “reaction time” or “thinking time” of the driver?
.....................................................................................................................................
(1)
(b)
(i)
What is the thinking time of the driver?
Time ........................ seconds
(1)
ii)
Calculate the distance travelled by the car in this thinking time.
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
Distance ..................................... m
(3)
(c)
Calculate the acceleration of the car after the brakes are applied.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
Acceleration ............................................
(4)
M5.
(a)
AB
for 1 mark
1
(b)
(i)
0.7
for 1 mark each
1
(ii)
16.8
gains 2 marks
2
but correct working
(d = v.t, d = 24 × 0.7, or in terms of area under graph)
gains 1 mark
1
(c)
a = (v-u)/t
= 24/4
=6
m/s2
(see marking of calculations)
(can work in terms of graph gradient)
4
(d)
Calculate the distance travelled by the car during braking.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
Distance ................................................ m
(3)
(e)
The mass of the car is 800 kg. Calculate the braking force.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
Braking force ........................................ N
(3)
(d)
d = v.t
= 24/2 × 4
= 48
(see marking of calculations)
(can work in terms of area under graph)
3
(e) F = ma
= 800 × 6
= 4800
(see marking of calculations)
3
P2 2.4 On the road
What is the resultant force on a vehicle travelling at constant
velocity?
What does the stopping distance of a vehicle depend on?
What factors can increase the stopping distance of a vehicle?
For any car travelling at constant velocity, the resultant force on
it is zero.
The braking force needed to stop a vehicle depends on a) the
velocity of the vehicle and b) the mass of the vehicle.
Stopping distance = thinking distance + the braking distance
Factors affecting stopping distances:
Tiredness, alcohol, drugs, how fast the vehicle is travelling,
adverse road conditions and poorly maintained vehicles.
LO: understand the factors that affect the stopping distance of a car
Stopping distance
The stopping distance of a car is the minimum distance
that a car can safely stop in
Stopping distance = thinking distance + braking distance
LO: understand the factors that affect the stopping distance of a car
Thinking distance
The thinking distance is the distance travelled by the
vehicle in the time it takes for the driver to react
alcohol
other drugs and some
medicines
distractions, such as
mobile phones
tiredness
speed
LO: understand the factors that affect the stopping distance of a car
Stopping distance
The stopping distance is the distance travelled by the
vehicle during the time the braking force acts
weather
condition of
tyres/brakes
condition of road
speed
(a)
A car driver makes an emergency stop.
The chart shows the ‘thinking distance’ and the ‘braking distance’ needed to stop the car.
Calculate the total stopping distance of the car.
....................................................................................................................................
Stopping distance = ................................................. m
(1
(b)
The graph shows how the braking distance of a car driven on a dry road changes with the
car’s speed.
15+38 = 53m
P2 2.5 Falling objects
What is the difference between weight and mass?
What is terminal velocity?
The weight of an object is the force of gravity on it (Newtons, N)
The mass of an object is the quantity of matter (kilograms, kg)
Gravitational field strength on Earth = The force of gravity on a
1kg object on Earth.
Weight (N) = mass (kg) x gravitational field strength (N/kg)
As an object falls, its acceleration decreases as the drag force
starts to increase. The object starts to travel at constant velocity
– this is called terminal velocity.
Terminal Velocity
Consider a skydiver:
1) At the start of his jump the air
resistance is ZERO so he
ACCELERATES downwards.
2) As his speed increases his air
resistance will INCREASE
3) Eventually the air resistance will be
big enough to EQUAL the
skydiver’s weight (force caused by
gravity). At this point the forces
are balanced so his speed becomes
CONSTANT - this is called
TERMINAL VELOCITY
Terminal Velocity
Consider a skydiver:
4) When he opens his parachute the
air resistance suddenly
INCREASES, causing him to start
SLOWING DOWN.
5) Because he is slowing down his air
resistance will DECREASE again
until it balances his WEIGHT. The
skydiver has now reached a new,
lower TERMINAL VELOCITY.
Velocity-time graph for terminal velocity…
Parachute opens – diver
slows down
Velocity
Speed
increases…
Terminal
velocity
reached…
Time
New, lower terminal
velocity reached
Diver hits the ground
P2 3.1 Energy, Work and Power
What do we mean by the word ‘work’ and ‘power’ in science?
