Transcript electric plum

P2 Revision
Learning Intentions:
• Basically all of P2.
P2.1 Forces and their effects
P2.1.1 Resultant forces
a) Whenever two objects interact, the forces
they exert on each other are equal and
opposite.
b) A number of forces acting at a point may be
replaced by a single force that has the same
effect on the motion as the original forces all
acting together. This single force is called the
resultant force.
c) A resultant force acting on an object may
cause a change in its state of rest or motion.
P2.1.1 Resultant forces
d) If the resultant force acting on a stationary
object is:
■ zero, the object will remain stationary
■ not zero, the object will accelerate in the
direction of the resultant force.
e) If the resultant force acting on a moving
object is:
■ zero, the object will continue to move at the
same speed and in the same direction
■ not zero, the object will accelerate in the
direction of the resultant force.
Reaction Force
• Forced act in pairs. When 2 forces
interact they are equal and opposite
in direction e.g. a person exerts a
force on the chair but the chair
applies an equal force upwards on
the person, a reaction force.
Mass or Weight
• Weight is also a force measured in
Newton's.
• Don’t confuse mass and weight as mass is
actually the amount of ‘stuff’ that makes up
an object measured in kilograms.
• Weight is the force calculated by
Weight(N)=Mass(kg)x Gravitational field strength (N/kg)
• On Earth g=10N/kg
• Or g=10m/s2
Resultant Forces
• When more than 1 force acts upon an object,
a resultant force can be calculated.
• This resultant force show how overall affect of
the force.
• This shows us in which direction the force is
accelerating in.
• If the resultant force is zero:
− A stationary object will not move.
− An object in motion will stay the same velocity.
Calculation force
Force(N)=Mass(kg)x Acceleration(m/s2)
P2.1.2 Forces and motion
a) The acceleration of an object is determined by the
resultant force acting on the object and the mass of the
object.
F=ma AND a=F/m
b) The gradient of a distance–time graph represents speed.
c) Calculation of the speed of an object from the
gradient of a distance–time graph.
d) The velocity of an object is its speed in a given direction.
e) The acceleration of an object is given by the equation:
a=(v-u)/t
f) The gradient of a velocity–time graph represents
acceleration.
g) Calculation of the acceleration of an object from the
gradient of a velocity–time graph.
h) Calculation of the distance travelled by an object
from a velocity–time graph.
Distance-time graph
• Distance-time graphs tell you how an objects
distance is changing over time.
• If there is a smooth slope (/) on your graph
then the object is moving at a constant speed.
• If there is a flat line (-)then there is no
movement.
• A steeper slope means a faster speed.
• If the slope is downwards the object is
returning to the starting position.
• If there is an upwards curve ( )on a distance
time graph then the object is accelerating, a
downward curve ( ) means it is decelerating.
• How do you
work out speed?
• Where is it
fastest?
• What is the
average speed?
Velocity-time graphs
• Velocity-time graphs tell you how an objects
velocity is changing over time.
• If there is a smooth slope (/) on your graph
then the object is accelerating.
• If there is a flat line (-) then the object is
moving at a constant speed.
• A steeper slope means a larger acceleration.
• If there is a downwards slope (\) then the
object is decelerating.
• The area under the velocity time graphs tells
you the distance travelled.
• To work out the acceleration from a section of
the slope you use the same method as for the
distance-time graph.
• Where are the
following:
• Acceleration
• Deceleration
• Constant Speed
Working out acceleration
• A velocity-time graph tells you how an
objects velocity changes over a certain
time.
• This is the acceleration.
