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

How does the resistance of a wire change if its length increases, and if its cross-sectional area increases (all else constant)?

Longer wire → greater resistance Thicker wire (larger cross-sectional area) → smaller resistance

If the diameter of a wire doubles, the cross-sectional area quadruples, so the resistance falls to .......... of its original value.

If the diameter of a wire doubles, the cross-sectional area quadruples, so the resistance falls to one quarter of its original value.

True or False? You need to memorise the resistivity values of common materials for the exam.

False. Resistivity values are given in the exam question — you don't need to memorise them.

Why is copper commonly used for electrical wiring?

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

Define insulator in terms of resistivity.

An insulator is a material with such a high resistivity that virtually no current will flow through it.

In this experiment, what is the independent variable and what is the dependent variable?

Independent variable: length , L , of the wire Dependent variable: the current , I , through the wire (Voltage across the wire and the wire material are controlled)

Why must the current be switched off between readings when investigating resistivity?

To prevent the wire heating up , since resistivity depends on temperature — a temperature rise would change the results.

Why is the diameter of the wire measured 5–10 times at different points along the wire, and then averaged?

To reduce random error in the diameter measurement, which is used to calculate the wire's cross-sectional area.

The .......... of the wire is measured using a micrometer screw gauge, then used to calculate the cross-sectional area.

The diameter of the wire is measured using a micrometer screw gauge, then used to calculate the cross-sectional area.

True or False? The current used in this experiment should be as large as possible, to give a strong ammeter reading.

False. Only small currents should be used, to keep the wire's temperature (and hence its resistivity) constant during readings.

Define drift velocity .

Drift velocity is the average velocity of the charge carriers travelling through a conductor.

For negative charge carriers (electrons), how does the direction of drift velocity compare with the direction of conventional current?

The drift velocity is in the opposite direction to the conventional current.

Why do conductors have a much lower resistivity than insulators, in terms of charge carriers?

Conductors have a large number density , n , of free charge carriers (e.g. free electrons in metals), while insulators have very few — a perfect insulator has n = 0.

Why does a semiconductor's resistivity change significantly with temperature?

At higher temperature, more delocalised electrons become available as charge carriers, increasing n and lowering resistivity.

The transport equation shows that drift velocity is .......... proportional to the number density of charge carriers, n .

The transport equation shows that drift velocity is inversely proportional to the number density of charge carriers, n .

True or False? Free electrons in a metal wire typically drift at speeds close to the speed of light.

False. Drift velocity is very slow, around 10 -3 m s -1 . Current appears to flow instantaneously because of the huge number density of charge carriers, not their speed.

Define potential difference .

Potential difference is the energy transferred (work done) per unit charge.

What is the unit of potential difference, and what SI unit combination is it equivalent to?

Potential difference is measured in volts (V) , equivalent to joules per coulomb (J C -1 ) .

How is a voltmeter connected in a circuit to measure potential difference?

A voltmeter is connected in parallel with (across) the component being measured.

Potential difference is defined as the .......... transferred per unit charge.

Potential difference is defined as the energy transferred per unit charge.

Define potential divider .

A potential divider is a circuit that produces an output voltage as a fraction of the input voltage, using two resistors in series to split the supply voltage in a chosen ratio.

State the electrical voltages rule for a closed circuit loop.

The sum of the e.m.f.s in a closed circuit loop is equal to the sum of the potential differences around that loop.

In a potential divider with two resistors in series, which resistor has the greater potential difference across it?

The resistor with the larger resistance has the greater potential difference across it.

List three purposes of a potential divider circuit.

To provide a variable potential difference To enable a specific potential difference to be chosen To split the potential difference of a power source between two or more components

In a potential divider, the ratio of potential differences across two series resistors is equal to the ratio of their .......... .

In a potential divider, the ratio of potential differences across two series resistors is equal to the ratio of their resistances .

Define potentiometer .

A potentiometer is a variable resistor consisting of a coil of wire with a sliding contact; it can act as a potential divider whose output voltage is varied by moving the slider.

How does the resistance of an LDR (light-dependent resistor) vary with light intensity?

Resistance decreases as light intensity increases , and increases as light intensity decreases .

How does the resistance of a thermistor vary with temperature?

Resistance decreases as temperature increases , and increases as temperature decreases .

Give one practical application each of an LDR potential divider circuit and a thermistor potential divider circuit.