What is the relationship between work and energy?
What happens to work done against frictional forces?
How do we calculate gravitational potential energy?
‘Work’ is done on an object if it moved by a force = energy
transferred
Work done (Joules, J) = force (N) X distance moved (m)
Work done to overcome friction is mainly transformed into heat
energy
Gravitational potential energy is a measure of the work done
against gravity.
GPE (J) = mass (kg) x gravity (N/kg) x height (m)
Power is the amount of energy transferred each second.
LO: understand how energy can be transferred
Calculating work
The work done by an object is equal to the
amount of energy that it transfers
Work done = force x distance
W=fxd
• W = work done(J)
• f = force (N)
• d = distance(m)
LO: understand the nature of gravitational potential energy
Gravitational Potential Energy
GPE = mass x Gravitational x height
Field strength
GPE = m x g x h
•
•
•
•
GPE = gravitational potential
energy (J)
m = mass (kg)
g = gravitational field strength
(N/kg)
h = height (m)
LO: understand how energy can be transferred
Calculating power
Power is the amount of work done/energy
transferred in a given time
Power = work done / time
P=W/t
• P = power (W)
• W = work done (J)
• t = time (s)
(a) A chair lift carries two skiers, Greg and Jill, to the top of a ski slope. Greg weighs 700 N and Jill
weighs 500 N.
(i)
Write down the equation that links distance moved, force applied and work done.
...........................................................................................................................
(1)
(ii)
Calculate the work done to lift Greg and Jill through a vertical height of 200 m. Show
clearly how you work out your answer and give the unit.
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
work done = ....................................................................
(3)
(b)
The chair takes 5 minutes to move from the bottom to the top of the ski slope.
Use the following equation to calculate the power required to lift Greg and Jill to the top of
the ski slope. Show clearly how you work out your answer.
power =
.....................................................................................................................................
.....................................................................................................................................
power = .................................................................. watts
(2)
(a)
(i)
work (done) = force (applied) × distance (moved)
accept W = F × s or W = F × d
accept
provided subsequent method is correct
1
(ii)
240 000 allow 1 mark for correct substitution or correct use of 1200 (N)
2
Joules accept J
do not accept j / Nm
1
(b)
800 (watts)
accept 0.8 kW
accept their (a)(ii) ÷ 300 correctly evaluated for 2 marks
allow 1 mark for correct substitution
(a)(ii) ÷ 5 correctly evaluated for 1 mark
2
P2 3.2 Kinetic and elastic energy
What are kinetic energy and elastic potential energy?
How does the kinetic energy of an object depend on its speed?
How can we calculate kinetic energy?
Kinetic energy (J) = ½ x mass (kg) x speed2 (m/s)2
Elastic potential energy is the energy stored in an elastic
object when work is done on it to change its shape.
LO: understand the nature of kinetic energy
Kinetic energy
KE = ½ x m x v²
•
•
•
KE = kinetic energy (J)
m = mass (kg)
v = velocity (m/s)
(b)
The car has a mass of 1200 kg.
Calculate the kinetic of the car when it travels at a speed of 12 m/s.
Write down the equation you use, and then show clearly how you work out your answer.
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
........................................................................................................................
Kinetic energy = ....................................... J
(2)
(b)
86 400
allow 1 mark for correct substitution into the correct equation
ie 1/2 × 1200 × 122
LO: understand the link between force and extension of an object
Stretching objects
When you stretch an
object, work is done to
change its shape.
While it remains
stretched the object
stores elastic potential
energy
The constant gradient here shows us
that the force and extension are
proportional
Most materials have a
range where the force
and extension are
proportional
LO: understand the link between force and extension of an object
Material properties
Beyond a point, the
material will start to show
plastic behaviour.
Beyond the proportional limit, the
material shows plastic behaviour. The
extension is now much harder to predict
A small increase in force
will give a large increase
in extension. The
deformation will be
irreversible (the material
will not go back to the
original shape when the
force is taken away)
LO: understand the link between force and extension of an object
Hooke’s Law
Hooke’s law states that:
The extension of an object
is directly proportional to
the force that is applied to
it provided that the limit of
proportionality is not
exceeded
LO: understand the link between force and extension of an object
Hooke’s Law
Hooke’s law can be written as:
F=kxe
•
•
•
F = Force (N)
k = spring constant (N/m)
e = extension (m)
(b)
A student makes a simple spring balance. To make a scale, the student uses a range of
weights. Each weight is put onto the spring and the position of the pointer marked
The graph below shows how increasing the weight made the pointer move further.
