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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FUNDAMENTAL Physics – Circuits: 3.7 Resistance of a Filament LampApparatus We will again use a ‘test circuit’.

Method

1. Set up the circuit as shown2. 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 0.42V5. 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 / A0.06 0.05 0.04 0.050.12 0.08 0.08 0.080.18 0.10 0.1 0.100.24 0.12 0.12 0.120.30 0.12 0.12 0.120.36 0.13 0.13 0.130.42 0.14 0.14 0.14

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A

V Safety.

Careful with overheated circuit. Keep current below a max of 0.5A.

FUNDAMENTAL Physics – Circuits: 3.7 Resistance of a Filament Lamp

Graph

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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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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 Aand 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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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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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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