Resistance is the opposition to the flow of current in a conductor.

One ohm is one volt per ampere (1 V A -1 ).

For a given potential difference, the .......... the resistance, the .......... the current.

For a given potential difference, the higher the resistance, the lower the current.

Why is copper commonly used for electrical wiring?

Copper has a low electrical resistance, making it a good conductor.

True or False? In exam calculations, the resistance of connecting wires is usually taken to be zero.

True. Although all electrical components have some resistance, the resistance of wires is taken as 0 in exam questions.

For a conductor at a constant temperature , the current through it is directly proportional to the potential difference across it.

What shape is the I–V graph for an ohmic conductor ?

A straight line through the origin.

Describe the I–V characteristic of a filament lamp , and explain its shape.

An 'S'-shaped curve . As current increases, the filament's temperature rises, increasing its resistance, so the current increases at a slower rate.

Describe the I–V characteristic of a thermistor , and explain its shape.

A curve with increasing gradient through the origin. As current increases, the thermistor's temperature rises, decreasing its resistance, so even more current can flow.

A semiconductor diode's I–V graph is a .......... line that then goes sharply upwards once in forward bias.

A semiconductor diode's I–V graph is a horizontal line that then goes sharply upwards once in forward bias.

True or False? A filament lamp obeys Ohm's law at all voltages.

False. A filament lamp only obeys Ohm's law for small voltages , where heating effects on its resistance are negligible.

In the experiment investigating electrical characteristics of components, what are the independent and dependent variables?

Independent: potential difference , V . Dependent: current , I .

Which three components are typically investigated in this experiment?

A fixed resistor , a filament lamp and a diode .

Why must the circuit be switched off between readings during this experiment?

To prevent the component and wires heating up , which would change their resistance.

How is random error in the current reading reduced for each voltage in this experiment?

By recording the current three times and calculating an average current.

To obtain readings for negative voltage and current, the terminals of the power supply are .......... .

To obtain readings for negative voltage and current, the terminals of the power supply are reversed .

True or False? A straight-line I–V graph indicates a non-ohmic component.

False. A straight line through the origin indicates an ohmic conductor, obeying Ohm's law: V = IR .

Why should the voltmeter and ammeter readings start from zero before the experiment begins?

To avoid a systematic (zero) error in the readings.

Define a light-dependent resistor (LDR) .

A non-ohmic component whose resistance changes depending on the light intensity falling on it.

LDRs are used in circuits which automatically switch on lights when it gets dark, such as .......... and garden lights.

LDRs are used in circuits which automatically switch on lights when it gets dark, such as street lighting and garden lights.

Compare the resistance of an LDR in bright light with its resistance in darkness.

In bright light, resistance is small (tens of ohms). In darkness, resistance is very large (millions of ohms).

True or False? An LDR is an ohmic conductor.

False. An LDR is a non-ohmic component whose resistance depends on light intensity, not a fixed ratio of V to I .

A property of a material describing the extent to which it opposes the flow of current, measured in Ω m ; it depends on temperature .

If the length of a wire is doubled, with all else constant, what happens to its resistance?

The resistance doubles (resistance is directly proportional to length).

A wire's diameter is doubled. What happens to its cross-sectional area and resistance?

The cross-sectional area quadruples (× 4), so the resistance drops to a quarter .

An increase in temperature causes the resistivity of a metal to .......... , but causes the resistivity of a semiconductor to .......... .

An increase in temperature causes the resistivity of a metal to increase , but causes the resistivity of a semiconductor to decrease .

True or False? Increasing the temperature of a thermistor increases its resistance.

False. A thermistor is a semiconductor component, so increasing temperature increases the number density of charge carriers, decreasing its resistance.

Explain why an increase in temperature increases the resistivity of a metal.

Ions vibrate with greater amplitude and frequency, so free electrons collide with them more often. This decreases the current for a given voltage, increasing resistance and resistivity.

In the experiment to determine the resistivity of a metal wire, what are the independent and dependent variables?

Independent: length , L , of the wire. Dependent: current , I , through the wire.

Why is the diameter of the wire measured 5–10 times with a micrometer?