The graph below shows how increasing the weight made the pointer move further.
(i)
Which one of the following is the unit of weight? Draw a ring around your answer.
joule
(ii)
kilogram
newton
watt
(1)
What range of weights did the student use?
........................................................................................................................... (1)
(iii)
How far does the pointer move when 4 units of weight are on the spring?
........................................................................................................................... (1)
(iv)
The student ties a stone to the spring. The spring stretches 10 cm. What is the
weight of the stone?
........................................................................................................................... (1)
(b)
(i)
newton
1
(ii)
0 – 5 (N) or 5
accept1 – 5 (N)
do not accept 4
1
(iii)
16 (cm)
1
(iv)
2.5 (N)
accept answer between 2.4 and 2.6 inclusive
1
P2 3.3 Momentum and impact
How can we calculate momentum?
What is its unit?
What happens to the total momentum of two objects when they collide?
What is the impact force and how can it be reduced?
Momentum of a moving object = its mass x velocity
Unit is kilogram metre/second (kgm/s)
When two objects collide momentum is conserved.
The impact force can be reduced by using air bags and crumple
zones.
LO: understand what is meant by momentum
Momentum
P=mxv
• P = momentum (kgm/s)
• m = mass (kg)
• v = velocity (m/s)
LO: understand what is meant by momentum
Conservation of momentum
In a closed system, the total
momentum before an event and
the total momentum after an event
are the same. This is called
conservation of momentum.
Events you may be asked about in
your exams are:
• Collisions
• Explosions
LO: explain how safety features on a car work
Brakes and crumple zones
Brakes, air bags and crumple zones are the main safety
features on a car.
They increase the time taken for the impact.
As F=ma and acceleration is change in velocity per second,
the longer the impact time, the less force is transferred to
the occupants of the car.
The diagram shows a child on a playground swing. The playground has a rubber safety surface.
(a)
The child, with a mass of 35 kg, falls off the swing and hits the ground at a speed of
6 m/s.
(i)
Use the equation in the box to calculate the momentum of the child as it hits the
ground.
momentum = mass × velocity
Show clearly how you work out your answer and give the unit.
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
Momentum = ............................................................
(3)
(ii)
After hitting the ground, the child slows down and stops in 0.25 s.
Use the equation in the box to calculate the force exerted by the ground on the
child.
force =
Show clearly how you work out your answer.
...........................................................................................................................
...........................................................................................................................
Force = ............................................................ N
(2)
(a)
(i)
210
allow 1 mark for correct substitution i.e. 35 × 6
2
kg m/s or Ns
do not accept n for N
accept 210 000g m/s for 3 marks
1
(ii)
840
if answer given is not 840 accept their (a)(i) in kg m/s ÷ 0.25
correctly calculated for both marks
allow 1 mark for correct substitution i.e. 210 ÷ 0.25 or their
(a)(i) ÷ 0.25
2
(b)
The diagram shows the type of rubber tile used to cover the playground surface.
Explain how the rubber tiles reduce the risk of children being seriously injured when they
fall off the playground equipment.
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
.....................................................................................................................................
(3)
(b)
increases the time to stop
accept increases impact time
do not accept any references to slowing down time
1
decreases rate of change in momentum
accept reduces acceleration/deceleration
reduces momentum is insufficient
1
reduces the force (on the child)
1
P2 4 Current Electricity.
What are charge, current and potential difference and how are
they linked?
What is Ohm’s Law?
What happens to current, potential difference and resistance in
series and parallel circuits?
Current is the flow of electrical charges around a circuit, measured in
Amps (A).
Potential difference is the difference in energy per change at two points
in a circuit, measured in volts (V).
Ohm’s Law states that Potential difference is proportional to current as
long as the temperature remains constant.
Ohm’s Law: V=IxR
Series circuits consist of one ‘loop’ of components through which
electrical current can pass.
Parallel circuits consist of many branches or paths for the electrical
current to take.
LO: understand static electricity
What is an atom made up of?
