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

What three factors determine the resistance of a wire?

length of the wire cross-sectional area through which the current passes resistivity of the material

The .......... the wire, the greater its resistance; the .......... the wire, the smaller its resistance

The longer the wire, the greater its resistance; the thicker the wire, the smaller its resistance

Explain, in terms of electrons, why a metal wire has resistance

As free electrons move through the wire they collide with metal ions , transferring some of their kinetic energy on collision. This causes electrical heating and resists the flow of charge

Why is copper used for electrical wires?

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

For a wire of fixed length and material, how does doubling the diameter affect its resistance?

Cross-sectional area is proportional to the diameter squared , so doubling the diameter quadruples the area. This makes the resistance fall to one quarter of its original value

True or False? A thicker wire has a greater resistance than a thinner wire of the same length and material

False. A thicker wire has a smaller resistance, because the greater cross-sectional area gives the charge carriers more room to flow

What are the independent, dependent, and control variables in Core Practical 2: Investigating Resistivity?

Independent variable – length L of the wire Dependent variable – current I through the wire Control variables – voltage across the wire and the material of the wire

The wire's diameter is measured using a .......... at several points along its length, and a .......... value is calculated

The wire's diameter is measured using a micrometer at several points along its length, and a mean value is calculated

Why should the current be switched off between readings?

The current heats the wire , and resistivity depends on temperature . Switching off between readings keeps the temperature constant, reducing this source of error

True or False? A graph of resistance against length for the wire is a straight line through the origin

True. Since R = ρL/A, resistance is proportional to length , giving a straight line through the origin with gradient ρ / A

Why must the fixed end of the wire start at 0 on the ruler?

If the wire does not start at 0 , every length measurement carries a zero error – a systematic error in the results

Define drift velocity

Drift velocity is the average velocity of the charge carriers travelling through a conductor. In metals these are usually free electrons , with a slow drift velocity of about 10 -3 m s -1

For a constant current, the drift velocity v is .......... proportional to the number density n

For a constant current, the drift velocity v is inversely proportional to the number density n

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

The drift velocity is in the opposite direction to the conventional current. For positive carriers, it is in the same direction

How does the number density of charge carriers n compare for conductors, semiconductors and insulators?

Conductors (metals) – large n from many free electrons, so low resistivity Semiconductors – small n , which increases with temperature Insulators – very few or no charge carriers, so extremely high resistivity

True or False? Individual free electrons drift through a metal at very high speeds

False. Free electrons travel only small distances before colliding with metal ions, giving a slow drift velocity of about 10 -3 m s -1 . Current appears instantaneous only because the number density n is very large

How does the number of free electrons in a semiconductor change with temperature?

The number of free electrons increases as temperature rises, so the resistivity decreases

Define potential difference

Potential difference across a component is the energy transferred (work done) per unit charge . It is measured in volts (V) , equivalent to joules per coulomb (J C -1 )

A voltmeter is always connected in .......... with the component whose potential difference is being measured

A voltmeter is always connected in parallel with the component whose potential difference is being measured

How does the potential difference across a uniform conductor vary with its length, at constant current and temperature?

As length increases, resistance increases (R = ρl/A), so from V = IR the potential difference increases uniformly with length

One volt is equivalent to what?

One volt equals one joule per coulomb (J C -1 ) – one joule of energy transferred per coulomb of charge

True or False? In a series circuit the supply's potential difference is the same across every component

False. The supply's potential difference is shared between the components in series; it is only equal across each one if they have equal resistance

For a uniform conductor at constant temperature, potential difference increases .......... with length

For a uniform conductor at constant temperature, potential difference increases uniformly with length

Define a potential divider

A potential divider is a circuit that uses two resistors in series to produce an output voltage that is a chosen fraction of the input voltage

State the three main purposes of a potential divider

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

In a potential divider, which resistor has the greater potential difference across it?