Final velocity (m / s )  initial velocity (m / s )
Acceleration (m / s ) 
time taken ( s )
2
P2.1.3 Forces and braking
a) When a vehicle travels at a steady speed the resistive
forces balance the driving force.
b) The greater the speed of a vehicle the greater the braking
force needed to stop it in a certain distance.
c) The stopping distance of a vehicle is the sum of the
distance the vehicle travels during the driver’s reaction time
(thinking distance) and the distance it travels under the
braking force (braking distance).
d) A driver’s reaction time can be affected by tiredness, drugs
and alcohol.
e) When the brakes of a vehicle are applied, work done by the
friction force between the brakes and the wheel reduces the
kinetic energy of the vehicle and the temperature of the
brakes increase.
f) A vehicle’s braking distance can be affected by adverse road
and weather conditions and poor condition of the vehicle.
Stopping distance
• How quickly a car can come to a stop
depends on the car and the driver. The
stopping distance is the thinking
distance (which depends on the drivers
reactions) and the braking distance
(which depends on the car and road
conditions).
Stopping distance = thinking distance + braking distance
Thinking and braking distance
• The thinking distance will be
increased if the driver is tired, been
drinking alcohol, been on drugs etc.
• The braking distance will depend on
the road surface, weather conditions
and how well the car responds e.g.
condition of brakes.
P2.1.4 Forces and terminal velocity
a) The faster an object moves through a fluid the
greater the frictional force that acts on it.
b) An object falling through a fluid will initially
accelerate due to the force of gravity. Eventually the
resultant force will be zero and the object will move
at its terminal velocity (steady speed).
c) Draw and interpret velocity-time graphs for
objects that reach terminal velocity, including a
consideration of the forces acting on the object.
d) Calculate the weight of an object using the force
exerted on it by a gravitational force:
W=mg
Terminal velocity
• An object falling through a fluid or gas will
initially accelerate due to the force of gravity.
• Eventually the force of gravity will be
balanced by the up thrust of the liquid/gas;
this makes the resultant force zero and the
object will move at its terminal velocity
(steady speed).
• The faster the object falls the greater the
frictional force that acts.
Terminal velocity
P2.1.5 Forces and elasticity
a) A force acting on an object may cause a
change in shape of the object.
b) A force applied to an elastic object such as
a spring will result in the object stretching
and storing elastic potential energy.
c) For an object that is able to recover its
original shape, elastic potential energy is
stored in the object when work is done on the
object to change its shape.
d) The extension of an elastic object is
directly proportional to the force applied,
provided that the limit of proportionality is not
exceeded:
F=ke
Hooke’s Law
• When a weight (force) is applied to a
spring it extends. The amount it
extends is proportional to the force
added. It is governed by the equation:
Force(N)=spring constant(N/m)x
extension(m)
Graph of Hooke’s law
• The spring constant can be determined
from the gradient (slope of the line) on
a force extension graph.
Extension (m)
Force extension graph for a spring
7
6
5
4
3
2
1
0
Limit of
proportionality
0
2
4
6
8
Force (N)
10
12
14
Graph of Hooke’s law
• Choose a section of the line and measure
the amount of force and the extension.
Then divide the force by the extension
• For example: In the sample graph the
section of the line chosen if for a force of
6N and an extension of 3m. k=F/e
k=6÷3=2N/m
• Also marked on the graph is the limit of
proportionality. This is the point at which
the spring can still return to its original
length. Beyond this point the spring can
never go back to its original length/shape.
P2.2 The kinetic energy of
objects speeding up or slowing
down
P2.2.1 Forces and energy
a) When a force causes an object to move
through a distance work is done.
b) Work done, force and distance are
related by the equation:
W=Fd
c) Energy is transferred when work is done.
d) Work done against frictional forces.
e) Power is the work done or energy
transferred in a given time.
P=E/t
P2.2.1 Forces and energy
• f) Gravitational potential energy is
the energy that an object has by
virtue of its position in a gravitational
field.
Ep=mgh
• g) The kinetic energy of an object
depends on its mass and its speed.
Ek=(1/2) mv2
Work done
When a force acts upon an object causing it to
move a through a distance energy is transferred
and work is done. The amount of work done is
equal to the amount of energy transferred. The
amount of work done is calculated by:
Work done (Joules, J) = Force applied (N) x distance moved (m)
Box moved from A to B
2N
5m
A
B
Work done = 2N x 5m = 10J
Power
Power is the amount of work done
(energy transferred) every second and is
calculated using the following equation:
Power(W)=Energy transformed(J)/time(s)
Elastic potential energy
• Work can also be done on other objects.
