Resistivity is a property of a material that describes the extent to which it opposes the flow of electric current. It depends on the material and its temperature , and is measured in Ω m .

How does increasing the length of a wire affect its resistance?

The longer the wire, the greater its resistance.

The .......... the wire, the .......... its resistance.

The thicker the wire, the smaller its resistance.

True or False? Doubling the diameter of a wire halves its resistance, for a constant length and resistivity.

False. Doubling the diameter quadruples the cross-sectional area, since area is proportional to diameter squared. This means the resistance drops to a quarter , not a half.

Why does virtually no current flow through an insulator?

Insulators have such a high resistivity that almost no current can flow through them.

Why is copper commonly used for electrical wires?

Copper has a relatively low resistivity at room temperature, so current flows through it very easily.

A thermistor is a non-ohmic conductor and sensory resistor whose resistance varies with temperature . Most thermistors are negative temperature coefficient (ntc) components.

How does the resistance of a metallic conductor change as temperature increases, and why?

Resistance increases . Higher temperature makes the metal's atoms vibrate faster , so free electrons collide with them more, reducing the current — and since V = IR , a lower current for the same V means higher resistance.

For a thermistor, an increase in temperature causes a .......... in resistance.

For a thermistor, an increase in temperature causes a decrease in resistance.

Give three applications of thermistors.

Ovens Fire alarms Digital thermometers

True or False? A thermistor's resistance increases as it gets hotter, in the same way as a metal wire.

False. A typical (ntc) thermistor's resistance decreases as it gets hotter — the opposite behaviour to a metal wire.

A thermistor is connected in series with a fixed resistor. When the temperature of the thermistor decreases, what happens to the potential difference across it?

The thermistor's resistance increases , so it takes a greater proportion of the potential difference, while the current in the circuit decreases .

Define superconductor .

A superconductor is a material with no resistance below a critical temperature .

Define critical temperature .

The critical temperature (also called the transition temperature ) is the temperature at which a material becomes superconducting .

Below its critical temperature, a superconductor's resistance drops to .......... .

Below its critical temperature, a superconductor's resistance drops to zero .

True or False? A superconductor has a very small amount of resistance below its critical temperature.

False. A superconductor's resistance is exactly zero below its critical temperature, not just reduced.

State two reasons superconductors are useful for applications requiring large electric currents.

The production of strong magnetic fields The reduction of energy loss / dissipation in the transmission of electric power

Name three applications of superconductors.

MRI scanners Maglev trains Particle accelerators Fusion reactors Electromagnets Power / electrical cables Microchips Transformers and generators Motors Monorail trains

Why can maglev trains reach such high speeds using superconducting electromagnets?

Extremely strong electromagnets allow the train to levitate despite its large mass, enabling speeds of up to 603 km/h .

What is the aim of this required practical?

To determine the resistivity of a two metre length of constantan wire .

What are the independent and dependent variables in this experiment?

Independent variable: length , L , of the wire Dependent variable: current , I , through the wire Controlled: voltage through the wire and the wire's material

In this experiment, the micrometer screw gauge has a resolution of .......... , while the ammeter has a resolution of .......... .

In this experiment, the micrometer screw gauge has a resolution of 0.01 mm , while the ammeter has a resolution of 0.01 A .

How should the diameter of the wire be measured to reduce random error?

Measure the diameter with a micrometer at 5 to 10 points randomly along the wire, then calculate the mean diameter.

Why should the current be switched off between readings?

To prevent the wire heating up , which would change its resistance and resistivity .

On a graph of wire length L (x-axis) against average resistance R (y-axis), what does the gradient represent, and how is resistivity found from it?

The gradient equals ρ / A . Resistivity is found by multiplying the gradient by the wire's cross-sectional area , A : ρ = gradient × A .

True or False? It does not matter where the fixed end of the wire is positioned relative to the 0 mark on the ruler.

False. The fixed end of the wire attached to the circuit must be at 0 on the ruler, otherwise this causes a zero error (a systematic error) in the length measurements.

A LevelPhysicsAQAFlashcardsElectricityResistance & Resistivity

Resistance & Resistivity (AQA A Level Physics): Flashcards

Exam code: 7408

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Define resistivity.

