Transcript New P4 P5 P6 Revision

P4 P5 P6 Revision
P4 Explaining Motion
P5 Electric Circuits
P6 Radioactive materials
P4
Explaining Motion
speed (m/s) = distance travelled (m) / time taken (s)
Usually when an object travels from ‘A’ to ‘B’ it’s
velocity will vary so a calculation of it’s velocity is
really an average velocity.
An instantaneous velocity is the velocity at a given moment.
Distances measured in one direction are positive,
and in the other, negative.
A negative velocity means moving in the opposite direction.
10
9
8
7
6
D
5
i
s
4
t
a
3
n
c
2
e
(m)
1
0
0 1
2 3
4 5
6 7 8 9 10 11 12 13 14 15 16
Time (s)
1. What is the velocity of the object at first ?
2. For how long was the object stationary ?
9  3 = 3 m/s
6s
3. What is the velocity in the last part ? 9  6 = - 1.5
m/s
1. A ball is thrown and takes 4 seconds for its velocity to steadily increase to
4 m/s and then travels at a constant velocity for 5 seconds. It then hits a
wall and rebounds at a constant velocity of 3 m/s for 5 s before it is caught.
Velocity
(m/s)
5
4
3
2
1
0
-1
-2
-3
-4
-5
0 1 2 3
4 5 6 7 8 9 10 11 12 13 14 15 16
Time (s)
2. An object moves at a velocity of 2 m/s for 3 seconds and then accelerates at 1
m/s2 for 2 seconds. It then moves at a constant velocity for 3 seconds and then
decelerates at 1 m/s2 until it is stationary. It remains stationary for 2 seconds and
then accelerates backwards at 2 m/s2 for 1 second. It then takes 2 seconds to
steadily decelerate till it stops.
5
4
3
V
2
e
l
1
o
c
0
i
-1
t
y
(m/s) -2
-3
-4
-5
0 1 2 3
4 5 6 7 8 9 10 11 12 13 14 15 16
Time (s)
A woman walks out onto the road
A car is travelling at 30 km/hr
Will she survive ?
8m
The driver has a reaction time of 1 second
30 km = 30,000 m
1 hr = 3,600 s
In 1 second the car would travel 30,000  3,600 = 8.33 m
The woman is hit BEFORE the driver applies the brakes !!!!
X
For a distance-time graph a steeper
gradient means a higher speed
distance
steeper gradient - faster
time
A tachometer continuously measures an objects
speed and can be used to make a tachograph.
If the speed of an object is increasing, we say
that it is accelerating.
Acceleration (m/s2) = change of velocity, m/s
time taken for the change (s)
A force arises from an interaction between two objects.
When one object exerts a force on another, it always
experiences a force in return (a reaction force).
A force and a reaction force are called an ‘interaction pair’.
The two forces in an interaction pair are equal in size and
opposite in direction and they act on different objects.
the box acts downwards on the table due to gravity
box
the table acts upwards on the box due to the reaction force
the two forces are equal and opposite
Walking
gravity
reaction
force of
the ground
acting on
the feet
reaction
force of
friction
on acting
on the
feet
force of
the feet
acting on
the ground
The horizontal motion of objects (like cars and bicycles)
can be analysed in terms of a driving force (produced by
the engine or the cyclist), and a counter force (due to
friction and air resistance).
driving force greater than counter force – speeding up
driving force equal to counter force – stationary
driving force less than counter force – slowing down
A resultant force takes into account all the acting forces.
50N
30N
resultant force = 20N
20
N
Friction is the interaction between two surfaces
when they slide over each other
There is a friction force on both objects involved
Friction is caused by the roughness of the sliding surfaces
Friction enables cars and people to get moving
momentum (kg m/s) = mass (kg) × velocity (m/s)
A car has a mass of 5,000 kg and a velocity of 4 m/s.
What is the car’s momentum ? 5,000 x 4 = 20,000 kg m/s
A cyclist cycling at 10 m/s has a momentum of 540 kg m/s.