Protons – Positively charged
particles found inside the
nucleus
Neutrons – Neutral particles
found inside the nucleus
Electrons – Negatively
charged particles that orbit
the nucleus
Circuit Electricity
LO: understand static electricity
Static electricity by friction
When you rub one of the rods with the cloths, you
create static electricity. This happens in one of two
ways.
For the polythene rod, the dry cloth transfers electrons
TO the surface of the rod and gives it a negative charge
LO: understand static electricity
Static electricity by friction
When you rub one of the rods with the cloths, you
create static electricity. This happens in one of two
ways.
For the perspex rod, the dry cloth transfers electrons
away from the surface of the rod. This gives it a
positive charge
LO: understand static electricity
Static electricity rules
1. Like (The same) charges repel
2. Unlike (The opposite) charges attract
LO: Understand how to create electrical circuits
Calculating current
Current: This is the flow of electric charges around a circuit.
The size of the current is dependent on the rate of flow of
electric charges.
Current = Charge/time
I = Q/t
• I = Current (Amps, A)
• Q = Charge (Coulombs, c)
• t = Time (s)
LO: Understand how to create electrical circuits
Calculating potential difference
Potential Difference (Voltage): The potential difference
between two points is the work done per unit charge
between two points
Potential difference = work done/charge
V = W/Q
• V = P.D. (Volts, V)
• W = Work done (Joules, J)
• Q = Charge (Coulombs, c)
LO: Understand how to create electrical circuits
Calculating energy
Energy = potential difference x Charge
E=VxQ
• E = Energy transferred (Joules, J)
• V = P.D. (Volts, V)
• Q = Charge (Coulombs, c)
LO: Understand the relationship-between current and voltage in a circuit
Ohm’s Law
Ohm’s Law states that the
current through a resistor is
directly proportional to the
potential difference (voltage)
provided the temperature is
constant
LO: Understand the relationship-between current and voltage in a circuit
Ohm’s Law
Potential difference = Current x Resistance
V = IR
• V = P.D. (V)
• I = Current (A)
• R = Resistance
(Ohms, Ω)
(b) The material, called conducting putty, is rolled into cylinders of different lengths but with equal
thicknesses. Graph 1 shows how the resistance changes with length.
Graph 1
(i)
Why has the data been shown as a line graph rather than a bar chart?
...........................................................................................................................
...........................................................................................................................
(1)
(ii)
The current through a 30 cm length of conducting putty was 0.15 A.
Use Graph 1 to find the resistance of a 30 cm length of conducting putty.
Resistance = ............................................... ohms
(1)
(iii)
Use your answer to (b)(ii) and the equation in the box to calculate the potential
difference across a 30 cm length of conducting putty.
potential difference = current × resistance
Show clearly how you work out your answer.
...........................................................................................................................
...........................................................................................................................
...........................................................................................................................
Potential difference = ............................................... volts
(2)
(b)
(i)
data is continuous (variable)
1
(ii)
36 (Ω) correct answer only
1
(iii)
5.4 or their (b)(ii) × 0.15 allow 1 mark for correct substitution
2
Series circuits
IN A SERIES CIRCUIT, EVERYTHING IS CONNECTED END TO END.
THERE IS NO PLACE FOR THE CURRENT TO SPLIT IN THE CIRCUIT.
1) The current through
each component in a
series circuit is the
same
2) The potential
difference of the
source is shared out
between the
components in a series
circuit
Parallel circuits
IN A PARALLEL CIRCUIT, THERE ARE BRANCHES THAT SEPERATE THE
CIRCUIT INTO SMALLER CIRCUITS. THERE IS MORE THAN ONE PATH
FOR THE CURRENT TO TAKE.
1) The potential
difference across each
component is the same
in a parallel circuit
2) The total current in the
circuit is the sum of the
currents through the
individual components
in the circuit
LO: Understand the relationship-between current and voltage in a circuit
Non-Ohmic Components1
An LED does not follow
Ohm’s law and is
designed to only allow
current to flow through in
one direction
LO: Understand the relationship-between current and voltage in a circuit
Non-Ohmic Components2
An LED does not follow
Ohm’s law and will only
light up when current to
flows through in the right
direction.
If current tries to flow in
the other direction it
encounters a MAHOOSIVE
resistance!
