By: Peter Brookes. Contents Representing Motion Force, Mass and Acceleration Weight and Friction...
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Additional Physics By: Peter Brookes
By: Peter Brookes. Contents Representing Motion Force, Mass and Acceleration Weight and Friction Kinetic Energy and Momentum Static Electricity Resistance
Contents Representing Motion Force, Mass and Acceleration
Weight and Friction Kinetic Energy and Momentum Static Electricity
Resistance and Resistors Mains Electricity
Slide 3
Representing Motion
Slide 4
The slope on a distance-time graph represents the speed of an
object. The velocity of an object is its speed in a particular
direction. The slope on a velocity-time graph represents the
acceleration of an object. The distance travelled is equal to the
area under a velocity-time graph.
Slide 5
Speed, Distance and Time When an object moves in a straight
line at a steady speed, you can calculate its speed if you know how
far it travels and how long it takes. This equation shows the
relationship between speed, distance travelled and time taken: For
example, a car travels 300 metres in 20 seconds. Its speed is 300
20 = 15m/s.
Slide 6
Distance-Time Graphs The vertical axis of a distance-time graph
is the distance travelled from the start. The horizontal axis is
the time from the start.
Slide 7
Features of the Graphs When an object is stationary, the line
on the graph is horizontal. When an object is moving at a steady
speed, the line on the graph is straight, but sloped. The diagram
shows some typical lines on a distance- time graph.
Slide 8
Velocity-Time Graphs You should be able to explain
velocity-time graphs for objects moving with a constant velocity or
constant acceleration.
Slide 9
Background Information The velocity of an object is its speed
in a particular direction. This means that two cars travelling at
the same speed, but in opposite directions, have different
velocities. The vertical axis of a velocity-time graph is the
velocity of the object. The horizontal axis is the time from the
start.
Slide 10
Features of the Graphs When an object is moving with a constant
velocity, the line on the graph is horizontal. When an object is
moving with a constant acceleration, the line on the graph is
straight, but sloped. The diagram shows some typical lines on a
velocity-time graph.
Slide 11
Velocity-Time Graphs The steeper the line, the greater the
acceleration of the object. The blue line is steeper than the red
line because it represents an object with a greater acceleration.
Notice that a line sloping downwards - with a negative gradient -
represents an object with a constant deceleration - slowing
down.
Slide 12
Acceleration You should be able to calculate the acceleration
of an object from its change in velocity and the time taken.
Slide 13
The Equation When an object moves in a straight line with a
constant acceleration, you can calculate its acceleration if you
know how much its velocity changes and how long this takes. This
equation shows the relationship between acceleration, change in
velocity and time taken: For example, a car accelerates in 5s from
25m/s to 35m/s. Its velocity changes by 35 - 25 = 10m/s. So its
acceleration is 10 5 = 2m/s 2.
Slide 14
Distance-Time Graph Gradients To calculate the gradient of the
line on a graph, divide the change in the vertical axis by the
change in the horizontal axis. The gradient of a line on a
distance-time graph represents the speed of the object. Study this
distance-time graph.
Slide 15
Velocity-Time Graph Gradients The gradient of a line on a
velocity-time graph represents the acceleration of the object.
Study this velocity-time graph.
Slide 16
The Area The area under the line in a velocity-time graph
represents the distance travelled. To find the distance travelled
in the graph above, we need to find the area of the light-blue
triangle and the dark-blue rectangle. 1- Area of light-blue
triangle The width of the triangle is 4 seconds and the height is 8
metres per second. To find the area, you use the equation: area of
triangle = 1 2 base height so the area of the light-blue triangle
is 1 2 8 4 = 16m. 2- Area of dark-blue rectangle The width of the
rectangle is 6 seconds and the height is 8 metres per second. So
the area is 8 6 = 48m. 3- Area under the whole graph The area of
the light-blue triangle plus the area of the dark-blue rectangle
is: 16 + 48 = 64m. This is the total area under the distance-time
graph. This area represents the distance covered.
