Define electromotive force (e.m.f.) .

The electromotive force of a power supply is the amount of chemical energy converted to electrical energy per unit charge when charge passes through it.

How can the e.m.f. of a cell be measured experimentally?

By connecting a high-resistance voltmeter across the terminals of the cell in an open circuit , so no current is flowing

E.m.f. is also equal to the potential difference across the cell when .......... current is flowing.

E.m.f. is also equal to the potential difference across the cell when no current is flowing.

True or False? E.m.f. is a genuine force, measured in newtons.

False. Despite its name, e.m.f. is not actually a force — it is a measure of energy transferred per coulomb of charge , and is measured in volts .

Define internal resistance .

Internal resistance ( r ) is the resistance between the terminals of a power supply, caused by the resistance of the materials inside it.

Why does a cell become warm after being used for a period of time?

Its internal resistance causes some electrical energy to be transformed into heat energy within the cell itself.

How can a cell with internal resistance be modelled in a circuit?

As a source of e.m.f. ( ε ) connected in series with a resistor representing the internal resistance ( r )

The lost volts ( V r ) is the voltage lost within a power supply due to its internal resistance, and is not available to the rest of the circuit.

For the same current, how does a higher internal resistance affect the lost volts?

A higher internal resistance results in a higher value for the lost volts.

In circuit calculations, the load resistance is treated as another resistor in .......... with the internal resistance.

In circuit calculations, the load resistance is treated as another resistor in series with the internal resistance.

True or False? The internal resistance of a cell is a separate physical component that can be removed from the circuit.

False. Internal resistance is not a separate resistor — it can be treated as though it were one for calculations, but it is inherent to the cell itself.

Define terminal potential difference .

The terminal potential difference is the potential difference across the terminals of a cell.

How does the terminal p.d. of a cell with internal resistance compare to its e.m.f.?

The terminal p.d. is always lower than the e.m.f., because some potential difference is lost across the internal resistance (the lost volts ).

What is the key difference between potential difference and e.m.f., in terms of energy transfer?

Potential difference describes energy transferred from electrical charges to other forms (e.g. heat). E.m.f. describes energy transferred to electrical charges, from the power supply.

If a cell has zero internal resistance, the terminal p.d. is .......... to the e.m.f.

If a cell has zero internal resistance, the terminal p.d. is equal to the e.m.f.

True or False? A battery of negligible internal resistance has a terminal p.d. equal to its e.m.f.

True. With negligible internal resistance there are no lost volts , so the terminal p.d. equals the e.m.f.

In the e.m.f. and internal resistance practical, what are the independent and dependent variables?

Independent variable : resistance, R (Ω), varied using a variable resistor Dependent variables : current, I (A) and voltage, V (V)

What should be plotted on each axis to determine e.m.f. and internal resistance graphically, and what does each feature represent?

Plot V (y-axis) against I (x-axis). Y-intercept = e.m.f. ( E ) Gradient = negative internal resistance (– r )

Why should the switch only be closed for as long as it takes to take each pair of readings?

To prevent the internal resistance of the cell from changing (heating up) during the experiment — this reduces a systematic error .

Give two ways to reduce random error in this experiment.

Wait for the voltmeter and ammeter readings to stabilise before recording Take at least three repeat readings for each voltage and current and calculate a mean

Only fairly .......... cells should be used, otherwise the e.m.f. and internal resistance can vary during the experiment.

Only fairly new cells should be used, otherwise the e.m.f. and internal resistance can vary during the experiment.

True or False? In this experiment, the e.m.f. of the cell is the independent variable.

False. The e.m.f. (and internal resistance) of the cell are control variables — the independent variable is the resistance , R , of the variable resistor.

Explain, in terms of the lattice, why the resistance of a metallic conductor increases with temperature.

As temperature increases, the ions in the lattice vibrate with greater frequency and amplitude , increasing the frequency of collisions with conduction electrons, which increases resistance.

A thermistor is a non-ohmic, sensory resistor whose resistance varies with temperature .