The resistor with the larger resistance has the greater potential difference across it, since V = IR and the current through both is the same

State the electrical voltages rule

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

True or False? If one resistor in a potential divider is increased in resistance, its share of the potential difference decreases

False. Increasing a resistor's resistance gives it a greater share of the potential difference, while the other resistor gets a smaller share (V = IR)

Define a potentiometer

A potentiometer is a single component that acts as a potential divider. It is a coil of wire with a sliding contact that provides a variable output voltage

How does the resistance of an LDR change with light intensity?

The higher the light intensity, the lower the resistance; the lower the light intensity, the higher the resistance

How does the resistance of a thermistor change with temperature?

The hotter the thermistor, the lower the resistance; the cooler the thermistor, the higher the resistance

In a potential divider, the output voltage across a sensory resistor is directly .......... to its resistance

In a potential divider, the output voltage across a sensory resistor is directly proportional to its resistance, from V = IR

Explain how an LDR potential divider switches on a street light at night

As light intensity falls , the LDR's resistance increases , so the output voltage across it increases . This rising voltage provides the voltage needed to switch on the lamp

True or False? As a thermistor gets hotter, its resistance increases

False. For a thermistor, resistance decreases as temperature increases

A LevelPhysicsEdexcelFlashcards3. Electric CircuitsResistance, Resistivity & Potential Dividers

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

Exam code: 9PH0

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FrontElectrical Resistivity
Define resistivity
FrontElectrical Resistivity
What three factors determine the resistance of a wire?
BackElectrical Resistivity
length of the wire
cross-sectional area through which the current passes
resistivity of the material

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Cards in this collection (40)