Slide 17
Summary the gradient of a velocity-time graph represents the
acceleration the area under a velocity-time graph represents the
distance covered
Slide 18
Force, Mass and Acceleration
Slide 19
A stationary object remains stationary if the sum of the forces
acting upon it - resultant force - is zero. A moving object with a
zero resultant force keeps moving at the same speed and in the same
direction. If the resultant force acting on an object is not zero,
a stationary object begins to accelerate in the same direction as
the force. A moving object speeds up, slows down or changes
direction. Acceleration depends on the force applied to an object
and the object's mass.
Slide 20
Resultant Force An object may have several different forces
acting on it, which can have different strengths and directions.
But they can be added together to give the resultant force. This is
a single force that has the same effect on the object as all the
individual forces acting together.
Slide 21
When the Resultant Force is Zero When all the forces are
balanced, the resultant force is zero. In this case: a stationary
object remains stationary a moving object keeps on moving at the
same speed in the same direction For example, in the diagram of the
weightlifter, the resultant force on the bar is zero, so the bar
does not move. Its weight acting downwards is balanced by the
upward force provided by the weightlifter. The longer the arrow,
the bigger the force. In this diagram, the arrows are the same
length, so we know they are the same size.
Slide 22
When the Resultant Force is Not Zero When all the forces are
not balanced, the resultant force is not zero. In this case: A
stationary object begins to move in the direction of the resultant
force. A moving object speeds up, slows down or changes direction
depending on the direction of the resultant force. In this diagram
of the weightlifter, the resultant force on the bar is not zero.
The upwards force is bigger than the downwards force. The resultant
force acts in the upwards direction, so the bar moves upwards. In
this next diagram of the weightlifter, the resultant force on the
bar is also not zero. This time, the upwards force is smaller than
the downwards force. The resultant force acts in the downwards
direction, so the bar moves downwards.
Slide 23
Diagrams
Slide 24
Forces and Acceleration You should know that objects accelerate
when the resultant force is not zero, and understand the factors
that affect the size of the acceleration.
Slide 25
Size of the Force An object will accelerate in the direction of
the resultant force. The bigger the force, the greater the
acceleration. Doubling the size of the (resultant) force doubles
the acceleration.
Slide 26
The Mass An object will accelerate in the direction of the
resultant force. A force on a large mass will accelerate it less
than the same force on a smaller mass. Doubling the mass halves the
acceleration.
Slide 27
Forces and Acceleration Calculations You should know the
equation that shows the relationship between resultant force, mass
and acceleration, and be able to use it.
Slide 28
The Equation Resultant force (newton, N) = mass (kg)
acceleration (m/s 2 ) You can see from this equation that 1N is the
force needed to give 1kg an acceleration of 1m/s 2. For example,
the force needed to accelerate a 10kg mass by 5m/s 2 is: 10 x 5 =
50N The same force could accelerate a 1kg mass by 50m/s 2 or a
100kg mass by 0.5m/s 2. Putting it simply, we can say that it takes
more force to accelerate a larger mass.
Slide 29
Four Typical Forces That I Could Be Asked On Air resistance -
drag When an object moves through the air, the force of air
resistance acts in the opposite direction to the motion. Air
resistance depends on the shape of the object and its speed.
Contact force This happens when two objects are pushed together.
They exert equal and opposite forces on each other. The contact
force from the ground pushes up on your feet even as you stand
still. This is the force you feel in your feet. You feel the ground
pushing back against your weight pushing down. Friction This is the
force that resists movement between two surfaces which are in
contact. Gravity This is the force that pulls objects towards the
Earth. We call the force of gravity on an object its weight. The
Earth pulls with a force of about 10 newtons on every kilogram of
mass.
Slide 30
Weight and Friction
Slide 31
Gravity is a force that attracts objects with mass towards each
other. The weight of an object is the force acting on it due to
gravity. The gravitational field strength of the Earth is 10 N/kg.