How does the resistance of a typical (NTC) thermistor change as temperature increases?

The resistance decreases , because the number density of charge carriers in the semiconductor increases with temperature.

Why does the I-V graph for a filament lamp curve with a decreasing gradient as current increases?

As current increases, more collisions raise the filament's temperature, increasing lattice vibrations and resistance, so current increases at a slower rate for each increase in voltage.

Most thermistors used in circuits are .......... temperature coefficient (ntc) components.

Most thermistors used in circuits are negative temperature coefficient (ntc) components.

True or False? An increase in temperature has the same effect on the resistance of a metallic conductor and a thermistor.

False. For a metal , increasing temperature increases resistance; for a thermistor , increasing temperature decreases resistance — they behave oppositely.

Define light-dependent resistor (LDR) .

A light-dependent resistor is a non-ohmic, sensory resistor whose resistance automatically changes depending on the light intensity (illumination) falling on it.

Explain why the resistance of an LDR decreases as light intensity increases.

Light absorbed by the semiconductor material makes more electrons available for conduction. More conduction electrons reduces the resistance.

Give an approximate resistance range for an LDR in darkness and in bright light.

In darkness: millions of ohms In bright light: tens of ohms

Give an everyday application of LDRs.

Automatic street lighting or garden lights, which switch on when it gets dark.

As the light intensity .......... , the resistance of an LDR decreases.

As the light intensity increases , the resistance of an LDR decreases.

True or False? An LDR is an ohmic conductor.

False. An LDR is a non-ohmic conductor — its resistance is not constant but depends on light intensity.

International A Level (IAL)PhysicsEdexcelFlashcards2. Waves & ElectricityE.M.F & Modelling Resistance

E.M.F & Modelling Resistance (Edexcel International A Level (IAL) Physics): Flashcards

Exam code: YPH11

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Define electromotive force (e.m.f.).

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Cards in this collection (38)