Define resistivity
Resistivity describes the extent to which a material opposes the flow of electric current through it. It is a property of the material and depends on temperature, and is measured in Ω m
What three factors determine the resistance of a wire?
- length of the wire
- cross-sectional area through which the current passes
- resistivity of the material
The .......... the wire, the greater its resistance; the .......... the wire, the smaller its resistance
The longer the wire, the greater its resistance; the thicker the wire, the smaller its resistance
Explain, in terms of electrons, why a metal wire has resistance
As free electrons move through the wire they collide with metal ions, transferring some of their kinetic energy on collision. This causes electrical heating and resists the flow of charge
Why is copper used for electrical wires?
Copper has a relatively low resistivity at room temperature, so current flows through it very easily
For a wire of fixed length and material, how does doubling the diameter affect its resistance?
Cross-sectional area is proportional to the diameter squared, so doubling the diameter quadruples the area. This makes the resistance fall to one quarter of its original value
True or False?
A thicker wire has a greater resistance than a thinner wire of the same length and material
False.
A thicker wire has a smaller resistance, because the greater cross-sectional area gives the charge carriers more room to flow
What are the independent, dependent, and control variables in Core Practical 2: Investigating Resistivity?
- Independent variable – length L of the wire
- Dependent variable – current I through the wire
- Control variables – voltage across the wire and the material of the wire
State the equation used to find the resistivity of the wire
$ρ = \frac{R A}{L}$
where R is resistance (Ω), A is cross-sectional area (m²) and L is length (m). Resistivity is measured in Ω m
How is the resistivity found from a graph of resistance R against length L?
Since R = ρL/A, the graph is a straight line with gradient = ρ / A. Multiply the gradient by the cross-sectional area A to find resistivity: $ρ = gradient \times A$
The wire's diameter is measured using a .......... at several points along its length, and a .......... value is calculated
The wire's diameter is measured using a micrometer at several points along its length, and a mean value is calculated
Why should the current be switched off between readings?
The current heats the wire, and resistivity depends on temperature. Switching off between readings keeps the temperature constant, reducing this source of error
How is the cross-sectional area A found from the wire's diameter d?
$A = \frac{π d^{2}}{4}$
True or False?
A graph of resistance against length for the wire is a straight line through the origin
True.
Since R = ρL/A, resistance is proportional to length, giving a straight line through the origin with gradient ρ / A
Why must the fixed end of the wire start at 0 on the ruler?
If the wire does not start at 0, every length measurement carries a zero error – a systematic error in the results
Define drift velocity
Drift velocity is the average velocity of the charge carriers travelling through a conductor. In metals these are usually free electrons, with a slow drift velocity of about 10^-3 m s^-1
State the transport equation and define each term
$I = n q v A$
- I = current (A)
- n = number density of charge carriers (m^-3)
- q = charge on each carrier (C)
- v = drift velocity (m s^-1)
- A = cross-sectional area (m²)
For a constant current, the drift velocity v is .......... proportional to the number density n
For a constant current, the drift velocity v is inversely proportional to the number density n
For negative charge carriers, how does the drift velocity direction compare with the conventional current?
The drift velocity is in the opposite direction to the conventional current. For positive carriers, it is in the same direction
How does the number density of charge carriers n compare for conductors, semiconductors and insulators?
- Conductors (metals) – large n from many free electrons, so low resistivity
- Semiconductors – small n, which increases with temperature
- Insulators – very few or no charge carriers, so extremely high resistivity
True or False?
Individual free electrons drift through a metal at very high speeds
False.
Free electrons travel only small distances before colliding with metal ions, giving a slow drift velocity of about 10^-3 m s^-1. Current appears instantaneous only because the number density n is very large
How does the number of free electrons in a semiconductor change with temperature?
The number of free electrons increases as temperature rises, so the resistivity decreases
Define potential difference
Potential difference across a component is the energy transferred (work done) per unit charge. It is measured in volts (V), equivalent to joules per coulomb (J C^-1)
A voltmeter is always connected in .......... with the component whose potential difference is being measured
A voltmeter is always connected in parallel with the component whose potential difference is being measured
How does the potential difference across a uniform conductor vary with its length, at constant current and temperature?
As length increases, resistance increases (R = ρl/A), so from V = IR the potential difference increases uniformly with length
One volt is equivalent to what?
One volt equals one joule per coulomb (J C^-1) – one joule of energy transferred per coulomb of charge
True or False?
In a series circuit the supply's potential difference is the same across every component
False.
The supply's potential difference is shared between the components in series; it is only equal across each one if they have equal resistance
For a uniform conductor at constant temperature, potential difference increases .......... with length
For a uniform conductor at constant temperature, potential difference increases uniformly with length
Define a potential divider
A potential divider is a circuit that uses two resistors in series to produce an output voltage that is a chosen fraction of the input voltage
State the three main purposes of a potential divider
- to provide a variable potential difference
- to enable a specific potential difference to be chosen
- to split the potential difference of a source between two or more components
In a potential divider, which resistor has the greater potential difference across it?
The resistor with the larger resistance has the greater potential difference across it, since V = IR and the current through both is the same
For two resistors in series, the ratio of their potential differences is equal to the ratio of their ..........
For two resistors in series, the ratio of their potential differences is equal to the ratio of their resistances: $\frac{V_{1}}{V_{2}} = \frac{R_{1}}{R_{2}}$
State the electrical voltages rule
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
True or False?
If one resistor in a potential divider is increased in resistance, its share of the potential difference decreases
False.
Increasing a resistor's resistance gives it a greater share of the potential difference, while the other resistor gets a smaller share (V = IR)
Define a potentiometer
A potentiometer is a single component that acts as a potential divider. It is a coil of wire with a sliding contact that provides a variable output voltage
How does the resistance of an LDR change with light intensity?
The higher the light intensity, the lower the resistance; the lower the light intensity, the higher the resistance
How does the resistance of a thermistor change with temperature?
The hotter the thermistor, the lower the resistance; the cooler the thermistor, the higher the resistance
In a potential divider, the output voltage across a sensory resistor is directly .......... to its resistance
In a potential divider, the output voltage across a sensory resistor is directly proportional to its resistance, from V = IR
Explain how an LDR potential divider switches on a street light at night
As light intensity falls, the LDR's resistance increases, so the output voltage across it increases. This rising voltage provides the voltage needed to switch on the lamp
True or False?
As a thermistor gets hotter, its resistance increases
False.
For a thermistor, resistance decreases as temperature increases

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