• If you change the shape of an object
then the energy gets stored in the
object, e.g. an elastic band.
• This is elastic potential energy.
Remember, potential energy is stored
energy that is ‘waiting’ to be used,
kinetic energy is movement energy.
Gravitational potential energy
• Gravitational potential energy is the
amount of energy an object has
when it is held above the ground. It
is calculated using the following
equation:
Ep(J)=m(kg)×g(N/kg)×h(m)
Kinetic energy
• To work out the kinetic energy a
body has you need to know it’s mass
and it’s velocity:
GPE and KE
• Gravitational potential energy and
kinetic energy are interchangeable.
• If you get a question about a falling
object the total energy is:
Total energy(J)=GPE(J)+KE(J)
P2.2.2 Momentum
a) Momentum is a property of moving
objects.
p=mv
b) In a closed system the total
momentum before an event is equal to
the total momentum after the event. This
is called conservation of momentum.
Momentum
• Momentum (has the symbol p) describes
how much motion an object has. It is
measured in kilogram metre per second
(kg m/s). Like velocity, momentum has
magnitude acting in a certain direction.
Momentum(kg m/s)=Mass(kg)xVelocity(m/s)
Momentum
Conservation of momentum
• In all situations, momentum is
conserved, providing there are no
external forces acting. For collisions, the
momentum before the collision is equal
to the momentum after the collision e.g.
snooker balls.
Cannon momentum
• Another example is cannon before being
fired and after being fired. Before the
cannon is fired the momentum is zero,
after it is fired the cannon ball moves
forward and the cannon moves back. The
momentum of the cannon ball is the same
as the momentum of the cannon moving
backwards.
• In this sort of example you should choose
one direction to be positive and the other
direction to be negative. The example
below illustrates this point. I will choose
the right to be positive and the left to be
negative.
Cannon momentum
• http://www.youtube.com/watch?v=sLoWQI6zzw
P2.3 Currents in electrical circuits
P2.3.1 Static electricity
a) When certain insulating materials are rubbed
against each other they become electrically
charged. Negatively charged electrons are rubbed
off one material and onto the other.
b) The material that gains electrons becomes
negatively charged. The material that loses
electrons is left with an equal positive charge.
c) When two electrically charged objects are
brought together they exert a force on each other.
d) Two objects that carry the same type of charge
repel. Two objects that carry different types of
charge attract.
e) Electrical charges can move easily through
some substances, eg metals.
P2.3.2 Electrical circuits
a) Electric current is a flow of electric charge.
The size of the electric current is the rate of
flow of electric charge. The size of the current is
given by the equation:
I=Q/t
b) The potential difference (voltage) between
two points in an electric circuit is the work done
(energy transferred) per coulomb of charge that
passes between the points.
V=W/Q
P2.3.2 Electrical circuits
I=Q/t
V=W/Q
P2.3.2 Electrical circuits
c) Circuit diagrams using standard
symbols. The following standard symbols
should be known:
P2.3.2 Electrical circuits
d) Current–potential difference graphs are used to show how the
current through a component varies with the potential difference
across it.
e) The current–potential difference graphs for a resistor at
constant temperature.
f) The resistance of a component can be found by measuring the
current through, and potential difference across, the component.
g) The current through a resistor (at a constant temperature) is
directly proportional to the potential difference across the resistor.
h) Calculate current, potential difference or resistance using the
equation:
V=IR
P2.3.2 Electrical circuits
i) The current through a component depends on its resistance.
The greater the resistance the smaller the
current for a given potential difference across the component.
j) The potential difference provided by cells connected in series
is the sum of the potential difference of each cell (depending
on the direction in which they are connected).
k) For components connected in series:
■ the total resistance is the sum of the resistance of each
component
■ there is the same current through each component
■ the total potential difference of the supply is shared between
the components.