The cyclist has a mass of 50 kg, what’s the mass of the bike ?
total mass = 540 / 10 = 54 kg mass of bike = 54 – 50 = 4 kg
If a resultant force acts on an object, it causes a
change of momentum in the direction of the force
If a resultant force on an object is zero
then there is no change of momentum
eg when the driving
force = friction
if it is stationary, it stays at rest
if it is already moving, it continues at a steady speed in a straight line
Total momentum before = total momentum afterwards
positive momentum = to the right
m1 = m2
negative momentum = to the left
momentum before = m1v1 + m2v2
v1 = -v2
= m1v1 + -m2v2
=0
m1
m2
momentum after = m1v1 + m2v2
= -m1v1 + m2v2
=0
m1 = m2
v2 = 0
momentum before = m1v1 + m2v2
= m1v1 + 0
= m1v1
m1
m2
momentum after = m1v1 + m2v2
= 0 + m2v2
= m2v2
therefore m1v1 = m2v2 ie all the momentum of
the first ball is transferred to the second ball
m2 > m1
momentum before = m1v1 + m2v2
v2 = 0
= m1v1 + 0
= m1v1
m1
m2
momentum after = m1v1 + m2v2
= -m1v1 + m2v2
m1 rebounds of m2 and transfers some of it’s momentum to m2
m1 > m2
momentum before = m1v1 + m2v2
v2 = 0
= 0+ 0
v1 = 0
m2 pushes off m1
m1
m2
momentum after = m1v1 + m2v2
= -m1v1 + m2v2
=0
therefore m1v1 = m2v2
When a force is applied to an object, its velocity increases
The longer the force is applied, the greater the change in velocity
The greater the force applied, the greater the change in velocity
momentum
= mass x velocity
increasing the velocity increases momentum
When a force is applied to an object, its momentum increases
The longer the force is applied, the greater the change in momentum
The greater the force applied, the greater the change in momentum
change of momentum = resultant force x time during which it acts
change of momentum = resultant force x
Increasing the time it takes
for a change in momentum
time for which it acts
reduces the force that causes
the change in momentum
If the time from impact to the velocity becoming zero is increased
then the impact force is reduced
which means less injury
Seat belts, air bags, crumple zones,
cycle helmets etc increase the time
during impact and therefore reduce
the impact force.
crumple zone
The energy of a moving object is called kinetic energy
When a force moves an object, work is done
work done (J) = force (N) × distance moved (m)
A braking force of 1000N is applied by a driver to
stop his car. The car covered 50m before it stopped.
How much work did the brakes do ?
1,000 x 50 = 50,000 J
When an object is lifted to a higher position above the
ground, work is done by the lifting force against the
gravitational force acting on the object (its weight)
As an object falls, its gravitational potential energy
decreases as it is transferred into kinetic energy and
heat (friction with the air)
When an object is lifted this increases the
object’s gravitational potential energy (GPE)
change in GPE (J) = weight (N) × height difference (m)
A crane is lifting a 50kg load up into the air with a
constant speed. If the load is raised by 20m how
much work has the crane done ?
remember that 1 kg has a weight of 10 N (on Earth)
50 kg = 500 N
work done = 500 x 20 = 10,000 J
kinetic energy (J) = ½ × mass (kg) × [velocity]2 (m/s2)
E = ½ m v2
A 70 kg boy runs at 10m/s. What is his kinetic energy ?
kinetic energy = ½ x 70 x 102 = ½ x 70 x 100 = 3,500 J
What is the kinetic energy of a 100g tennis ball being
thrown at a speed of 5m/s ?
100g = 0.1 kg
kinetic energy = ½ x 0.1 x 52 = ½ x 0.1 x 25 = 1.25 J
A parachutist with a total mass of 70 kg jumps from a
helicopter at a height of 1,500 m. He pulls the cord of
the parachute when he is 1,000 m above the ground.