The stopping distance of a car depends on two things: the thinking
distance and the braking distance.
Slide 32
Weight Weight is not the same as mass. Mass is a measure of how
much stuff is in an object. Weight is a force acting on that stuff.
You have to be careful. In physics, the term weight has a specific
meaning, and is measured in newtons. Mass is measured in kilograms.
The mass of a given object is the same everywhere, but its weight
can change.
Slide 33
Gravitational Field Strength Weight is the result of gravity.
The gravitational field strength of the Earth is 10 N/kg (ten
newtons per kilogram). This means an object with a mass of 1kg
would be attracted towards the centre of the Earth by a force of
10N. We feel forces like this as weight. You would weigh less on
the Moon because the gravitational field strength of the Moon is
one-sixth of that of the Earth. But note that your mass would stay
the same.
Slide 34
Weight On Earth, if you drop an object it accelerates towards
the centre of the planet. You can calculate the weight of an object
using this equation: weight (N) = mass (kg) gravitational field
strength (N/kg)
Slide 35
Falling Objects You should be able to describe the forces
affecting a falling object at different stages of its fall.
Usually, you need to think about two forces: 1- The weight of the
object. This is a force acting downwards, caused by the objects
mass the Earths gravitational field. 2- Air resistance. This is a
frictional force acting in the opposite direction to the movement
of the object.
Slide 36
Three Stages of Falling When an object is dropped, we can
identify three stages before it hits the ground: 1- At the start,
the object accelerates downwards because of its weight. There is no
air resistance. There is a resultant force acting downwards. 2- As
it gains speed, the objects weight stays the same, but the air
resistance on it increases. There is a resultant force acting
downwards. 3- Eventually, the objects weight is balanced by the air
resistance. There is no resultant force and the object reaches a
steady speed, called the terminal velocity.
Slide 37
Terminal Velocity What happens if you drop a feather and a coin
together? The feather and the coin have roughly the same surface
area, so when they begin to fall they have about the same air
resistance. As the feather falls, its air resistance increases
until it soon balances the weight of the feather. The feather now
falls at its terminal velocity. But the coin is much heavier, so it
has to travel quite fast before air resistance is large enough to
balance its weight. In fact, it probably hits the ground before it
reaches its terminal velocity.
Slide 38
On the Moon An astronaut on the Moon carried out a famous
experiment. He dropped a hammer and a feather at the same time and
found that they landed together. The Moon's gravity is too weak for
it to hold onto an atmosphere, so there is no air resistance. When
the hammer and feather were dropped, they fell together with the
same acceleration.
Slide 39
Stopping Distances The following are factors that affect
stopping distance of cars.
Slide 40
Thinking Distance It takes a certain amount of time for a
driver to react to a hazard and start applying the brakes. During
this time, the car is still moving. The faster the car is
travelling, the greater this thinking distance will be. The
thinking distance will also increase if the driver's reactions are
slower because they are: under the influence of alcohol under the
influence of drugs tired
Slide 41
Braking Distance The braking distance is the distance the car
travels from where the brakes are first applied to where the car
stops. If the braking force is too great, the tyres may not grip
the road sufficiently and the car may skid. The faster the car is
travelling, the greater the braking distance will be. The braking
distance will also increase if: The brakes or tyres are worn. The
weather conditions are poor, such as an icy or wet road. The car is
more heavily laden, for example, with passengers and luggage.
Slide 42
Stopping Distance The stopping distance is the thinking
distance added to the braking distance. The graph shows some
typical stopping distances.
Slide 43
Kinetic Energy and Momentum
Slide 44
Work done and energy transferred are measured in joules (J).
The work done on an object can be calculated if the force and
distance moved are known. A change in momentum happens when a force
is applied to an object that is moving or is able to move. The
total momentum in an explosion or collision stays the same.
Slide 45
Work, Force and Distance You should know, and be able to use,
the relationship between work done, force applied and distance
moved.