P2 5 Mains Electricity.
What is the difference between A.C. and D.C?
The features of a Plug.
How fuses and circuit breakers (RCCBs) work to make
electrical appliances safe.
Alternating current (A.C.) oscillated forwards and backwards.
Direct Current (D.C.) only flows in one direction.
A 3 pin plug contains a live, neutral and earth wire, along with
a fuse, and cable grip.
Fuses melt when too much current passes through them,
breaking the circuit.
Circuit Breakers measure the difference between the live
and neutral wires and ‘trip’ when this is too big.
LO: describe features of mains electricity
AC vs DC
If you turn on any battery
powered device the
electricity will only ever
flow in one direction.
This is called DIRECT
CURRRENT (d.c.) as the
electricity goes around in
just one direction.
Voltage / V
Direct current is what comes out of a battery. It
only flows in one direction and should remain at a
constant value throughout the life of the battery.
If you connected this sort of supply to an
oscilloscope it would look like this.
time
© Boardworks Ltd 2001
LO: describe features of mains electricity
AC vs DC
However, the same isn’t
true for mains electricity.
Mains electricity uses
ALTERNATING CURRENT
(a.c.) which repeatedly
flows in one direction and
then reverses its flow. The
frequency is how many
times it changes direction
in one second
Voltage / V
+320
0
One cycle
(1/50th second)
-320
Time
Frequency (Hz) = 1 ÷ time taken for one cycle (s)
© Boardworks Ltd 2001
LO: describe features of mains electricity
Key points
1. Mains electricity uses
a.c.
2. Mains electricity is at
230V
3. Mains electricity has a
frequency of 50Hz. This
means it changes
direction 50 times in
one second
LO: describe features of mains electricity
Cables and Plugs
Cables and wires are
designed to allow people to
use them without risk of
hurting themselves. Most
appliances are supplied with
three-core cable. This means
the cable is made up of three
separate wires.
LO: describe features of mains electricity
Components of a plug and cable
1) Live wire (brown) – This
carries the current to the
appliance.
2) Neutral wire (blue) – This
completes the circuit and is
usually at 0V
3) Earth wire (green/yellow) –
This ‘earths’ the appliance in
case of a fault
4) Fuse – This stops the flow of
current if it gets too high
.
(4)
(a)
The diagram shows a three-pin plug and electrical cable.
Name a suitable material to make:
the plug casing .................................................................
the inner cores of the cable. ..............................................
Give the reason for your choice of each material.
plug casing .....................................................................................................
........................................................................................................................
inner core of the cable ...................................................................................
........................................................................................................................
.
(a)
plastic accept rubber
1
as it is a good electrical insulator
accept as it is a poor electrical conductor
any mention of heat negates this mark
1
copper
1
as it is a good electrical conductor any mention of heat negates this mark
1
LO: describe features of mains electricity
Earthing
Components are earthed to make
sure you don’t get an electric
shock if the live wire accidentally
touches the casing. The electricity
will flow harmlessly through the
earth wire instead of through you
when you touch the casing.
However, appliances with plastic
cases (hairdryers etc.) don’t have
earth wires.
LO: describe features of mains electricity
Fuses
A fuse is a component that
has a wire running through
it made of a different
material/thickness than the
rest of the circuit. It is
designed to stop current
that is too high flowing
through it.
LO: describe features of mains electricity
Fuses
Fuses have a rating based on the
amount of current they will allow
through. For example, a 13A fuse
will allow a maximum of 13 amps
of current to flow through. If
MORE than this tries to flow
through, the wire heats up and
melts, breaking the circuit and
protecting the appliance
The Circuit breaker
What happens if the
current is too large?
Too much current
causes the electromagnet to
produce a magnetic field
strong enough to open the
switch.
switch
electromagnet
(The circuit break is said to
‘trip’).
This switches off the current.
© Boardworks Ltd 2003
LO: describe features of mains electricity
Circuit Breakers
Circuit breakers are fitted in
newer homes. They measure the
difference in current in the live
and neutral wires. If the
difference is too great, an
electromagnetic switch opens
(‘trips’) which stops the flow of
current. They work a lot faster
than fuses and can be reset easily
LO: describe features of mains electricity
Calculating electrical power
Power = Potential difference x Current
P=VxI
• P = Power (w)
• V = P.D. (V)
• I = Current (A)
2 (a) (iii) The bulb is at full brightness when the potential difference across the bulb is 12 V.