(a) Ignoring air resistance, what is the speed of the
parachutist just as he pulls the cord ?
You will need to use the formula E = ½ m v2. You are given the mass (70kg) in
the question and you can work out E (energy) by using GPE = weight x height.
Remember that 70kg = 700N.
GPE = weight x difference in height
GPE = 700 x (1,500 – 1,000) = 700 x 500 = 350,000 J
E = ½ m v2
350,000 = ½ m v2
700,000 = m v2
700,000 = 70 x v2
700,000 / 70 = v2
{replacing E with 350,000}
v2 = 10,000
{multiplied both sides by 2}
v = √10,000
{replacing m with 70}
v = 100 m/s
{divided both sides by 70}
{getting the square root}
(b) Why doesn’t the parachutist actually reach the speed
calculated in part (a) ? [2 marks]
because of air resistance [1], some of the gravitational potential energy
is dissipated as heat [1]
(c) The parachutist actually reached the velocity of 40
m/s before the using the parachute. How much energy
was dissipated ?
total energy = 350,000 J
velocity (without taking air resistance into account) = 100 m/s
velocity (taking air resistance into account) = 40 m/s = 40%
therefore 60% of the energy is dissipated
350,000 x 60 / 100 = 21,000 J
(d) What principle is used to calculate part (c) ?
the conservation of energy (all of the energy is accounted for)
{B to C = steady speed and
C to D = fastest speed}
1.65 hrs
144 – 112 = 32
gradient / slope
24.6 m/s
4.0 – 2.7 = 1.3
speed = 32 / 1.3 = 24.6 m/s
Lance
outlier / anomoly /anomalous

(38 + 41 + 40 + 37) / 4 = 39

kinetic energy = ½ m v2

1875
no change / nothing / stays the same
P4 Explaining Motion
P5 Electric Circuits
P6 The Wave Model of Radiation
P5
Electric Circuits
Electric charge – objects become charged when
electrons are transferred to or from them, for
example, by rubbing
Two types of charge are positive and negative (these
names are just labels)
Two objects with the same charge repel each other
Two objects with different charges attract each other
Metal wire
+
metal ions
Normally the free electrons in a metal
move around slowly at random.
electrons
potential difference
The potential difference (voltage) provides energy which makes the electrons
move through the metal ie it generates a current.
The electrons experience resistance when they flow through the metal.
The symbol for voltage is V
The symbol for current is I
The symbol for resistance is R
current =
voltage
resistance
SERIES
V
I
i2
v1
I = i1 = i2
V = v1 + v2
v2
i1
The current is the same everywhere
The sum of the voltages across each
component equals the supply voltage
Resistance
R =
r1 +
r2
PARALLEL Current
I
I3
i4
i1
i2
I = I3
i5
I1 = I4
I2 = I 5
I = I1 + I2
Total current = the sum of the currents through each component
Current does not get used up
PARALLEL
V
v1
v2
Voltage = energy per unit of charge
V = v1 = v2
The voltage across each component is
the same as the supply voltage.
If more bulbs are added in parallel to a circuit then
they will all be as bright as normal and more current
is drawn from the power supply
The potential difference is largest across the
component with the greatest resistance, because more
energy is transferred by the charge flowing through a
large resistance than through a small one
The current is smallest through the component with
the largest resistance, because the same battery
voltage causes more current through a smaller
resistance than a bigger one
SERIES
12 V
2A
i3 = 2 A
4V
5V
1 ohm
3 ohm i3
v3 = 3 V
v3
r3
r3 = 2 ohm
12 V
PARALLEL
4A
12 ohm
i3 = 2 A
v2 = 12 V
1A
v2
1A
i3
r3
12
ohm
Current is a flow of electrons
Electrons have charge (negative)
So current is a flow of charge
How do we quantify current ?
Current is the amount of charge flowing in a
particular amount of time
Voltage provides energy to the electrons
Electrons have charge (negative)
So Voltage provides energy to the charge
How do we quantify voltage ?