Slide 46
Background Work and energy are measured in the same unit, the
joule (J). When an object is moved by a force, energy is
transferred and work is done. But work is not a form of energy - it
is one of the ways in which energy can be transferred.
Slide 47
The Equation This equation shows the relationship between work
done, force applied and distance moved: work done (joule, J) =
force (newton, N) distance (metre, m ) The distance involved is the
distance moved in the direction of the applied force.
Slide 48
Gravitational Potential Energy Any object that is raised
against the force of gravity stores gravitational potential energy.
For example, if you lift a book up onto a shelf, you have to do
work against the force of gravity. The book has gained
gravitational potential energy.
Slide 49
Elastic Potential Energy Elastic objects such as elastic bands
and squash balls can change their shape. They can be stretched or
squashed, but energy is needed to change their shape. This energy
is stored in the stretched or squashed object as elastic potential
energy.
Slide 50
Kinetic Energy Every moving object has kinetic energy
(sometimes called movement energy). The more mass an object has,
and the faster it is moving, the more kinetic energy it has. You
should be able to discuss the transformation of kinetic energy to
other forms of energy.
Slide 51
Example 1- The Bouncing Ball Several energy transfers happen
when a squash ball is dropped onto a table and bounces up again.
When the ball is stationary above the table, its gravitational
potential energy (GPE) is at a maximum. It has no kinetic energy
(KE), or elastic potential energy (EPE). As the ball falls, its GPE
is transferred to KE and the ball accelerates towards the table.
When the ball hits the table, the KE is transferred to EPE as the
ball squashes. As the ball regains its shape, the EPE is
transferred to KE and it bounces upwards. When the ball reaches the
top of its travel, all the KE has been transferred to GPE again.
Note that the ball will be lower than it was when it was first
dropped, because some energy is also transferred as heat and sound
to the surroundings.
Slide 52
Example 1 Diagrams High Up GPE- Maximum KE- None EPE- None
Falling GPE- Decreasing KE- Increasing EPE- None On Table GPE-
Minimum KE- None EPE- Maximum
Slide 53
Example 2- The Pendulum The pendulum is a simple machine for
transferring gravitational potential energy to kinetic energy, and
back again. When the bob is at the highest point of its swing, it
has no kinetic energy, but its gravitational potential energy is at
a maximum. As the bob swings downwards, gravitational potential
energy is transferred to kinetic energy, and the bob accelerates.
At the bottom of its swing, the bobs kinetic energy is at a maximum
and its gravitational potential energy is at a minimum. As the bob
swings upwards, its kinetic energy is transferred to gravitational
potential energy again. At the top of its swing, it once again has
no kinetic energy, but its gravitational potential energy is at a
maximum. Note that the bobs swing will become lower with each
swing, because some energy is also transferred as heat to the
surroundings.
Slide 54
Example 2- Diagram
Slide 55
Momentum A moving object has momentum. This is the tendency of
the object to keep moving in the same direction. It is difficult to
change the direction of movement of an object with a lot of
momentum. You can calculate momentum using this equation: momentum
(kg m/s) = mass (kg) velocity (m/s) Notice that momentum has:
magnitude - an amount because it depends on the objects mass
direction - because it depends on the velocity of the object
Slide 56
Conservation of Momentum So long as no external forces are
acting on the objects involved, the total momentum stays the same
in explosions and collisions. We say that momentum is conserved.
You can use this idea to work out the mass, velocity or momentum of
an object in an explosion or collision.
Slide 57
Example A bullet with a mass of 0.03 kg leaves a gun at 1000
m/s. If the guns mass is 1.5 kg, what is the velocity of the recoil
on the gun? momentum of bullet = mass velocity = 0.03 kg 1,000 m/s
= 30 kg m/s Rearrange the equation: velocity = momentum mass
velocity of recoil on gun = 30 kg m/s 1.5 kg = 20 m/s
Slide 58
Safety Features in Vehicles When there is a car crash, the car,
its contents, and the passengers, decelerate rapidly. They
experience great forces because of the change in momentum, which
can cause injury. If the time taken for the change in momentum on
the body is increased, the forces on the body are reduced too. Seat
belts and crumple zones are designed to reduce the forces on the
body if there is a collision.
