The charge stored per unit potential difference between the plates.

There is commonly a .......... between the plates of a capacitor to ensure charge does not flow across them.

There is commonly a dielectric between the plates of a capacitor to ensure charge does not flow across them.

Define the farad (F) .

The unit of capacitance . 1 F is a very large unit, so capacitance is often quoted in microfarads ( µF ), nanofarads ( nF ) or picofarads ( pF ).

True or False? The charge Q stored by a capacitor is the charge of the capacitor itself.

False. Q is the charge stored on the plates — equal but opposite on each plate — not the charge of the capacitor itself.

For a given potential difference, how does a greater capacitance affect the charge stored?

The greater the capacitance , the greater the charge stored on the plates.

Why does the potential difference across a capacitor increase as it charges?

As charge builds on the negative plate, electrostatic repulsion means more work must be done to add further charge, so the potential difference increases as the charge increases .

The energy stored by a capacitor is equal to the .......... under a potential difference–charge graph.

The energy stored by a capacitor is equal to the area under a potential difference–charge graph.

How is the charge stored Q related to the potential difference V across a capacitor?

Q is directly proportional to V (since Q = CV ), so a graph of charge against potential difference is a straight line through the origin .

True or False? The area under a potential difference–charge graph gives the charge stored on the capacitor.

False. The area gives the energy stored (equal to ½QV). The charge is read directly from an axis, not from the area.

Define the time constant of a discharging capacitor.

The time taken for the charge, current or potential difference of a discharging capacitor to decrease to 37% (1/e) of its original value.

A capacitor is discharged through a .......... , with no power supply present.

A capacitor is discharged through a resistor , with no power supply present.

True or False? When a capacitor charges, the current, potential difference and charge all increase.

False. The potential difference and charge increase to a maximum, but the current is largest at the start and decreases exponentially to zero.

How does increasing the circuit resistance affect the time a capacitor takes to discharge?

Higher resistance means the current decreases more slowly and charge flows off the plates more slowly, so the capacitor takes longer to discharge.

Why does the charging current of a capacitor fall to zero?

As negative charge builds on the plate, electrostatic repulsion means fewer electrons are pushed on, until no more can be added and the current stops.

What is the aim of Core Practical 11: Investigating Capacitor Charge & Discharge?

To calculate the capacitance of a capacitor by investigating its discharge.

In Core Practical 11, what are the independent and dependent variables?

Independent variable = time (t) ; dependent variable = potential difference (V) across the capacitor.

To obtain a straight line, a graph of .......... against time is plotted from the discharge data.

To obtain a straight line, a graph of ln(V) against time is plotted from the discharge data.

Why should a resistor with a large resistance be used in this experiment?

So the capacitor discharges slowly enough for the potential difference and time readings to be taken accurately, reducing random error.

True or False? Once the power supply is removed, a capacitor holds no charge and is safe to handle immediately.

False. Capacitors can retain charge after power is removed, risking an electric shock. They should be fully discharged before handling.

When a capacitor discharges through a resistor, the charge stored on it decreases .......... .

When a capacitor discharges through a resistor, the charge stored on it decreases exponentially .

On a graph of ln(Q) against t for a discharging capacitor, what does the gradient represent?

The gradient equals −1/RC . The y-intercept represents ln Q₀ .

True or False? A graph of charge against time for a discharging capacitor is a straight line.

False. Charge decreases exponentially with time, giving a curve. It is the graph of ln(Q) against t that is a straight line.

A LevelPhysicsEdexcelFlashcards7. Electric & Magnetic FieldsCapacitance

Capacitance (Edexcel A Level Physics): Flashcards

Exam code: 9PH0

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FrontCapacitance
Define capacitance.

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Cards in this collection (32)