Define electromotive force (e.m.f.).
The electromotive force of a power supply is the amount of chemical energy converted to electrical energy per unit charge when charge passes through it.
What is the equation for e.m.f. in terms of energy transferred and charge?
$ε = \frac{W}{Q}$
where ε = e.m.f. (V), W = chemical energy converted to electrical energy (J), Q = charge (C)
What are the SI base units of the volt?
J C^-1
How can the e.m.f. of a cell be measured experimentally?
By connecting a high-resistance voltmeter across the terminals of the cell in an open circuit, so no current is flowing
E.m.f. is also equal to the potential difference across the cell when .......... current is flowing.
E.m.f. is also equal to the potential difference across the cell when no current is flowing.
True or False?
E.m.f. is a genuine force, measured in newtons.
False.
Despite its name, e.m.f. is not actually a force — it is a measure of energy transferred per coulomb of charge, and is measured in volts.
Define internal resistance.
Internal resistance (r) is the resistance between the terminals of a power supply, caused by the resistance of the materials inside it.
Why does a cell become warm after being used for a period of time?
Its internal resistance causes some electrical energy to be transformed into heat energy within the cell itself.
How can a cell with internal resistance be modelled in a circuit?
As a source of e.m.f. (ε) connected in series with a resistor representing the internal resistance (r)
Define lost volts.
The lost volts (V_r) is the voltage lost within a power supply due to its internal resistance, and is not available to the rest of the circuit.
For the same current, how does a higher internal resistance affect the lost volts?
A higher internal resistance results in a higher value for the lost volts.
In circuit calculations, the load resistance is treated as another resistor in .......... with the internal resistance.
In circuit calculations, the load resistance is treated as another resistor in series with the internal resistance.
True or False?
The internal resistance of a cell is a separate physical component that can be removed from the circuit.
False.
Internal resistance is not a separate resistor — it can be treated as though it were one for calculations, but it is inherent to the cell itself.
Define terminal potential difference.
The terminal potential difference is the potential difference across the terminals of a cell.
How does the terminal p.d. of a cell with internal resistance compare to its e.m.f.?
The terminal p.d. is always lower than the e.m.f., because some potential difference is lost across the internal resistance (the lost volts).
Write the equation relating e.m.f. to terminal p.d. and lost volts.
$ε = V_{R} + V_{r}$
where V_R = terminal p.d. across the load and V_r = lost volts across the internal resistance
What is the key difference between potential difference and e.m.f., in terms of energy transfer?
Potential difference describes energy transferred from electrical charges to other forms (e.g. heat). E.m.f. describes energy transferred to electrical charges, from the power supply.
If a cell has zero internal resistance, the terminal p.d. is .......... to the e.m.f.
If a cell has zero internal resistance, the terminal p.d. is equal to the e.m.f.
True or False?
A battery of negligible internal resistance has a terminal p.d. equal to its e.m.f.
True.
With negligible internal resistance there are no lost volts, so the terminal p.d. equals the e.m.f.
In the e.m.f. and internal resistance practical, what are the independent and dependent variables?
- Independent variable: resistance, R (Ω), varied using a variable resistor
- Dependent variables: current, I (A) and voltage, V (V)
What should be plotted on each axis to determine e.m.f. and internal resistance graphically, and what does each feature represent?
Plot V (y-axis) against I (x-axis).
- Y-intercept = e.m.f. (E)
- Gradient = negative internal resistance (–r)
Rearrange E = I(R + r) into the form of a straight-line graph of V against I.
$V = - r I + E$
Why should the switch only be closed for as long as it takes to take each pair of readings?
To prevent the internal resistance of the cell from changing (heating up) during the experiment — this reduces a systematic error.
Give two ways to reduce random error in this experiment.
- Wait for the voltmeter and ammeter readings to stabilise before recording
- Take at least three repeat readings for each voltage and current and calculate a mean
Only fairly .......... cells should be used, otherwise the e.m.f. and internal resistance can vary during the experiment.
Only fairly new cells should be used, otherwise the e.m.f. and internal resistance can vary during the experiment.
True or False?
In this experiment, the e.m.f. of the cell is the independent variable.
False.
The e.m.f. (and internal resistance) of the cell are control variables — the independent variable is the resistance, R, of the variable resistor.
Explain, in terms of the lattice, why the resistance of a metallic conductor increases with temperature.
As temperature increases, the ions in the lattice vibrate with greater frequency and amplitude, increasing the frequency of collisions with conduction electrons, which increases resistance.
Define thermistor.
A thermistor is a non-ohmic, sensory resistor whose resistance varies with temperature.
How does the resistance of a typical (NTC) thermistor change as temperature increases?
The resistance decreases, because the number density of charge carriers in the semiconductor increases with temperature.
Why does the I-V graph for a filament lamp curve with a decreasing gradient as current increases?
As current increases, more collisions raise the filament's temperature, increasing lattice vibrations and resistance, so current increases at a slower rate for each increase in voltage.
Most thermistors used in circuits are .......... temperature coefficient (ntc) components.
Most thermistors used in circuits are negative temperature coefficient (ntc) components.
True or False?
An increase in temperature has the same effect on the resistance of a metallic conductor and a thermistor.
False.
For a metal, increasing temperature increases resistance; for a thermistor, increasing temperature decreases resistance — they behave oppositely.
Define light-dependent resistor (LDR).
A light-dependent resistor is a non-ohmic, sensory resistor whose resistance automatically changes depending on the light intensity (illumination) falling on it.
Explain why the resistance of an LDR decreases as light intensity increases.
Light absorbed by the semiconductor material makes more electrons available for conduction. More conduction electrons reduces the resistance.
Give an approximate resistance range for an LDR in darkness and in bright light.
- In darkness: millions of ohms
- In bright light: tens of ohms
Give an everyday application of LDRs.
Automatic street lighting or garden lights, which switch on when it gets dark.
As the light intensity .........., the resistance of an LDR decreases.
As the light intensity increases, the resistance of an LDR decreases.
True or False?
An LDR is an ohmic conductor.
False.
An LDR is a non-ohmic conductor — its resistance is not constant but depends on light intensity.

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