I) For components connected in parallel:
■ the potential difference across each component is the same
■ the total current through the whole circuit is the sum of the
currents through the separate components.
P2.3.2 Electrical circuits
m) The resistance of a filament bulb
increases as the temperature of the
filament increases.
n) The current through a diode flows in
one direction only. The diode has a very
high resistance in the reverse direction.
o) An LED emits light when a current flows through it in
the forward direction.
p) The resistance of a light-dependent resistor (LDR)
decreases as light intensity increases.
q) The resistance of a thermistor decreases as the
temperature increases.
Static electricity
• In static electricity when two objects
are rubbed together the electrons
move from one object to another.
This causes one object to have an
overall positive charge and the other
object to have an overall negative
charge
Static electricity
• Like charges repel
• Unlike charges attract
• Neutral objects are attracted
to both positively and
negatively charged objects.
If you wanted to test if an object was charged then you could
check if it attracted bits of paper, hair etc. It could attract or repel
another charged object.
If an object becomes highly charged then the potential difference
between then object and the ground increases and the objects
will discharge. When a charged object discharges (goes to
ground) then a spark might occur. This is the electrons jumping
from the object to the earthed conductor.
Current
• Current (symbol I, measured in amperes,
A) is the rate of flow of electrical charges
(symbol Q) or electrons i.e. The number of
charges per second.
• Current is the amount of charges (measured in
Coulombs) that flow every second, it is
represented by the equation:
Current (Ampere,A)=Charge (Coulombs, C)÷Time(s)
Voltage
• Voltage or potential difference (symbol V,
measured in volts, v) is the amount of energy
transferred by the charges i.e. the amount of energy
per charge.
• If there is a 2V cell or battery in a circuit then it gives
2 joules of energy to every coulomb of charge. When
these charges get to the device in the circuit e.g. a
bulb, then the energy gets transferred to the device.
To calculated potential difference/voltage you use the
following equation
Resistance
• Resistance (symbol R, measured in
ohms, Ω) is something that apposes the
flow of current.
• Voltage, current and resistance related by
the equation: V = I x R
Current-volt graphs
• Current- potential difference graphs tell you
how the current through a component varies
with voltage.
Series Circuits
• The total resistance is the sum of the
resistance of each component in the circuit.
– Total resistance (Rtotal) = R1 + R2
• The current is the same at every point in the
circuit.
• The voltage is shared between each
component in the circuit.
– Total voltage (Vtotal) = V1 + V2
Parallel circuit
• The voltage is the same across each branch
– Vtotal = V1 = V2
• The total current through the circuit is the
sum of the current through each component
– Total current (Itotal)= I1 + I2
P2.4 Using mains electricity
safely and the power of electrical
appliances
P2.4.1 Household electricity
a) Cells and batteries supply current that always
passes in the same direction. This is called direct
current (d.c.).
b) An alternating current (a.c.) is one that is
constantly changing direction.
c) Mains electricity is an a.c. supply. In the UK it has a
frequency of 50 cycles per second (50 hertz) and is
about 230V.
d) Most electrical appliances are connected to the
mains using cable and a three-pin plug.
e) The structure of electrical cable.
f) The structure and wiring of a three-pin plug.
P2.4.1 Household electricity
g) If an electrical fault causes too great a current,
the circuit is disconnected by a fuse or a circuit
breaker in the live wire.
h) When the current in a fuse wire exceeds the rating
of the fuse it will melt, breaking the circuit.
i) Some circuits are protected by Residual Current
Circuit Breakers (RCCBs).
j) Appliances with metal cases are usually earthed.
k) The earth wire and fuse together protect the
wiring of the circuit.
Direct current
• In circuits which are powered by
cells/batteries the current only flows
in one direction, this is called direct
current (d.c.).
Alternating current
• Alternating current (a.c.) is what we
receive from power station and what comes
out of plug sockets. This current changes
direction i.e. the current move back and forth
in the circuit.
