GRIFFIN EDUCATION working with
EDU-LAB and The RADMASTE Centre
UNIT P1a TOPIC 9 PRODUCING AND MEASURING ELECTRICITY
OHM’S LAW
TEACHER NOTES
OVERVIEW OF THE ACTIVITY
In this Activity learners are going to investigate the
relationship between potential difference and current in a
resistor and find its resistance. They will set up a circuit
A
with one resistor (RA) and a 3 V battery. They will use a
multimeter as an ammeter to measure the current through
RA, and another multimeter as a voltmeter to measure the
RA
potential difference across RA. Then they will add a second
RB
RC
resistor (RB) in series with RA and move the lead of the
W
Z
X
Y
ammeter to include RB in the circuit. They will then repeat
the measurements of current through RA and potential
V
difference across RA. Finally they will add a third resistor
(RC) in series and repeat the measurements of current and potential difference across RA with RC included
in the circuit.
To discuss before you start
1
By adding resistors we decrease the current in the circuit.
2
The potential difference is being measured across the resistor (RA). It would be different if it was
measured across other parts of the circuit. We want to look at the relationship between potential
difference and current in the resistor and not of the complete circuit.
3
An increase in temperature affects the resistance of conductors and can lead to inaccurate results.
To keep the temperature constant we need to stop for a few minutes between the readings to
allow the resistor to reach room temperature again.
4
The current is the independent variable. The potential difference is the dependent variable.
What to do
1
Here are two examples of measurements using 220 ohm and 20 ohm resistors.
using 220 ohm resistors
Ohm’s Law
using 20 ohm resistors
No of
resistors
PDWX
(V)
I
(x 10-3 A)
PDWX
(V)
I
(x 10-3 A)
1
2.8
12.5
2.5
124
2
1.43
6.4
1.35
67
3
0.96
4.3
0.92
46
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Ohm’s Law
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Ohm’s Law
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2
Following is a graph of the measurements given in the table above.
An example of the
slope/gradient of the graph is:
3
2.5
(2.25 - 0 )V
(10 x 1000 - 0) A
2
Potential difference
(volts)
= 225 
This value is very close to the
value of 220  found for one of
the brown resistors in the kit,
using the colour code.
1.5
1
0.5
0
0
2
4
6
8
10
12
14
Current
(Amperes 0
x 1000)
3
The results of this activity are usually good. Your learners will find their resistance results are
very close to the value determined using the table.
4
Again the results will be comparable. If you use 220 ohm resistors, set the multimeter/ohmmeter
on the 20k or 2000 setting. If you use 20 ohm resistors, set the multimeter/ohmmeter on the 200
setting.
Assessment
One of the performance indicators of the outcome SO1 is ‘following written instructions supported by
diagrams’. You can assess the learners in their constructions of the circuits and where they place the
multimeters.
In the table below are some examples of possible levels for the performance indicator.
Level 1
Level 2
The set up does not
resemble the diagram
and the learners do
not get any readings
on the multimeters
The set up resembles
the diagram, but the
multimeters are
connected incorrectly
Ohm’s Law
Level 3
The set up resembles
the diagram but
learners have
difficulty following
written instructions
Level 4
The set up resembles
the diagram; the
written instructions
are carefully followed
and meaningful results
are obtained
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OHM’S LAW
Ohm’s Law
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STUDENT WORKSHEET
Many years ago a famous German physicist, Georg Simon Ohm (1787-1854), discovered the relationship
between the current in a wire and the potential difference across the ends of the wire. When this
potential difference
current
relationship is expressed as a ratio,
the ratio value is always the same.
Because the ratio is constant it can be written as an equation. This constant is equal to the resistance (R)
of the wire. This is known as Ohm’s law.
resistance = potential difference
current
R =
V
I
To discuss before you start
Work with the other members of your group to discuss the following:
1
2
3
4
5
How are we changing the current in this circuit?
Across which points is the potential difference
being measured?
Ohm’s Law applies to a given conductor only when
the temperature of the conductor remains
constant. How can we keep the temperature of the
resistor constant? Is it in fact necessary? Explain.
In this Activity, which is the independent variable,
the current or the potential difference? Explain.
Plan a table in which to record your readings.
What you need: a basic micro-electricity kit and 2 multimeters.
What to do
1
Set up the circuit using the micro-electricity kit as shown in the diagram..
2
Join W to the positive terminal of your battery.
3
Join the negative terminal of the battery to the ammeter at V.
4
Move the free lead on the ammeter from X to Y to Z in turn. Read the potential difference across
RA and the current in RA each time.
5
Plot a graph which you can use to find the resistance (in ohms) of R A between W and X on the
graph paper supplied.
6
Use the coloured bands on RA and the guidelines and the table next page to work out the
resistance of RA. How does this compare with the resistance you measured from your graph?
Ohm’s Law
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7
Use the multimeter as an ohmmeter to measure the resistance of RA. How does this confirm with
the resistance you measured from the graph?
HOW TO USE THE COLOURS ON YOUR RESISTOR
TO WORK OUT ITS RESISTANCE
(IN OHMS)
orange - the THIRD band
Your resistor is likely to show FOUR bands
which may or may not be of different colours.
The first three bands tell you what the
resistance of your resistor is in ohms. The
fourth band tells you how accurate this
resistance is.
red - the FIRST band
This is the FOURTH BAND.
It is either silver or gold.
Gold tells us that the accuracy is 5%.
Silver tells us that the
accuracy is 10 %
violet - the SECOND band
The GOLD or SILVER band tells us the
accuracy to which the resistor was made.
If the resistor has a gold band, the accuracy is 5%. If the resistor has (according to the colours on the first
three bands) a resistance of 20 , then, its resistance will vary from 19 , to 21 ,. If the resistor’s
colour code tells us that it has a resistance of 20  with a silver band, its resistance will be in the range
from 18  to 22 .
The table below shows the numerical values for each of the colours.
0
black
5
green
1
brown
6
blue
2
red
7
violet
3
orange
8
grey
4
yellow
9
white
The colour of the FIRST band gives you a number which you can read from the table. The colour of the
SECOND band gives you a colour which you can read from the table. The colour of the THIRD band tells you
how many zeros (0's) there are after the first two numbers. Use the table to work out the resistance (in
ohms) of the resistor in the diagram above.
(It is 27 000 .)
Ohm’s Law
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