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Cards in this collection (29)

Define resistivity.
Resistivity is a property of a material that describes the extent to which it opposes the flow of electric current. It depends on the material and its temperature, and is measured in Ω m.
State the equation linking resistance to resistivity, length and cross-sectional area.
$R = \frac{ρ L}{A}$
where R = resistance (Ω), ρ = resistivity (Ω m), L = length (m), A = cross-sectional area (m²).
How does increasing the length of a wire affect its resistance?
The longer the wire, the greater its resistance.
The .......... the wire, the .......... its resistance.
The thicker the wire, the smaller its resistance.
True or False?
Doubling the diameter of a wire halves its resistance, for a constant length and resistivity.
False.
Doubling the diameter quadruples the cross-sectional area, since area is proportional to diameter squared. This means the resistance drops to a quarter, not a half.
Why does virtually no current flow through an insulator?
Insulators have such a high resistivity that almost no current can flow through them.
Why is copper commonly used for electrical wires?
Copper has a relatively low resistivity at room temperature, so current flows through it very easily.
Define thermistor.
A thermistor is a non-ohmic conductor and sensory resistor whose resistance varies with temperature. Most thermistors are negative temperature coefficient (ntc) components.
How does the resistance of a metallic conductor change as temperature increases, and why?
Resistance increases. Higher temperature makes the metal's atoms vibrate faster, so free electrons collide with them more, reducing the current — and since V = IR, a lower current for the same V means higher resistance.
For a thermistor, an increase in temperature causes a .......... in resistance.
For a thermistor, an increase in temperature causes a decrease in resistance.
Give three applications of thermistors.
- Ovens
- Fire alarms
- Digital thermometers
True or False?
A thermistor's resistance increases as it gets hotter, in the same way as a metal wire.
False.
A typical (ntc) thermistor's resistance decreases as it gets hotter — the opposite behaviour to a metal wire.
A thermistor is connected in series with a fixed resistor. When the temperature of the thermistor decreases, what happens to the potential difference across it?
The thermistor's resistance increases, so it takes a greater proportion of the potential difference, while the current in the circuit decreases.
Define superconductor.
A superconductor is a material with no resistance below a critical temperature.
Define critical temperature.
The critical temperature (also called the transition temperature) is the temperature at which a material becomes superconducting.
What is the critical temperature of mercury, a common superconducting material?
4.2 K
Below its critical temperature, a superconductor's resistance drops to ...........
Below its critical temperature, a superconductor's resistance drops to zero.
True or False?
A superconductor has a very small amount of resistance below its critical temperature.
False.
A superconductor's resistance is exactly zero below its critical temperature, not just reduced.
State two reasons superconductors are useful for applications requiring large electric currents.
- The production of strong magnetic fields
- The reduction of energy loss / dissipation in the transmission of electric power
Name three applications of superconductors.
- MRI scanners
- Maglev trains
- Particle accelerators
- Fusion reactors
- Electromagnets
- Power / electrical cables
- Microchips
- Transformers and generators
- Motors
- Monorail trains
Why can maglev trains reach such high speeds using superconducting electromagnets?
Extremely strong electromagnets allow the train to levitate despite its large mass, enabling speeds of up to 603 km/h.
What is the aim of this required practical?
To determine the resistivity of a two metre length of constantan wire.
What are the independent and dependent variables in this experiment?
- Independent variable: length, L, of the wire
- Dependent variable: current, I, through the wire
- Controlled: voltage through the wire and the wire's material
In this experiment, the micrometer screw gauge has a resolution of .........., while the ammeter has a resolution of ...........
In this experiment, the micrometer screw gauge has a resolution of 0.01 mm, while the ammeter has a resolution of 0.01 A.
How should the diameter of the wire be measured to reduce random error?
Measure the diameter with a micrometer at 5 to 10 points randomly along the wire, then calculate the mean diameter.
Why should the current be switched off between readings?
To prevent the wire heating up, which would change its resistance and resistivity.
On a graph of wire length L (x-axis) against average resistance R (y-axis), what does the gradient represent, and how is resistivity found from it?
The gradient equals ρ / A. Resistivity is found by multiplying the gradient by the wire's cross-sectional area, A: ρ = gradient × A.
What equation is used to calculate the cross-sectional area of the wire from its diameter?
$A = \frac{π d^{2}}{4}$
True or False?
It does not matter where the fixed end of the wire is positioned relative to the 0 mark on the ruler.
False.
The fixed end of the wire attached to the circuit must be at 0 on the ruler, otherwise this causes a zero error (a systematic error) in the length measurements.

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