• In the UK we use 230V at a frequency of 50Hz.
Mains Plug
Green/
Yellow
BLUE
BROWN
Structure of an Electrical Cable
• Electrical cabling has 3
main sections.
1) Stranded copper wire.
2) Coloured inner
insulation.
3) Harden outer
insulation.
Fuses
• A fuse is a small safety device that
contains a length of wire that is
designed to melt if the current in
the circuit gets too high.
•
•
Describe how fuses and circuit breakers are used in electrical safety.
Be able to correctly wire a plug.
Earth
• The Earth is a low resistance path for
the current to flow through.
• This means if the electricity has a
choice of going through us or the Earth
wire, it will flow through the earth wire.
Circuit breakers
• Much more sensitive.
• Much faster.
• Can be reset.
• There are also Residual Current Circuit Breaker
(RCCB) which, like circuit breakers, but work
much faster than circuit breakers and fuses.
P2.4.2 Current, charge and power
a) When an electrical charge flows through a
resistor, the resistor gets hot.
b) The rate at which energy is transferred by
an appliance is called the power.
P=E/t
c) Power, potential difference and current are
related by the equation:
P=IV
d) Energy transferred, potential difference
and charge are related by the equation:
E=VQ
Power
• Power is the amount of energy that
is transferred in 1 second.
• Power is measured in Watts.
• Power(W) = Energy(J)/Time(s)
Calculating Power from current
and voltage
• Power (W) = Current (A) x Voltage (V)
Power
(W)
Voltage Current
(V)
(A)
Energy transfer
• Electrons are ‘pushed’ through an
electrical circuit by the battery or other
electrical supply.
• Potential difference (voltage) is a
measure of the electrical ‘push’.
• The amount of energy transferred by 1
coulomb of charge (lots of electrons)
depends on the p.d. that pushes it.
E=VQ
P2.5 What happens when
radioactive substances decay,
and the uses and dangers of
their emissions
P2.5.1 Atomic structure
a) The basic structure of an atom is a small central
nucleus composed of protons and neutrons
surrounded by electrons.
b) The relative masses and relative electric charges of
protons, neutrons and electrons.
c) In an atom the number of electrons is equal to the
number of protons in the nucleus. The atom has no
overall electrical charge.
d) Atoms may lose or gain electrons to form charged
particles called ions.
e) The atoms of an element always have the same
number of protons, but have a different number of
neutrons for each isotope. The total number of
protons in an atom is called its atomic number. The
total number of protons and neutrons in an atom is
called its mass number.
Proton Charge = +1
Neutron Charge = 0
Electron Charge = -1
The discovery of the nucleus
Dalton’s Atomic Theory: Atoms are indestructible
and indivisible (cannot be divided into smaller
particles).
John Dalton (1766-1804)
Thomson’s Plum Pudding Model of the
Atom: Thompson (1856-1940)
Thompson discovered the ‘electron’ .
So Dalton’s model of the atom was no longer
acceptable because now there is something
inside the atom.
Thomson believed the atom was made of
positively charged matter with negatively
charged electrons scattered throughout like
plums in a plum pudding (or chocolate chips
in chocolate chip cookie).
Ernest Rutherford (1871-1937) wanted to test
Thompson’s plum pudding model of the atom using his
newly discovered alpha particles.
He carried out the ‘gold foil experiment in 1910:
He bombarded thin gold foil with a beam of ‘alpha’
particles. He expected it to be just like firing bullets at
a tissue paper. “If the positive charge was evenly
spread out like Thompson says, the beam should have
easily passed through”.
Expected
Found
Conclusion: All of the positive charge, and most of the
mass of an atom are concentrated in a small core,
called the nucleus.
Proton Number and Mass Number
• The number of protons in an atom tells you
what element it is.
• The number of neutrons tells you if it is an
isotope of an element.
Number of Protons
and neutrons
Number of Protons (Also
the number of electrons)
Mass number = Number of protons + Number of
neutrons.