Slide 59
Seat Belts Seat belts stop you tumbling around inside the car
if there is a collision. However, they are designed to stretch a
bit in a collision. This increases the time taken for the bodys
momentum to reach zero, so reduces the forces on it.
Slide 60
Air Bags Air bags increase the time taken for the heads
momentum to reach zero, so reduce the forces on it. They also act a
soft cushion and prevent cuts.
Slide 61
Kinetic Energy The equation This equation shows the
relationship between kinetic energy (J), mass (kg) and speed (m/s):
kinetic energy = 1 2 mass speed 2
Slide 62
Momentum-Higher You need to be able to calculate the force
involved in changing the momentum of an object. Here is the
equation you need: The force is measured in newtons, N. The time is
measured in seconds, s.
Slide 63
Static Electricity
Slide 64
Some insulating materials become electrically charged when they
are rubbed together. Charges that are the same repel, while unlike
charges attract. Electrostatic precipitators, photocopiers and
laser printers make practical use of electrostatic charges.
Slide 65
Moving Charges When you rub two different insulating materials
against each other they become electrically charged. This only
works for insulated objects - conductors lose the charge to earth.
When the materials are rubbed against each other: 1- negatively
charged particles called electrons move from one material to the
other 2- the material that loses electrons becomes positively
charged 3- the material that gains electrons becomes negatively
charged 3- both materials gain an equal amount of charge, but the
charges are opposite
Slide 66
Detecting Charges If two charged objects with the same type of
charge are brought close together, they will repel each other -
that is, if they are both positive or both negative. They will
attract each other if they have opposite charges. The only way to
tell if an object is charged is to see if it repels another charged
object. This is because charged objects will also attract small
uncharged objects.
Slide 67
Discharge A charged object can be discharged by connecting it
to earth with a metal wire or other conductor. If the potential
difference (voltage) is very large, a spark may jump across the gap
between the charged object and the conductor. This can be
dangerous. For example, it could cause an explosion in a petrol
station.
Slide 68
Electrostatic Precipitators Many power stations burn fossil
fuels such as coal and oil. Smoke is produced when these fuels
burn. Smoke comprises tiny solid particles, such as unreacted
carbon, which can damage buildings and cause breathing
difficulties. To avoid this, the smoke is removed from waste gases
before they pass out of the chimneys. The electrostatic
precipitator is the device used for this job.
Slide 69
Electrostatic Precipitators The flow chart outlines how an
electrostatic precipitator works. 1.Smoke particles pick up a
negative charge. 2.Smoke particles are attracted to the collecting
plates. 3.Collecting plates are knocked to remove the smoke
particles.
Slide 70
Photocopiers The flow chart outlines how a photocopier works. A
laser printer works in a similar way.
Slide 71
Resistance and Resistors
Slide 72
Resistance is measured in ohms. It can be calculated from the
potential difference across a component and the current flowing
through it. The total resistance of a series circuit is the sum of
the resistances of the components in the circuit. Resistors,
filament lamps and diodes produce different current-potential
difference graphs. The resistance of thermistors depends on the
temperature, while the resistance of light-dependent resistors
(LDRs) depends on the light intensity.
Slide 73
Why Do We Get Resistance? An electric current flows when
electrons move through a conductor. The moving electrons can
collide with the atoms of the conductor. This makes it more
difficult for the current to flow, and causes resistance. Electrons
collide with atoms more often in a long wire than they do in a
short one. A thin wire has fewer electrons to carry the current
than a thick wire. This means the resistance in a wire increases
as: 1- the length of the wire increases 2- the thickness of the
wire decreases
Slide 74
Calculating Resistance Resistance is measured in ohms, You can
calculate resistance using this equation: potential difference
(volt, V) = current (ampere, A) resistance (ohm, )
Slide 75
Series Circuits When components are connected in series, their
total resistance is the sum of their individual resistances. For
example, if a 2 resistor, a 1 resistor and a 3 resistor are
connected side by side, their total resistance is 2 + 1 + 3 = 6 .