To calculate a mean diameter , reducing random error in the cross-sectional area calculation.

What is plotted on the analysis graph in this experiment, and what does the gradient represent?

Resistance , R , (y-axis) against length , L , (x-axis). Gradient = resistivity ÷ cross-sectional area ( ρ / A ).

The resistivity of the wire is calculated by multiplying the .......... of the R against L graph by the cross-sectional area.

The resistivity of the wire is calculated by multiplying the gradient of the R against L graph by the cross-sectional area.

True or False? Only small currents should be used in this experiment.

True. Larger currents heat the wire, changing its resistance and resistivity and introducing error.

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

Most thermistors are negative temperature coefficient (ntc) components, meaning that as the temperature increases , the resistance .......... .

Most thermistors are negative temperature coefficient (ntc) components, meaning that as the temperature increases , the resistance decreases .

Give three applications of thermistors in circuits.

Thermistors are used in ovens , fire alarms and digital thermometers .

True or False? As the temperature of a thermistor increases, its resistance also increases.

False. Most thermistors are negative temperature coefficient (ntc) components, so as temperature increases , resistance decreases .

Why does a thermistor's temperature-resistance graph never touch the x-axis?

Touching the x-axis would imply zero resistance , which is only possible in superconductors .

A LevelPhysicsOCRFlashcards4. Electrons, Waves & PhotonsResistance & Resistivity

Resistance & Resistivity (OCR A Level Physics): Flashcards

Exam code: H556

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Define resistance.

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Cards in this collection (46)