Define capacitance.
The charge stored per unit potential difference between the plates.
State the equation that defines capacitance.
$C = \frac{Q}{V}$
Where C = capacitance (F), Q = charge stored (C) and V = potential difference across the plates (V).
There is commonly a .......... between the plates of a capacitor to ensure charge does not flow across them.
There is commonly a dielectric between the plates of a capacitor to ensure charge does not flow across them.
Define the farad (F).
The unit of capacitance. 1 F is a very large unit, so capacitance is often quoted in microfarads (µF), nanofarads (nF) or picofarads (pF).
True or False?
The charge Q stored by a capacitor is the charge of the capacitor itself.
False.
Q is the charge stored on the plates — equal but opposite on each plate — not the charge of the capacitor itself.
For a given potential difference, how does a greater capacitance affect the charge stored?
The greater the capacitance, the greater the charge stored on the plates.
State the three equations for the energy stored by a capacitor.
$W = \frac{1}{2} Q V$
$W = \frac{1}{2} C V^{2}$
$W = \frac{Q^{2}}{2 C}$
Why does the potential difference across a capacitor increase as it charges?
As charge builds on the negative plate, electrostatic repulsion means more work must be done to add further charge, so the potential difference increases as the charge increases.
Using $W = \frac{1}{2} C V^{2}$ , how does the energy stored change if the potential difference across a capacitor doubles?
Energy stored is proportional to V², so doubling the potential difference quadruples the energy stored.
The energy stored by a capacitor is equal to the .......... under a potential difference–charge graph.
The energy stored by a capacitor is equal to the area under a potential difference–charge graph.
How is the charge stored Q related to the potential difference V across a capacitor?
Q is directly proportional to V (since Q = CV), so a graph of charge against potential difference is a straight line through the origin.
True or False?
The area under a potential difference–charge graph gives the charge stored on the capacitor.
False.
The area gives the energy stored (equal to ½QV). The charge is read directly from an axis, not from the area.
Define the time constant of a discharging capacitor.
The time taken for the charge, current or potential difference of a discharging capacitor to decrease to 37% (1/e) of its original value.
State the equation for the time constant.
$τ = R C$
Where τ = time constant (s), R = resistance (Ω) and C = capacitance (F).
A capacitor is discharged through a .........., with no power supply present.
A capacitor is discharged through a resistor, with no power supply present.
True or False?
When a capacitor charges, the current, potential difference and charge all increase.
False.
The potential difference and charge increase to a maximum, but the current is largest at the start and decreases exponentially to zero.
How does increasing the circuit resistance affect the time a capacitor takes to discharge?
Higher resistance means the current decreases more slowly and charge flows off the plates more slowly, so the capacitor takes longer to discharge.
In one time constant, to what percentage of its maximum does a charging capacitor's charge rise?
63% of its maximum value.
Why does the charging current of a capacitor fall to zero?
As negative charge builds on the plate, electrostatic repulsion means fewer electrons are pushed on, until no more can be added and the current stops.
What is the aim of Core Practical 11: Investigating Capacitor Charge & Discharge?
To calculate the capacitance of a capacitor by investigating its discharge.
In Core Practical 11, what are the independent and dependent variables?
Independent variable = time (t); dependent variable = potential difference (V) across the capacitor.
To obtain a straight line, a graph of .......... against time is plotted from the discharge data.
To obtain a straight line, a graph of ln(V) against time is plotted from the discharge data.
How is the capacitance found from the graph of ln(V) against time?
The gradient equals −1/RC, so the capacitance is $C = - \frac{1}{gradient \times R}$
Why should a resistor with a large resistance be used in this experiment?
So the capacitor discharges slowly enough for the potential difference and time readings to be taken accurately, reducing random error.
True or False?
Once the power supply is removed, a capacitor holds no charge and is safe to handle immediately.
False.
Capacitors can retain charge after power is removed, risking an electric shock. They should be fully discharged before handling.
State the exponential decay equation for the charge on a discharging capacitor.
$Q = Q_{0} e^{- t / R C}$
Where Q = charge remaining, Q₀ = initial charge, t = time, R = resistance, C = capacitance.
What are the decay equations for potential difference and current in a discharging capacitor?
$V = V_{0} e^{- t / R C}$
$I = I_{0} e^{- t / R C}$
When a capacitor discharges through a resistor, the charge stored on it decreases ...........
When a capacitor discharges through a resistor, the charge stored on it decreases exponentially.
How can the exponential decay equation for charge be turned into a straight-line form?
By taking the natural logarithm of both sides:
$\ln Q = - \frac{1}{R C} t + \ln Q_{0}$
This has the form y = mx + c.
On a graph of ln(Q) against t for a discharging capacitor, what does the gradient represent?
The gradient equals −1/RC. The y-intercept represents ln Q₀.
True or False?
A graph of charge against time for a discharging capacitor is a straight line.
False.
Charge decreases exponentially with time, giving a curve. It is the graph of ln(Q) against t that is a straight line.
How can the initial current I₀ be found from the initial potential difference V₀ and resistance R?
Using $I_{0} = \frac{V_{0}}{R}$ (rearranged from V₀ = I₀R).

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