Voltage is the amount of energy a particular amount
of charge has
What about resistance ?
All components will offer resistance to a flow of electrons
How do we quantify resistance ?
If a current of 1A flows through a component when the
voltage across it is 1V then the component is said to have
a resistance of 1 ohm [ 1 W ]
I=
V
R
Multiply both sides by R
Rx I=
V
xR
R
RI=V
Take
V=IR
and divide both sides by I
Or
V=IR
V
IR
=
I
I
V
I
= R
or
R =
V
I
I=
V
R
current = voltage / resistance
V
R =
I
V=IR
voltage = current x resistance
resistance = voltage / current
V
I
R
Q. A current of 4 A flows through a circuit with resistance 3 W. What is the voltage ?
use
V=IR
V=4x3
Voltage = 12 V
V
I
R
Q. A current of 5 A flows through a circuit with voltage 10 V. What is the resistance ?
use
V
R =
I
10
R =
5
resistance = 2 W
Q. A circuit with voltage of 6 V has a resistance of 2 W . What current should flow ?
use
V
I =
R
6
I =
2
current = 3 A
V
I
I=
R
V
R
V=IR
V
R =
I
Q. A current of 4 A flows through a circuit with voltage 12 V. What is the resistance ?
use
V
R =
I
12
R =
4
resistance = 3 W
Q. A circuit with voltage of 8 V has a resistance of 2 W . What current should flow ?
use
V
I =
R
8
I =
2
current = 4 A
Q. A current of 60 A flows through a circuit with resistance 4 W. What is the voltage ?
use
V=IR
V = 60 x 4
Voltage = 240 V
V
I
I=
R
V
R
V=IR
V
R =
I
Q. A current of 2 A flows through a circuit with voltage 16 V. What is the resistance ?
use
V
R =
I
16
R =
2
resistance = 8 W
Q. A circuit with voltage of 230 V has a resistance of 5 W . What current should flow ?
use
V
I =
R
230
I =
5
current = 46 A
Q. A current of 25 A flows through a circuit with resistance 3 W. What is the voltage ?
use
V=IR
V = 25 x 3
Voltage = 75 V
The higher the temperature the
lower the resistance
The greater the light intensity the
lower the resistance
Variable resistors
Resistors are used in circuits to control the size
of the current
Two resistors in series have a larger resistance than
one on its own.
Connecting two resistors in parallel makes a smaller
total resistance
Two resistors in series make a potential divider
Current (A)
Current is less here due to
the extra resistance of the
heating effect
Current through
a filament bulb
Voltage (V)
Power =
current X voltage
(watt,W) (ampere, A)
(volt, V)
If you know the power, it is easy to calculate how much
work is done (or how much energy is transferred) in a
given period of time:
Work done (or energy transferred) = power
(joule, J)
x
time
(watt, W) (second, s)
Generators produce a voltage by a process called
electromagnetic induction
AC generator
AC = alternating current
The size of the induced voltage can
be increased by:
• increasing the speed of rotation of
the magnet or electromagnet or coil;
• increasing the strength of its
magnetic field;
• increasing the number of turns on
the coil;
• placing an iron core inside the coil
DC generator
If a magnet is moving out of the coil, or the other pole of
the magnet is moving into it, there is a voltage induced in
the opposite direction
Transformer
8 turns
4 turns
Voltage across primary coil
Voltage across secondary coil
=
Number of turns primary coil
Number of turns secondary coil
Vp / Vs = Np / Ns
Rate/speed of rotation
Strength of magnet/ magnetic field
Number of turns/coils of wire
a.c / alternating current



Energy = power x time
Power = energy / time

£0.78
2990
30 ohm


P4 Explaining Motion
P5 Electric Circuits
P6 Radioactive materials
Radioactive materials
An atom
proton +
electron -
neutron 0
nucleus
orbit / shell
energy level
Animation completed
The nucleus is positively charged so it attracts the negative electron
How do scientists know about the structure of atoms?