The current through the bulb is then 3 A.
Calculate the power of the bulb when it is at full brightness and give the unit.
Use the correct equation from the Physics Equations Sheet.
............................................................................................................................................
............................................................................................................................................
............................................................................................................................................
Power = ..................................................
(3 marks)
P2 6 Radioactivity.
The discovery of the nucleus.
The different types of ionising radiation.
What happens during nuclear fission and nuclear fusion.
How to calculate half life.
Rutherford discovered the presence of the nucleus after
preforming his alpha scattering experiment.
Ionising radiation occurs in the form of alpha particles, beta
particles and gamma waves.
In nuclear fission, heavy unstable nuclei split to release
ionising radiation and lots of energy.
In nuclear fusion, helium nuclei fuse to become hydrogen
nuclei, creating LOTS of energy.
Half life is the time taken for half of a quantity of
radioactive isotope to decay, It is useful in dating materials.
Radioactivity
LO: understand the nature of radioactive decay
The Plum Pudding Model - 1897
LO: understand the nature of radioactive decay
Enter Rutherford
Ernest Rutherford fired alpha particles at gold foil.
Alpha particles have a positive charge and he
expected them to go through the particle, with a
small amount of deviation from their path
LO: understand the nature of radioactive decay
Gold Foil Experiment - 1911
The results are very different!
Most alpha particles go straight through with no
deviation! Some, however, are diverted through very
large angles! The physics community is flummoxed by
this finding!
LO: understand the nature of radioactive decay
Gold Foil Experiment - 1911
The results are very different!
Most alpha particles go straight through with no
deviation! Some, however, are diverted through very
large angles! The physics community is flummoxed by
this finding!
LO: understand the nature of radioactive decay
Conclusions
Most of the fast, highly charged
alpha particles went whizzing
straight through undeflected.
SUGGESTS THAT MOST OF THE
ATOM IS EMPTY SPACE!!
LO: understand the nature of radioactive decay
Conclusions
Some of the alpha particles were
deflected back through large angles.
A very small number of alpha
particles were deflected backwards!
SUGGESTS THAT THERE IS A
CONCENTRATED POSITIVE MASS
SOMEWHERE IN THE ATOM.
LO: understand the nature of radioactive decay
Conclusions
A very small number of alpha
particles were deflected backwards!
SUGGESTS THAT THE CONCENTRATED
MASS IS MINISCULE COMPARED TO
THE SIZE OF THE REST OF THE ATOM,
BUT CONTAINS MOST OF THE MASS
LO: understand the nature of radioactive decay
Types of Radiation
There are three different kinds of radiation. Each one has a
unique nature and penetration
Alpha radiation:
This particle is made up of two
protons and two neutrons (i.e. a
Helium nucleus). It has a charge
of +2 and moves slowly because
of it’s large mass. It can be
stopped by a few cm of air or by
a piece of paper
LO: understand the nature of radioactive decay
Types of Radiation
There are three different kinds of radiation. Each one has a
unique nature and penetration
Beta radiation:
During beta radiation, a neutron turns into
a proton inside the nucleus and gives off
an electron, which is fired from the
nucleus. The electron is small and light
and so moves very fast! Beta particles can
be stopped by a thin sheet of aluminium
LO: understand the nature of radioactive decay
Types of Radiation
There are three different kinds of radiation. Each one has a
unique nature and penetration
Gamma radiation:
Gamma radiation usually follows
alpha or beta decay. It is NOT a
particle like the other two. It is a high
energy EM wave that travels at the
speed of light (the fastest that
anything can travel Joel). It can only
be stopped by a very thick piece of
lead or concrete.
LO: understand the nature of fusion and fission
Isotopes
An isotope of an element has the same number of
protons and neutrons as the original, but a
different number of neutrons.
LO: understand the nature of fusion and fission
Radioactivity of a substance
LO: understand the nature of fusion and fission
Radioactivity of a substance
As a radioactive
substance decays, the
number of particles left
in it will start to reduce.
Therefore the
radioactivity of the
substance will begin to
decrease. It will
continue to decrease,
until the radioactivity
has reached zero!
