Download 2462-Electricity for OCR 21st

OCR 21st
Module P5 Electricity
TASK 1: ELECTRIC CHARGE
What is electricity? It is some kind of flow through the wires in the circuit but
what is it that flows? To find out more you need to do some background reading.
Using an appropriate text book for guidance
Explain ....
a) what is meant by an electric current
b) the difference between 'conventional current direction' and the direction of
electron flow
c) the rule which links current, charge and time
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Module P5 Electricity
TASK 2: VOLTAGE
Using a voltmeter
Voltmeters look very much like ammeters but it is important that you don't confuse one for the
other. They are used in quite different ways and measure completely different things.
The voltage of a battery is a measure of the electrical 'push' which it provides. A larger voltage
means that the electrons have more energy when they leave the battery and that more current
will flow in the circuit.
To measure the voltage applied to a lamp you do not break the circuit. You place the voltmeter
along side the lamp and connect the two terminals of the voltmeter to the two terminals of the
lamp. Remember to connect the positive (red) terminal of the voltmeter to the lamp terminal
which is nearest to the positive side of the battery.
Experiment 1
Use a digital voltmeter for this. Check that you are using the correct (orange)
adaptor.
Set up the circuit shown.
Now connect the voltmeter with the positive (+) terminal to A
and the negative (-) terminal to B. If the voltmeter reads
negative then you have connected these the wrong way
round.
Reading = .....................
Now connect the voltmeter between points C and D.
Reading = .....................
Now connect the voltmeter to the terminals of the lab-pack, between points E and F.
Reading = .....................
You should find that the first two readings add up to the third reading (there may
be a slight difference in the meter readings).
Experiment 2
Set up the parallel circuit shown.
Connect the voltmeter between points
A and B.
Reading = ..................
Now connect the voltmeter between C and D.
Reading = ..................
Now between E and F
Reading = ..................
You should find that all these readings are the same.
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Module P5 Electricity
Explanation
The voltmeter is connected between two points in the circuit.
Its reading shows how much energy the electrons lose as they travel through the
circuit between these two points.
In the first experiment the electrons have to pass through both lamps, and so
part of the energy is lost in each lamp (not really lost - just converted into heat
and light). The total amount of energy lost is equal to the amounts lost in each
lamp added together.
In the second experiment the electrons pass through only one lamp when they
travel round the circuit. All their energy is 'lost' in that lamp - whichever one they
pass through. So the full battery voltage is applied to each lamp and they both
light brightly.
The general rule for any series circuit:
If you add up the voltages across each componentthis sum will equal the
supply voltage.
The general rule for any parallel circuit:
When two components are connected in parallel the same voltage is applied
to each.
Questions
In the circuits shown below all the lamps are of the same type.
1. A voltmeter connected between X and Y reads 12V.
What would a voltmeter read if connected between:
a) A and B ..........
b) A and C...........
c) A and D ..........
d) A and E ..........
2.What will the reading on the voltmeter V2 be ?
.........................................................................
Check your answers before you go on.
.
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Module P5 Electricity
TASK 3: ELECTRICAL RESISTANCE - Part 1
In some of the following experiments you will need a length of 'resistance wire'.
This is a piece of wire about 30cm long, which you should pin to a board. This is
so that it cannot coil up and touch itself. You can connect the wire into your
circuits by means of crocodile clips.
Experiment 1
Set up circuit 1 using 30cm of the wire labelled 'A'. The crocodile clips should be
connected to the ends of the resistance wire.
What difference does the resistance wire make to
to the lamp ?
...............................................................................
What is the reading on the ammeter ?
...............................
Now slowly slide one of the clips along the wire so that the length of wire
between the clips is decreased. What happens ?
.................................................................................................................................
The electricity only flows through the piece of wire between the clips. As you
decrease this length there is less resistance in the circuit. Less resistance
generally means more current.
Experiment 2
Repeat experiment 1 using the second type of wire labelled 'B'. Make sure that
you use exactly the same length as before.
This wire is slightly thicker than wire 'A'. What effect does the thickness have on
the resistance of the wire ?
.................................................................................................................................
Draw the symbol for a resistor in the space below.
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Module P5 Electricity
TASK 3: RESISTANCE -Part 2
Resistivity
You should now know that the resistance of a piece of wire increases
i) as its length increases
ii) as its thickness decreases.
substance
resistivity
The resistance also depends on what
the wire is made of. Each substance
has a certain resistivity. A substance
with a low resistivity makes a good
electrical conductor.
silver
copper
gold
aluminium
iron
steel
carbon
Using a variable resisto (rheostat)
1.6
1.7
2.4
2.7
10
15
3000
It is often necessary to be able to
control the amount of current flowing in a circuit. This can be done by adjusting
the voltage of the power supply but it can also by done by using a variable
resistor. A variable resistor may also be called a rheostat (ree-oh-stat) or a
potentiometer.
This is simply a long length of wire wound into
a coil (see diagram).
If connections are made to A and B (or A and
C) then the resistance depends on the
position of the sliding contact.
If the slider is moved towards B then the
resistance of AB is decreased because the
length of wire between A and B is shorter. At
the same time the resistance of AC increases.
Note that the resistance between B and C is always the same. (It is a common
mistake to connect to B and C.)
Practical Task
Build the circuit shown. Adjust the rheostat so
that the 12V lamp has a current of exactly 1A
flowing through it. Get your circuit checked.
There are two alternative symbols for a variable resistor. Make sure you can
recognise them..
To summarise the lower the resistance the greater the current and the brighter
the lamp
If you did not get your circuit checked, get your work checked now!