If you increase the number of lamps in a series circuit, the total
resistance will increase and less current will flow.
Slide 76
Variable Resistors The resistance in a circuit can also be
altered using variable resistors. For example, these components may
be used in dimmer switches, or to control the volume of a CD
player.
Slide 77
Current-Potential Difference Graphs A graph of current -
vertical axis - against potential difference - horizontal axis -
shows you how the current flowing through a component varies with
the potential difference across it. You should be able to recognise
these graphs for resistors at constant temperature, for filament
lamps, and for diodes.
Slide 78
Resistor At Constant Temperature The current flowing through a
resistor at a constant temperature is directly proportional to the
potential difference across it. A component that gives a graph like
the one to the right is said to follow Ohms Law.
Slide 79
The Filament Lamp The filament lamp is a common type of light
bulb. It contains a thin coil of wire called the filament. This
heats up when an electric current passes through it, and produces
light as a result. The filament lamp does not follow Ohms Law. Its
resistance increases as the temperature of its filament increases.
So the current flowing through a filament lamp is not directly
proportional to the voltage across it. This is the graph of current
against voltage for a filament lamp.
Slide 80
The Diode Diodes are electronic components that can be used to
regulate the potential difference in circuits and to make logic
gates. Light-emitting diodes (LEDs) give off light and are often
used for indicator lights in electrical equipment such as computers
and television sets. The diode has a very high resistance in one
direction. This means that current can only flow in the other
direction. This is the graph of current against potential
difference for a diode. A Diode
Slide 81
Thermistors and LDRs You should be able to recognise the
circuit symbols for the thermistor and the LDR (light-dependent
resistor), and know how the resistance of these components can be
changed.
Slide 82
The Thermistor Thermistors are used as temperature sensors -
for example, in fire alarms. Their resistance decreases as the
temperature increases: 1- At low temperatures, the resistance of a
thermistor is high and little current can flow through them. 2- At
high temperatures, the resistance of a thermistor is low and more
current can flow through them. A Thermistor
Slide 83
The LDR LDRs (light-dependent resistors) are used to detect
light levels, for example, in automatic security lights. Their
resistance decreases as the light intensity increases: 1- In the
dark and at low light levels, the resistance of an LDR is high and
little current can flow through it. 2- In bright light, the
resistance of an LDR is low and more current can flow through it.
Light Dependant Resistor (LDR)
Slide 84
Mains Electricity
Slide 85
The UK mains electricity supply is about 230V and can kill if
not used safely. Electrical circuits, cables, plugs and appliances
are designed to reduce the chances of receiving an electric shock.
The more electrical energy used, the greater the cost. Electrical
supplies can be direct current (d.c.) or alternating current
(a.c.).
Slide 86
The Cable A mains electricity cable contains two or three inner
wires. Each has a core of copper, because copper is a good
conductor of electricity. The outer layers are flexible plastic,
because plastic is a good electrical insulator. The inner wires are
colour coded: ColourWire BlueNeutral BrownLive Green and Yellow
StripesEarth
Slide 87
The Plug The features of a plug are: 1- The case is made from
tough plastic or rubber, because these materials are good
electrical insulators. 2- The three pins are made from brass, which
is a good conductor of electricity. 3- There is a fuse between the
live terminal and the live pin. 4- The fuse breaks the circuit if
too much current flows. 5- The cable is secured in the plug by a
cable grip. This should grip the cable itself, and not the
individual wires inside it.
Slide 88
A Plug Diagram
Slide 89
Where Does Each Wire Go? There is an easy way to remember where
to connect each wire. Take the second letters of the words blue,
brown and striped. This reminds you that when you look into a plug
from above: blue goes left, brown goes right and striped goes to
the top.