LO: understand the nature of fusion and fission
Half-life
The half-life of a
substance is the time it
takes for HALF of the
particles in a sample to
decay or for the
radioactivity of a
substance to decrease
by HALF.
LO: understand the nature of fusion and fission
Nuclear Fission
Nuclear fission is a process that uses
atoms to generate VAST amounts of
energy.
LO: understand the nature of fusion and fission
Nuclear Fission
To begin with, we have a
simple Uranium nucleus.
Uranium is used because it
is already unstable.
A slow moving neutron is
fired at the Uranium.
Neutron
Uranium
nucleus
LO: understand the nature of fusion and fission
Nuclear fusion
Although the names sound very similar,
fission and fusion are VERY DIFFERENT
PROCESSES.
LO: understand the nature of fusion and fission
Nuclear fusion
In nuclear fusion, two nuclei are fused
together to release energy. It is the
opposite of nuclear fission.
LO: understand the nature of fusion and fission
Where does this happen?
The sun is made up of
mainly hydrogen. The
high temperature on
the sun allows the
hydrogen to fuse
together and make
helium, releasing
massive amounts of
energy in the process
P2 7 Stars.
The features of the life cycle of a main sequence star like
the sun.
What could happen in the life of a star larger than then sun.
Main sequence stars go through the following stages:
Nebulae – protostar – main sequence – red giant – white dwarf
– black dwarf.
Larger stars could form super red giants, these will then turn
in to a supernova – neutron star/black hole.
LO: understand the lifecycle of a star
Nebula
All stars start their
lives as part of a
nebula. Nebulae
are large clouds of
dust and gas
(mainly hydrogen).
LO: understand the lifecycle of a star
Protostar
Over millions of years,
gravity will cause the dust
and gas in the nebula to
come together. As it does
this, the temperature
increases until hydrogen can
fuse. When this happens, a
protostar is born. This is
kind of like a ‘baby’ star.
LO: understand the lifecycle of a star
Main sequence star
The main sequence star is
the next stage after a
protostar. Hydrogen fusion
is now in full flow and the
star is much hotter and
brighter than the protostar.
LO: understand the lifecycle of a star
Red Giant star
When a star runs out of
hydrogen, it begins to fuse
other, heavier elements.
This releases more energy,
causing the star to expand.
It also gives off red light,
giving it the name ‘Red
Giant’.
LO: understand the lifecycle of a star
White dwarf
When the red giant has run
out of all fuel and can fuse
nothing more, it will lose its
outer layers. This leaves just
the core, which is still
extremely hot. It is so hot it
glows white hot, giving the
name to this stage – the
‘white dwarf’.
LO: understand the lifecycle of a star
Black dwarf
After a long enough time,
the white dwarf will cool
down enough so that it
stops glowing white hot. It
is now called a ‘black
dwarf’.
LO: understand the lifecycle of a star
Red Super Giant star
Following the main
sequence, the star begins to
fuse together heavier
elements. However, as it
has far more fuel, it
expands to a much larger
size and gives off much
more energy.
LO: understand the lifecycle of a star
Supernova
For very heavy stars, once
they have run out of fuel,
the star begins to collapse
in on itself. It continues to
collapse until it reaches a
critical point when it can’t
collapse any more. This
causes a MASSIVE
shockwave!
LO: understand the lifecycle of a star
Supernova
The shockwave is so large
that the outer layers
EXPLODE outwards! The
explosion only lasts
seconds, but can release as
much energy in those
seconds as the star has
released up to that point! It
can be as bright as the light
from 10billion stars.
LO: understand the lifecycle of a star
Neutron star
After a supernova, only the
star’s core is left behind.
During the collapsing
process, this core is turned
into just neutrons.
The resulting ‘neutron star’
is very very dense. One
spoonful of a neutron star
would weigh more than the
Earth!
LO: understand the lifecycle of a star
Black hole
In some very very rare
cases, the core of a star left
over after a supernova will
continue to collapse. It will
keep getting smaller and
smaller until the whole star
has collapsed into an
infinitely small point.
LO: understand the lifecycle of a star
Black hole
This ‘singularity’ has an
immense gravitational
force. It’s attraction is so
strong that not even light
can escape from it. Hence
the name ‘black hole’.