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1. FUNDAMENTAL Physics – Circuits: 3.7 Resistance of a Filament Lamp
Investigating the Resistance of a Filament Lamp
Learning Outcomes
• I know how current varies with potential difference for a filament lamp (and
any metal wire)
• I know and can use the formula for calculating resistance
• I can describe the effect of Ohmic Heating
• I can describe the effect of a temperature change on the resistance of a
metallic conductor
Research Question:
Investigate the relationship between current and potential difference for a
filament lamp and by doing so conclude if a filament lamp is OHMIC.
Remember - a device is Ohmic is there is a ……….. …………… between current
and the potential difference, which is evidenced by a ……….. line through the
…….. .
Independent Variable – Potential Difference
Dependent Variable – Current
Control Variables:
• Filament Lamp – Use the same resistor
• Wires and other components – use the same circuit and same components
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2. FUNDAMENTAL Physics – Circuits: 3.7 Resistance of a Filament Lamp
Apparatus
We will again use a ‘test circuit’.
Method
1. Set up the circuit as shown
2. Select a potential difference on the powerpack to get the PD close to 0.06V,
then use the variable resistor to ‘fine tune’ the voltage to exactly 0.06V.
3. Measure and record the current.
4. Continue for voltages of 0.12V, 0.18V, 0.24V, 0.30V, 0.36V and 0.42V
5. Repeat one more time.
6. Record all readings in a table and calculate average current.
7. Plot a suitable graph to answer the research question.
Results
P.D. across lamp Current in lamp Current in lamp Average Current in lamp
/ V Repeat 1 / A Repeat 2 / A / A
0.06 0.05 0.04 0.05
0.12 0.08 0.08 0.08
0.18 0.10 0.1 0.10
0.24 0.12 0.12 0.12
0.30 0.12 0.12 0.12
0.36 0.13 0.13 0.13
0.42 0.14 0.14 0.14
2
A
V
Safety.
Careful with overheated
circuit. Keep current
below a max of 0.5A.

3. FUNDAMENTAL Physics – Circuits: 3.7 Resistance of a Filament Lamp
Graph
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4. FUNDAMENTAL Physics – Circuits: 3.7 Resistance of a Filament Lamp
Conclusion
When the potential difference …….. current also ………..
The line goes up. This is a …positive.. correlation.
The line is NOT a …straight.. line.
This means that PD and current are NOT …directly proportional…!
This means that the filament lamp is not ……….
A filament lamp is ………..-…………….. .
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5. FUNDAMENTAL Physics – Circuits: 3.7 Resistance of a Filament Lamp
Calculating Resistance
The formula for calculating resistance is:
Resistance = p.d. / current
Is the resistance different at different voltages?
On the graph 0.1V gives 0.06 A
and 0.6V gives 0.12 A
R = V / I
R = 0.1 /
R = 1.7 Ω
R = V / I
R = 0.6 /
R = 5 Ω
The resistance of the filament lamp is INCREASING .
This is why the graph is a curve and why the filament lamp is ….-……..
Different values of PD and current from the graph will give different
results because a filament lamp has ………… resistance at higher
temperatures.
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6. FUNDAMENTAL Physics – Circuits: 3.7 Resistance of a Filament Lamp
Why is the filament lamp changing its resistance?
Think of the electrons in the metal?
• The P.D. ………….. the electrons and gives them KE.
• The electrons ………… with the metal particles, transferring KE
to the particles and making them ………… faster.
• The particles are now at a higher average ………. Energy.
• This means the metal is now at a higher ………..
• This is called ………. HEATING.
What does this do to the resistance of the metal?
• The increased ……….. means that it is harder for electrons to pass
through the metal.
• This means that resistance ………… when …………. increases
for metallic conductors such as a Tungsten filament.
Homework – Read P 184 and 185 which is
prep for a later investigation.
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7. FUNDAMENTAL Physics – Circuits: 3.7 Resistance of a Filament Lamp
Why is the filament lamp changing its resistance?
Think of the electrons in the metal?
• The P.D. ………….. the electrons and gives them KE.
• The electrons ………… with the metal particles, transferring KE
to the particles and making them ………… faster.
• The particles are now at a higher average ………. Energy.
• This means the metal is now at a higher ………..
• This is called ………. HEATING.
What does this do to the resistance of the metal?
• The increased ……….. means that it is harder for electrons to pass
through the metal.
• This means that resistance ………… when …………. increases
for metallic conductors such as a Tungsten filament.
Homework – Read P 184 and 185 which is
prep for a later investigation.
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