Slide 90
Earthing Many electrical appliances have metal cases, including
cookers, washing machines and refrigerators. The earth wire creates
a safe route for the current to flow through if the live wire
touches the casing. You will get an electric shock if the live wire
inside an appliance, such as a cooker, comes loose and touches the
metal casing. However, the earth terminal is connected to the metal
casing so that the current goes through the earth wire instead of
causing an electric shock. A strong current surges through the
earth wire because it has a very low resistance. This breaks the
fuse and disconnects the appliance.
Slide 91
Earthing Of An Electric Cooker
Slide 92
The Circuit Breaker Many electrical appliances have metal
cases, including cookers, washing machines and refrigerators. The
earth wire creates a safe route for the current to flow through if
the live wire touches the casing. You will get an electric shock if
the live wire inside an appliance, such as a cooker, comes loose
and touches the metal casing. However, the earth terminal is
connected to the metal casing so that the current goes through the
earth wire instead of causing an electric shock. A strong current
surges through the earth wire because it has a very low resistance.
This breaks the fuse and disconnects the appliance.
Slide 93
The Fuse The fuse breaks the circuit if a fault in an appliance
causes too much current flow. This protects the wiring and the
appliance if something goes wrong. The fuse contains a piece of
wire that melts easily. If the current going through the fuse is
too great, the wire heats up until it melts and breaks the circuit.
Fuses in plugs are made in standard ratings. The most common are
3A, 5A and 13A. The fuse should be rated at a slightly higher
current than the device needs: 1- if the device works at 3A, use a
5A fuse 2- if the device works at 10A, use a 13A fuse Cars also
have fuses. An electrical fault in a car could start a fire, so all
the circuits have to be protected by fuses.
Slide 94
A 13A Fuse With A Low Melting Point
Slide 95
Power Power is a measure of how quickly energy is transferred.
The unit of power is the watt (W). You can work out power using
this equation:
Slide 96
Energy In Circuits The more energy that is transferred in a
certain time, the greater the power. A 100W light bulb transfers
more electrical energy each second than a 60W light bulb. The
equation below shows the relationship between power, potential
difference (voltage) and current: power (watts) = current (amps) x
potential difference (volts)
Slide 97
Direct Current (D.C.) If the current flows in only one
direction it is called direct current, or d.c. Batteries and cells
supply d.c. electricity, with a typical battery supplying maybe
1.5V. The diagram shows an oscilloscope screen displaying the
signal from a d.c. supply.
Slide 98
Alternating Current (A.C.) If the current constantly changes
direction, it is called alternating current, or a.c.. Mains
electricity is an a.c. supply, with the UK mains supply being about
230V. It has a frequency of 50Hz (50 hertz), which means it changes
direction, and back again, 50 times a second. The diagram shows an
oscilloscope screen displaying the signal from an a.c. supply.
Slide 99
Alternating Current (A.C.)- Higher The potential difference of
the live terminal varies between a large positive value and a large
negative value. However, the neutral terminal is at a potential
difference close to earth, which is zero. The diagram shows an
oscilloscope screen displaying the signals from the mains supply.
The red trace is the live terminal and the blue trace the neutral
terminal. Note that, although the mean voltage of the mains supply
is about 230V, the peak voltage is higher.
Slide 100
Charge, Current and Time Electrical charge is measured in
coulomb, C. The amount of electrical charge that moves in a circuit
depends on the current flow and how long it flows for. The equation
below shows the relationship between charge, current and time:
charge (coulomb, C) = current (ampere, A) time (second, s)
Slide 101
Energy Transferred, Potential Difference and Charge For a given
amount of electrical charge that moves, the amount of energy
transformed increases as the potential difference (voltage)
increases. The equation below shows the relationship between energy
transformed, potential difference and charge: energy transformed
(joule, J) = potential difference (volt, V) charge (coulomb,
C)