LDR : street or security lights (switching on a lamp when it gets dark) Thermistor : fire alarms, ovens, digital thermometers

As the slider of a potentiometer moves to increase the resistance of one part, the potential difference across that part .......... .

As the slider of a potentiometer moves to increase the resistance of one part, the potential difference across that part increases .

True or False? In a thermistor potential divider circuit, the output voltage V out across the thermistor increases as temperature rises.

False. As temperature rises, the thermistor's resistance decreases , so V out across the thermistor decreases (not increases).

International A Level (IAL)PhysicsEdexcelFlashcards2. Waves & ElectricityResistance, Resistivity & Potential Dividers

Resistance, Resistivity & Potential Dividers (Edexcel International A Level (IAL) Physics): Flashcards

Exam code: YPH11

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FrontElectrical Resistivity
Define resistivity.
FrontElectrical Resistivity
What equation links resistance to resistivity, length and cross-sectional area?
BackElectrical Resistivity
$R = \frac{ρ l}{A}$
R = resistance (Ω)
ρ = resistivity (Ω m)
l = length (m)
A = cross-sectional area (m²)
FrontElectrical Resistivity
How does the resistance of a wire change if its length increases, and if its cross-sectional area increases (all else constant)?
BackElectrical Resistivity
Longer wire → greater resistance
Thicker wire (larger cross-sectional area) → smaller resistance

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Cards in this collection (42)