The Rutherford Scattering Experiment
Alpha
particles
(positive
charge)
Thin gold
foil
Some particles passed
through, some were
deflected backwards
Particles passing through the foil indicated atoms have large amounts of space.
The particles that were deflected back indicated the alpha particles had passed
close to something positively charged within the atom (the nucleus)
When an unstable nucleus changes, what can happen ?
alpha
radiation
nucleus
beta
radiation
gamma
radiation
Radioactive isotopes release radiation and the nucleus changes
The behaviour of radioactive materials
(radioactive decay) cannot be changed by
chemical or physical processes
Isotopes
An isotope is an atom with a different number of neutrons:
A “radioisotope” is simply an isotope that is radioactive –
e.g. carbon 14, which is used in carbon dating.
Radioactive changes – some nuclei that are unstable
can become stable by emitting an alpha or beta particle
1) Alpha () – an atom decays into a new atom and emits an alpha
particle (2 protons and 2 neutrons – the nucleus of a helium atom)
2) Beta () – an atom decays into a new atom by changing a neutron
into a proton and electron. The fast moving, high energy electron is
called a beta particle.
3) Gamma – after  or  decay surplus energy is sometimes emitted.
This is called gamma radiation and has a very high frequency with
short wavelength. The atom is not changed.
Alpha decay
Example
Radium-226 undergoing alpha decay forms
Radon-222, an alpha particle and releases
energy.
Beta decay
Example
Polonium-218 undergoing beta decay forms
Astatine-218, an electron and releases energy.
Radioactivity
If a substance is capable of ALWAYS emitting radiation under any
conditions we say it is radioactive. There are three types of radiation:
ALPHA, BETA and GAMMA.



Sheet of
paper
Few mm of
aluminium
Few cm of
lead
Sources of background radiation
Background radiation
Radiation dose measures the possible harm the radiation could do to
the body. It is measures in millisieverts (mSv).
The potential harm done depends on
•the amount of radiation
•the type of radiation
Exposure to a radiation source outside your body is
called irradiation
If a radiation source enters your body, or gets on skin
or clothes, it is called contamination
Alpha particles are the most ionising so they are the
most dangerous inside your body
Employers must ensure that radiation workers receive a
Radiation dose “as low as reasonably achievable”.
Precautions taken are
•use protective clothing and screens
•.wear gloves and aprons
•wear special devices to monitor their dose
Uses of gamma radiation
treating cancer
it can kill cancer cells
sterilising
equipment
it can penetrate the outer
casing and kill microbes
sterilising food
it can kill microbes
without harming the food
Radon gas is harmful because it is radioactive. It
produces ionising radiation that can damage cells.
Medical imaging and treatment
Radioactive materials cane be used to diagnose and
cure many health problems.
Radiotherapy is used to kill cancer cells
Half life
The HALF-LIFE of an atom is the time taken for HALF of
the radioisotopes in a sample to decay…
= radioisotope
At start there
are 16
radioisotopes
After 1 half
life half have
decayed
(that’s 8)
= new atom formed
After 2 half
lives another
half have
decayed (12
altogether)
After 3 half
lives another
2 have
decayed (14
altogether)
A radioactive decay graph
Count
1 half life
Time
A substance is considered safe once its activity drops to the
same level as background radiation.
Nuclear fission
The energy released can be
calculated from Einstein’s
equation : E = mc²
ENERGY
neutron
U-235
nucleus
The fission of one atom can
set off several more causing Smaller nucleus
a chain reaction
neutrons
Nuclear waste
Nuclear reactor
High level waste – this is “spent” fuel rods
Intermediate level waste – HLW decays to become ILW
Low level waste – protective clothing and medical equipment
Nuclear fusion – the nuclei of two hydrogen atoms join
together and energy is released.
Protons and neutrons in a nucleus are held together by a
strong nuclear force, which acts against the electrical
repulsive force between protons