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Module P5 Electricity
TASK 4: VOLTAGE, CURRENT AND RESISTANCE (Core and Extension)
(Some of this is hard work)
You will have seen that the resistance of a component can be worked out from the formula
Resistance =
voltage applied to component
V
-------------------------------------------- or R= ------current flowing through it
I
(Read this again if you don't remember it.)
The question you are to answer now is ....
Does the resistance of a wire change when the current through it is
increased ?`
Experiment
You will need
a voltmeter
an ammeter
a length of resistance wire 'A'
leads
2 crocodile clips
a lab-pack.
Connect up the circuit shown (It is advisable to connect up the circuit without the voltmeter first.
When you are sure that it works add the voltmeter.)
Use the rheostat to adjust the readings of the ammeter and voltmeter. Take at least eight pairs of
readings of voltage and current. Do not allow the current to exceed 1A. Put your values in the
table below.
voltage
(in volts)
current
(in
amperes)
Work out the values for the resistance for each pair of readings.
resistance
(in ohms)
(Extension)
Now, using a 'How to plot graphs' sheet, plot a graph of current (y-axis) against voltage (xaxis).Use graph paper and don't join up the points.
Look at the graph and decide whether it is a straight line (allowing for slight
inaccuracies) or a curve.
------ GET YOUR WORK CHECKED -----
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Module P5 Electricity
OHM'S LAW
If the resistance of the wire does not change, then when the voltage applied to
the wire is doubled the current flowing through it should also double.
e.g. if the resistance is 4 ohm....
a voltage of 2V will produce a current of 0.5A
a voltage of 4V will produce a current of 1A
So a constant resistance will give a proportional graph (a straight line through the
origin)
Check your graph to see if it is a straight line - it should be!
If you know how, calculate the gradient of the best-fit straight line (if not ask)
For most metals, the resistance does not change as the current is increased as
long as the temperature of the wire does not change. (That is why you were told
to keep the current low.) This general rule is known as Ohm's Law after (George
Ohm - 1826)
There are two ways that is can be stated...
either
The resistance of a metal conductor is the same whatever current is
flowing - provided that the temperature doesn't change.
Problems involving resistance(Extension)
Resistance is found from the formula
Voltage applied (in volts)
Resistance = --------------------------------------------Current flowing (in amps)
If the potential difference (voltage) is measured in volts and the current is measured in amps then
the resistance is in ohms, and the abbreviation for ohms is  (the greek letter omega).
For larger resistances the kilohm ( k ) is used. 1k = 1000
So we now have the formula for calculating the resistance of a part of a circuit:-
R
=
V
----I
Where R stands for resistance, V stands for potential difference, and I stands for current.
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Module P5 Electricity
If you are told any two of these values (R,V or I) you can work out the third by using a
triangle rule to find the formula.
Now try these questions:
1. A resistor is connected to a 6V battery and a current of 3A flows
through it.
What is its resistance?
.......................................................................................
.......................................................................................
.......................................................................................
2. What voltage is needed to drive a current of 0.5A through a 10
ohm resistor ?
.......................................................................................
.......................................................................................
.......................................................................................
3. What voltage is needed to drive a current of 10A through a 24
ohm resistor ?
.......................................................................................
.......................................................................................
.......................................................................................
4. What current flows in a 6 ohm resistor when it is connected to
a 12V supply ?
.......................................................................................
.......................................................................................
.......................................................................................
5. Fill in the missing entries in the table below:Resistance
Current
Voltage
2
6A
?
10
?
10V
0.1
10A
?
?
3A
24V
?
0.1A
240V
An important worked example
Questions involving resistance can be more
difficult than this. Sometimes the voltage of the
battery or lab-pack is not the same as the p.d.
across the resistor.
In the circuit shown here the 12V is shared
between the resistor and the lamp. You should
remember this from the experiments you did
using voltmeters.
If the voltages on the meters are as shown you
can easily work out the current.
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Module P5 Electricity
The potential difference across the resistor is 4V.
So to find the current through the resistor use I=V/R
I = 4/8
= 0.5A
This current must also flow through the lamp. So the resistance of the lamp must be
R = V/I
= 8/(0.5) = 16
TASK 6: SWITCHES
a) The single-pole switch.
This is the simplest type of switch.
If the switch is 'off' there is a gap in the circuit.
If the switch is 'on' the gap is joined up.
b) The two-way switch.
A two-way switch has three terminals.
A central terminal can be connected to
double pole (two-way) switch of the other
two, but not to both.
Design Problems
The following problems involve you in the design of some practical circuits.
In each case you have to:
1. Work out how to do it.
2. Draw the circuit diagram.
3. Try it out using simple components to represent the different parts of
the system, e.g.
a small lamp to represent a car headlight
a buzzer to represent a car horn or a doorbell
(If in doubt, ask for your circuit diagram to be checked)
Use standard symbols for your circuit diagrams where possible.
If you don't know the symbol, use a box with the name of the component inside it.
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Module P5 Electricity
1.The Zig-Zag.
This is the old Fete amusement where you have to move a wire loop along a zigzag wire. If the loop touches the wire a bell rings.
2.Houselights.
Two rooms in a house each have a single light, each controlled by its own switch.
They use the same electrical supply but each room can be lit independently.
3.Door bells.
A house has one bell push at the front door and another at the back door, but
there is only one bell.
4.Staircase Lights.
One lamp on the staircase can be controlled by two light switches, one at the
top and the other at the bottom of the stairs.
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Module P5 Electricity
ELECTRICITY BOOKLET
NAME.................................................
GROUP............................
TEACHER........................
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