What happens when an atom
loses or gain an electron?
• They become ions.
• Atoms with a charge that is not zero.
• If an electron is lost the charge is
positive.
• If an electron is gained the charge is
negative.
P2.5.2 Atoms and radiation
a) Some substances give out radiation
from the nuclei of their atoms all the
time, whatever happens to them. These
substances are said to be radioactive.
b) The origins of background radiation.
c) Identification of an alpha particle as
two neutrons and two protons, the
same as a helium nucleus, a beta
particle as an electron from the nucleus
and gamma radiation as
electromagnetic radiation.
d) Nuclear equations to show single
alpha and beta decay. (HT)
P2.5.2 Atoms and radiation
e) Properties of the alpha, beta and gamma
radiations limited to their relative ionising
power, their penetration through materials
and their range in air.
f) Alpha and beta radiations are deflected by
both electric and magnetic fields but
gamma radiation is not.
g) The uses of and the dangers associated
with each type of nuclear radiation.
h) The half-life of a radioactive isotope is the
average time it takes for the number of
nuclei of the isotope in a sample to halve,
or the time it takes for the count rate from
a sample containing the isotope to fall to
half its initial level.
Types of radiation
Half-Life
If the number of counts recorded on
a Geiger counter starts at 100 and 3
hours later it counts 50; then the half
life of that substance will be 3 hours.
The nuclei of radioactive
atoms are unstable. They
break down and change
into a completely
different type of atom.
This is called radioactive
decay. For example,
carbon-14 decays to
nitrogen-14 when it
emits beta radiation.
It is not possible to
predict when an
individual atom might
decay. But it is possible
to measure how long it
takes for half the nuclei
of a piece of radioactive
material to decay. This is
called the half-life of the
radioactive isotope.
Only Alpha
and beta
radiation are
affected by
electric and
magnetic
fields.
P2.6 Nuclear fission and
nuclear fusion
P2.6.1 Nuclear fission
a) There are two fissionable substances in
common use in nuclear reactors:
uranium-235 and plutonium-239.
b) Nuclear fission is the splitting of an
atomic nucleus.
c) For fission to occur, the uranium-235 or
plutonium-239 nucleus must first absorb
a neutron.
d) The nucleus undergoing fission splits
into two smaller nuclei and two or three
neutrons and energy is released.
e) The neutrons may go on to start a chain
reaction.
P2.6.2 Nuclear fusion
a) Nuclear fusion is the joining of two
atomic nuclei to form a larger one.
b) Nuclear fusion is the process by which
energy is released in stars.
c) Stars form when enough dust and gas
from space is pulled together by
gravitational attraction. Smaller masses
may also form and be attracted by a
larger mass to become planets.
d) During the ‘main sequence’ period of its
life cycle a star is stable because the
forces within it are balanced.
P2.6.2 Nuclear fusion
e) A star goes through a life cycle. This
life cycle is determined by the size of
the star.
f) Fusion processes in stars produce all
of the naturally occurring elements.
These elements may be distributed
throughout the Universe by the
explosion of a massive star
(supernova) at the end of its life.
Nuclear Fission
Nuclear fission occurs when a
Uranium-235 nucleus or a
Plutonium-239 nucleus splits.
When a nucleus undergoes
fission, it releases two or three
neutrons which go on to cause
further fission resulting in a
chain reaction.
The energy released could be
used to generate electricity.
the waste product is highly
radioactive substances (Barium
and Krypton) which need to be
disposed of safely.
Nuclear Fusion
Nuclear fusion occurs when
two small nuclei are forced
close enough together so they
join to make large nucleus.
Nuclear fusion is the process
by which energy is released in
the Sun. Energy is released
when two nuclei are fused
together.
The energy released could be
used to generate electricity.
The waste product is Helium
which is a harmless gas.
On the other hand it has
technical difficulties as a very
high temperatures are needed
to start the fusion of nuclei.
Black Dwarf
Main
Sequence
Stars