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

The amount of chemical energy converted to electrical energy per unit charge when charge passes through a power supply.

What unit is e.m.f. measured in?

e.m.f. is measured in volts (V) , which is J C -1 in SI units.

What happens to charge as it passes through a power supply such as a battery?

It gains electrical energy — chemical energy in the supply is converted to electrical energy carried by the charge.

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

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

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

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

True or False? e.m.f. is a type of force.

False. e.m.f. is not actually a force — despite its name it is a measure of the energy transferred per coulomb of charge. Its symbol is ε (epsilon).

Define internal resistance .

The resistance between the terminals of a power supply (symbol r ), which causes some electrical energy to be transformed to heat energy within the supply itself.

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

Its internal resistance transforms some electrical energy into heat energy inside the power supply.

A cell can be thought of as a source of e.m.f. with an internal resistance connected in .......... .

A cell can be thought of as a source of e.m.f. with an internal resistance connected in series .

What are the 'lost volts' ?

The voltage lost across the internal resistance of a power supply (symbol V r ) — energy not available to the rest of the circuit.

What effect does a higher internal resistance have on the lost volts?

A higher internal resistance produces a higher value of lost volts.

True or False? Internal resistance is a separate resistor built into the cell.

False. Internal resistance is a property of the cell itself, not a separate component. It can be treated as though it were a separate resistor in series to make calculations easier.

Define terminal potential difference .

The potential difference across the terminals of a cell. When a load resistor is connected it equals the p.d. across that load resistor.

For a cell with internal resistance, how does the terminal p.d. compare with the e.m.f.?

The terminal p.d. is always lower than the e.m.f. They are equal only if the cell has no internal resistance .

The e.m.f. is equal to the sum of the terminal p.d. and the .......... .

The e.m.f. is equal to the sum of the terminal p.d. and the lost volts .

What is the key difference between potential difference and e.m.f. ?

They describe opposite energy transfers per unit charge. e.m.f. is energy transferred from the power supply to the charges; potential difference is electrical energy transferred from the charges to other forms in a component.

True or False? Terminal p.d. always equals the e.m.f.

False. Terminal p.d. equals the e.m.f. only when the cell has negligible internal resistance (or no current flows). Otherwise it is lower, because of the lost volts across the internal resistance.

What are the independent and dependent variables in Core Practical 3: Investigating e.m.f. & Internal Resistance?

Independent: resistance R , set with a variable resistor Dependent: voltage V and current I Control: e.m.f. and internal resistance of the cell

For the analysis, the cell and its internal resistance r are connected in .......... and treated as a single cell.

For the analysis, the cell and its internal resistance r are connected in series and treated as a single cell.

True or False? Keeping the switch closed longer improves accuracy.

False. Leaving the switch closed heats the cell and changes its internal resistance, causing a systematic error. Close it only long enough to record each pair of readings.

How can random errors be reduced in this experiment?

Use fairly new cells, wait for the voltmeter and ammeter readings to stabilise, and take at least 3 repeat readings for each value to find a mean.

When V is plotted against I , how are the e.m.f. and internal resistance found from the graph?

The y-intercept gives the e.m.f. E ; the gradient equals the negative internal resistance (– r ).

Why does the resistance of a metallic conductor increase as its temperature rises?

The lattice ions vibrate with greater frequency and amplitude, increasing collisions between free electrons and ions. This impedes electron flow, raising the resistance.

As free electrons move through a metal wire, they collide with .......... , transferring kinetic energy which causes electrical heating.

As free electrons move through a metal wire, they collide with ions , transferring kinetic energy which causes electrical heating.

Why does the I – V graph for a filament lamp curve rather than being a straight line?

As current increases the filament heats up, so ion vibrations and collisions increase and its resistance rises. The current therefore increases at a slower rate, seen as a curve.

A non-ohmic sensory resistor made from a semiconductor, whose resistance varies with temperature.

How does the resistance of an NTC thermistor change as temperature increases?

Its resistance decreases — the number density of charge carriers increases with rising temperature in a semiconductor.

True or False? A metal and a thermistor respond to temperature the same way.

False. They respond in opposite ways. A metallic conductor's resistance increases with temperature, whereas an NTC thermistor's resistance decreases .

State two uses of thermistors.

Temperature-sensing circuits such as thermometers , thermostats , ovens and fire alarms.

Define light-dependent resistor (LDR) .

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

How does the resistance of an LDR change as light intensity increases?

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

Why does absorbing light reduce the resistance of the semiconductor in an LDR?

Absorbed light frees more electrons for conduction. A greater number of conduction electrons lowers the resistance.

In the dark an LDR's resistance is very .......... , but in bright light its resistance is very .......... .

In the dark an LDR's resistance is very large (millions of ohms), but in bright light its resistance is very small (tens of ohms).

Give a practical use of an LDR.

As a light sensor in circuits that switch lights on automatically when it gets dark, such as street lighting and garden lights.

True or False? An LDR's resistance rises when more light hits it.

False. An LDR's resistance decreases as light intensity increases, because the absorbed light frees more conduction electrons.