Define resistivity.
Resistivity is a property of a material that describes the extent to which it opposes the flow of electric current through it. It depends on temperature and is measured in Ω m.
What equation links resistance to resistivity, length and cross-sectional area?
$R = \frac{ρ l}{A}$
- R = resistance (Ω)
- ρ = resistivity (Ω m)
- l = length (m)
- A = cross-sectional area (m²)
How does the resistance of a wire change if its length increases, and if its cross-sectional area increases (all else constant)?
- Longer wire → greater resistance
- Thicker wire (larger cross-sectional area) → smaller resistance
If the diameter of a wire doubles, the cross-sectional area quadruples, so the resistance falls to .......... of its original value.
If the diameter of a wire doubles, the cross-sectional area quadruples, so the resistance falls to one quarter of its original value.
True or False?
You need to memorise the resistivity values of common materials for the exam.
False.
Resistivity values are given in the exam question — you don't need to memorise them.
Why is copper commonly used for electrical wiring?
Copper has a relatively low resistivity at room temperature, so current flows through it very easily.
Define insulator in terms of resistivity.
An insulator is a material with such a high resistivity that virtually no current will flow through it.
Define how resistivity is calculated in this practical.
Resistivity is calculated using $ρ = gradient \times A$ where the gradient is taken from a graph of wire length against resistance, and A is the cross-sectional area of the wire.
In this experiment, what is the independent variable and what is the dependent variable?
- Independent variable: length, L, of the wire
- Dependent variable: the current, I, through the wire
(Voltage across the wire and the wire material are controlled)
How is the resistance of each length of wire calculated in this experiment?
$R = \frac{V}{I}$
using the average current I measured for that length, and the potential difference V across the circuit.
What does the gradient of a graph of wire length (L) against resistance (R) represent, and how is resistivity found from it?
- Gradient = ρ / A
- Resistivity is found by multiplying the gradient by the cross-sectional area, A:
$ρ = gradient \times A$
Why must the current be switched off between readings when investigating resistivity?
To prevent the wire heating up, since resistivity depends on temperature — a temperature rise would change the results.
Why is the diameter of the wire measured 5–10 times at different points along the wire, and then averaged?
To reduce random error in the diameter measurement, which is used to calculate the wire's cross-sectional area.
The .......... of the wire is measured using a micrometer screw gauge, then used to calculate the cross-sectional area.
The diameter of the wire is measured using a micrometer screw gauge, then used to calculate the cross-sectional area.
True or False?
The current used in this experiment should be as large as possible, to give a strong ammeter reading.
False.
Only small currents should be used, to keep the wire's temperature (and hence its resistivity) constant during readings.
Define drift velocity.
Drift velocity is the average velocity of the charge carriers travelling through a conductor.
State the transport equation for current, defining each symbol.
$I = n q v A$
- I = current (A)
- n = number density of charge carriers (m^-3)
- q = charge of the charge carrier (C)
- v = drift velocity (m s^-1)
- A = cross-sectional area (m²)
For negative charge carriers (electrons), how does the direction of drift velocity compare with the direction of conventional current?
The drift velocity is in the opposite direction to the conventional current.
Why do conductors have a much lower resistivity than insulators, in terms of charge carriers?
Conductors have a large number density, n, of free charge carriers (e.g. free electrons in metals), while insulators have very few — a perfect insulator has n = 0.
Why does a semiconductor's resistivity change significantly with temperature?
At higher temperature, more delocalised electrons become available as charge carriers, increasing n and lowering resistivity.
The transport equation shows that drift velocity is .......... proportional to the number density of charge carriers, n.
The transport equation shows that drift velocity is inversely proportional to the number density of charge carriers, n.
True or False?
Free electrons in a metal wire typically drift at speeds close to the speed of light.
False.
Drift velocity is very slow, around 10^-3 m s^-1. Current appears to flow instantaneously because of the huge number density of charge carriers, not their speed.
Define potential difference.
Potential difference is the energy transferred (work done) per unit charge.
What is the unit of potential difference, and what SI unit combination is it equivalent to?
Potential difference is measured in volts (V), equivalent to joules per coulomb (J C^-1).
How is a voltmeter connected in a circuit to measure potential difference?
A voltmeter is connected in parallel with (across) the component being measured.
How does the resistance of a uniform conductor at constant temperature change as its length increases, and why?
Resistance increases, because $R = \frac{ρ l}{A}$ so resistance is directly proportional to length, l.
For a conductor at constant temperature and constant current, how does potential difference vary with the length of the conductor?
Potential difference increases uniformly with length, since $V = I R$ and R increases proportionally with length.
Potential difference is defined as the .......... transferred per unit charge.
Potential difference is defined as the energy transferred per unit charge.
True or False?
Doubling the length of a uniform wire at constant temperature doubles its resistance, but the potential difference across it (for a constant current) stays the same.
False.
Since $V = I R$ , doubling R (from doubling the length) also doubles the potential difference across the wire for the same current.
Define potential divider.
A potential divider is a circuit that produces an output voltage as a fraction of the input voltage, using two resistors in series to split the supply voltage in a chosen ratio.
State the electrical voltages rule for a closed circuit loop.
The sum of the e.m.f.s in a closed circuit loop is equal to the sum of the potential differences around that loop.
In a potential divider with two resistors in series, which resistor has the greater potential difference across it?
The resistor with the larger resistance has the greater potential difference across it.
List three purposes of a potential divider circuit.
- To provide a variable potential difference
- To enable a specific potential difference to be chosen
- To split the potential difference of a power source between two or more components
In a potential divider, the ratio of potential differences across two series resistors is equal to the ratio of their ...........
In a potential divider, the ratio of potential differences across two series resistors is equal to the ratio of their resistances.
True or False?
In a potential divider circuit, the output voltage V_out is independent of the resistance of the resistor it is measured across.
False.
V_out is proportional to the resistance of that resistor, since $V = I R$ and the current is constant through both resistors.
Define potentiometer.
A potentiometer is a variable resistor consisting of a coil of wire with a sliding contact; it can act as a potential divider whose output voltage is varied by moving the slider.
How does the resistance of an LDR (light-dependent resistor) vary with light intensity?
Resistance decreases as light intensity increases, and increases as light intensity decreases.
How does the resistance of a thermistor vary with temperature?
Resistance decreases as temperature increases, and increases as temperature decreases.
In an LDR potential divider circuit, why does V_out across the LDR decrease when light intensity increases?
The LDR's resistance decreases as light intensity increases, and since $V = I R$ (constant current), a smaller resistance gives a smaller potential difference across it.
Give one practical application each of an LDR potential divider circuit and a thermistor potential divider circuit.
- LDR: street or security lights (switching on a lamp when it gets dark)
- Thermistor: fire alarms, ovens, digital thermometers
As the slider of a potentiometer moves to increase the resistance of one part, the potential difference across that part ...........
As the slider of a potentiometer moves to increase the resistance of one part, the potential difference across that part increases.
True or False?
In a thermistor potential divider circuit, the output voltage V_out across the thermistor increases as temperature rises.
False.
As temperature rises, the thermistor's resistance decreases, so V_out across the thermistor decreases (not increases).

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