Define resistance.
Resistance is the opposition to the flow of current in a conductor.
Give the equation for resistance in terms of potential difference and current.
$R = \frac{V}{I}$
Define the ohm (Ω).
One ohm is one volt per ampere (1 V A^-1).
For a given potential difference, the .......... the resistance, the .......... the current.
For a given potential difference, the higher the resistance, the lower the current.
Why is copper commonly used for electrical wiring?
Copper has a low electrical resistance, making it a good conductor.
True or False?
In exam calculations, the resistance of connecting wires is usually taken to be zero.
True.
Although all electrical components have some resistance, the resistance of wires is taken as 0 in exam questions.
Calculate the potential difference across a resistor of resistance 10 Ω carrying a current of 0.3 A.
$V = I R = 0.3 \times 10 = 3 V$
Define Ohm's law.
For a conductor at a constant temperature, the current through it is directly proportional to the potential difference across it.
What shape is the I–V graph for an ohmic conductor?
A straight line through the origin.
Describe the I–V characteristic of a filament lamp, and explain its shape.
An 'S'-shaped curve. As current increases, the filament's temperature rises, increasing its resistance, so the current increases at a slower rate.
Describe the I–V characteristic of a thermistor, and explain its shape.
A curve with increasing gradient through the origin. As current increases, the thermistor's temperature rises, decreasing its resistance, so even more current can flow.
A semiconductor diode's I–V graph is a .......... line that then goes sharply upwards once in forward bias.
A semiconductor diode's I–V graph is a horizontal line that then goes sharply upwards once in forward bias.
True or False?
A filament lamp obeys Ohm's law at all voltages.
False.
A filament lamp only obeys Ohm's law for small voltages, where heating effects on its resistance are negligible.
How can the resistance of an ohmic conductor be found from its I–V graph?
$R = \frac{1}{gradient}$
In the experiment investigating electrical characteristics of components, what are the independent and dependent variables?
Independent: potential difference, V. Dependent: current, I.
Which three components are typically investigated in this experiment?
A fixed resistor, a filament lamp and a diode.
Why must the circuit be switched off between readings during this experiment?
To prevent the component and wires heating up, which would change their resistance.
How is random error in the current reading reduced for each voltage in this experiment?
By recording the current three times and calculating an average current.
To obtain readings for negative voltage and current, the terminals of the power supply are ...........
To obtain readings for negative voltage and current, the terminals of the power supply are reversed.
True or False?
A straight-line I–V graph indicates a non-ohmic component.
False.
A straight line through the origin indicates an ohmic conductor, obeying Ohm's law: V = IR.
Why should the voltmeter and ammeter readings start from zero before the experiment begins?
To avoid a systematic (zero) error in the readings.
Define a light-dependent resistor (LDR).
A non-ohmic component whose resistance changes depending on the light intensity falling on it.
What happens to the resistance of an LDR as light intensity increases?
The resistance decreases.
LDRs are used in circuits which automatically switch on lights when it gets dark, such as .......... and garden lights.
LDRs are used in circuits which automatically switch on lights when it gets dark, such as street lighting and garden lights.
Compare the resistance of an LDR in bright light with its resistance in darkness.
In bright light, resistance is small (tens of ohms). In darkness, resistance is very large (millions of ohms).
True or False?
An LDR is an ohmic conductor.
False.
An LDR is a non-ohmic component whose resistance depends on light intensity, not a fixed ratio of V to I.
Define resistivity.
A property of a material describing the extent to which it opposes the flow of current, measured in Ω m; it depends on temperature.
State the equation linking resistance, resistivity, length and cross-sectional area.
$R = \frac{ρ L}{A}$
If the length of a wire is doubled, with all else constant, what happens to its resistance?
The resistance doubles (resistance is directly proportional to length).
A wire's diameter is doubled. What happens to its cross-sectional area and resistance?
The cross-sectional area quadruples (× 4), so the resistance drops to a quarter.
An increase in temperature causes the resistivity of a metal to .........., but causes the resistivity of a semiconductor to ...........
An increase in temperature causes the resistivity of a metal to increase, but causes the resistivity of a semiconductor to decrease.
True or False?
Increasing the temperature of a thermistor increases its resistance.
False.
A thermistor is a semiconductor component, so increasing temperature increases the number density of charge carriers, decreasing its resistance.
Explain why an increase in temperature increases the resistivity of a metal.
Ions vibrate with greater amplitude and frequency, so free electrons collide with them more often. This decreases the current for a given voltage, increasing resistance and resistivity.
In the experiment to determine the resistivity of a metal wire, what are the independent and dependent variables?
Independent: length, L, of the wire. Dependent: current, I, through the wire.
Why is the diameter of the wire measured 5–10 times with a micrometer?
To calculate a mean diameter, reducing random error in the cross-sectional area calculation.
What is plotted on the analysis graph in this experiment, and what does the gradient represent?
Resistance, R, (y-axis) against length, L, (x-axis). Gradient = resistivity ÷ cross-sectional area (ρ/A).
The resistivity of the wire is calculated by multiplying the .......... of the R against L graph by the cross-sectional area.
The resistivity of the wire is calculated by multiplying the gradient of the R against L graph by the cross-sectional area.
True or False?
Only small currents should be used in this experiment.
True.
Larger currents heat the wire, changing its resistance and resistivity and introducing error.
Give the equation for the cross-sectional area of the wire in terms of its diameter, d.
$A = \frac{π d^{2}}{4}$
A constantan wire has cross-sectional area 2.87 × 10^-8 m². A graph of R against L has gradient 15.71 Ω m^-1. Calculate the resistivity of the wire.
$ρ = gradient \times A = 15.71 \times 2.87 \times 10^{- 8} = 4.51 \times 10^{-7} Ω m$
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, meaning that as the temperature increases, the resistance ...........
Most thermistors are negative temperature coefficient (ntc) components, meaning that as the temperature increases, the resistance decreases.
Give three applications of thermistors in circuits.
Thermistors are used in ovens, fire alarms and digital thermometers.
A thermistor and a fixed resistor R are connected in series with a battery. If the temperature of the thermistor decreases, what happens to the potential difference across the thermistor?
The p.d. across the thermistor increases. As the current I is the same throughout a series circuit, and the thermistor's resistance increases as its temperature decreases, $V = I R$ shows that V across it must also increase.
True or False?
As the temperature of a thermistor increases, its resistance also increases.
False.
Most thermistors are negative temperature coefficient (ntc) components, so as temperature increases, resistance decreases.
Why does a thermistor's temperature-resistance graph never touch the x-axis?
Touching the x-axis would imply zero resistance, which is only possible in superconductors.

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