A LevelPhysicsEdexcelFlashcards3. Electric CircuitsE.M.F & Modelling Resistance

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

Exam code: 9PH0

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

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Cards in this collection (37)

Define electromotive force (e.m.f.).
The amount of chemical energy converted to electrical energy per unit charge when charge passes through a power supply.
What unit is e.m.f. measured in?
e.m.f. is measured in volts (V), which is J C^-1 in SI units.
What happens to charge as it passes through a power supply such as a battery?
It gains electrical energy — chemical energy in the supply is converted to electrical energy carried by the charge.
e.m.f. is also equal to the potential difference across the cell when .......... is flowing.
e.m.f. is also equal to the potential difference across the cell when no current is flowing.
How can the e.m.f. of a cell be measured?
By connecting a high-resistance voltmeter across the terminals of the cell in an open circuit, so no current flows.
True or False?
e.m.f. is a type of force.
False.
e.m.f. is not actually a force — despite its name it is a measure of the energy transferred per coulomb of charge. Its symbol is ε (epsilon).
Define internal resistance.
The resistance between the terminals of a power supply (symbol r), which causes some electrical energy to be transformed to heat energy within the supply itself.
Why does a cell become warm after being used for a period of time?
Its internal resistance transforms some electrical energy into heat energy inside the power supply.
A cell can be thought of as a source of e.m.f. with an internal resistance connected in ...........
A cell can be thought of as a source of e.m.f. with an internal resistance connected in series.
What are the 'lost volts'?
The voltage lost across the internal resistance of a power supply (symbol V_r) — energy not available to the rest of the circuit.
What effect does a higher internal resistance have on the lost volts?
A higher internal resistance produces a higher value of lost volts.
True or False?
Internal resistance is a separate resistor built into the cell.
False.
Internal resistance is a property of the cell itself, not a separate component. It can be treated as though it were a separate resistor in series to make calculations easier.
Define terminal potential difference.
The potential difference across the terminals of a cell. When a load resistor is connected it equals the p.d. across that load resistor.
For a cell with internal resistance, how does the terminal p.d. compare with the e.m.f.?
The terminal p.d. is always lower than the e.m.f. They are equal only if the cell has no internal resistance.
The e.m.f. is equal to the sum of the terminal p.d. and the ...........
The e.m.f. is equal to the sum of the terminal p.d. and the lost volts.
How are the lost volts V_r calculated?
$V_{r} = I \times r$
where I is the current in the circuit and r is the internal resistance.
What is the key difference between potential difference and e.m.f.?
They describe opposite energy transfers per unit charge. e.m.f. is energy transferred from the power supply to the charges; potential difference is electrical energy transferred from the charges to other forms in a component.
True or False?
Terminal p.d. always equals the e.m.f.
False.
Terminal p.d. equals the e.m.f. only when the cell has negligible internal resistance (or no current flows). Otherwise it is lower, because of the lost volts across the internal resistance.
What are the independent and dependent variables in Core Practical 3: Investigating e.m.f. & Internal Resistance?
Independent: resistance R, set with a variable resistor
Dependent: voltage V and current I
Control: e.m.f. and internal resistance of the cell
What equation relates e.m.f., current, load resistance and internal resistance?
$E = I (R + r)$
which rearranges to $V = - r I + E$ , matching the straight-line form y = mx + c.
For the analysis, the cell and its internal resistance r are connected in .......... and treated as a single cell.
For the analysis, the cell and its internal resistance r are connected in series and treated as a single cell.
True or False?
Keeping the switch closed longer improves accuracy.
False.
Leaving the switch closed heats the cell and changes its internal resistance, causing a systematic error. Close it only long enough to record each pair of readings.
How can random errors be reduced in this experiment?
Use fairly new cells, wait for the voltmeter and ammeter readings to stabilise, and take at least 3 repeat readings for each value to find a mean.
When V is plotted against I, how are the e.m.f. and internal resistance found from the graph?
The y-intercept gives the e.m.f. E; the gradient equals the negative internal resistance (–r).
Why does the resistance of a metallic conductor increase as its temperature rises?
The lattice ions vibrate with greater frequency and amplitude, increasing collisions between free electrons and ions. This impedes electron flow, raising the resistance.
As free electrons move through a metal wire, they collide with .........., transferring kinetic energy which causes electrical heating.
As free electrons move through a metal wire, they collide with ions, transferring kinetic energy which causes electrical heating.
Why does the I–V graph for a filament lamp curve rather than being a straight line?
As current increases the filament heats up, so ion vibrations and collisions increase and its resistance rises. The current therefore increases at a slower rate, seen as a curve.
Define thermistor.
A non-ohmic sensory resistor made from a semiconductor, whose resistance varies with temperature.
How does the resistance of an NTC thermistor change as temperature increases?
Its resistance decreases — the number density of charge carriers increases with rising temperature in a semiconductor.
True or False?
A metal and a thermistor respond to temperature the same way.
False.
They respond in opposite ways. A metallic conductor's resistance increases with temperature, whereas an NTC thermistor's resistance decreases.
State two uses of thermistors.
Temperature-sensing circuits such as thermometers, thermostats, ovens and fire alarms.
Define light-dependent resistor (LDR).
A non-ohmic sensory resistor whose resistance automatically changes depending on the light intensity (illumination) falling on it.
How does the resistance of an LDR change as light intensity increases?
As light intensity increases, the resistance of an LDR decreases.
Why does absorbing light reduce the resistance of the semiconductor in an LDR?
Absorbed light frees more electrons for conduction. A greater number of conduction electrons lowers the resistance.
In the dark an LDR's resistance is very .........., but in bright light its resistance is very ...........
In the dark an LDR's resistance is very large (millions of ohms), but in bright light its resistance is very small (tens of ohms).
Give a practical use of an LDR.
As a light sensor in circuits that switch lights on automatically when it gets dark, such as street lighting and garden lights.
True or False?
An LDR's resistance rises when more light hits it.
False.
An LDR's resistance decreases as light intensity increases, because the absorbed light frees more conduction electrons.

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