Capacitance is the charge stored per unit potential difference between the plates of a capacitor.

A capacitor is made from two conductive metal plates, usually separated by a .......... , which prevents charge from flowing between them.

A capacitor is made from two conductive metal plates, usually separated by a dielectric , which prevents charge from flowing between them.

What is the SI unit of capacitance?

The farad (F) . In practice, capacitance is often quoted in microfarads (μF), nanofarads (nF) or picofarads (pF), since 1 F is a very large unit.

Name three practical uses of capacitors.

Any three from: producing a bright flash in cameras , smoothing currents, electronic timing circuits , providing backup power during power cuts, and storing information in devices with memory (e.g. calculators ).

True or False? The charge stored by a capacitor is the total charge on both plates added together.

False. The charge stored refers to the magnitude of charge on each plate individually, not the sum of both plates.

When a capacitor charges, from which plate are electrons pulled and onto which plate are they pushed?

Electrons are pulled from the plate connected to the positive terminal (which becomes positively charged), and pushed onto the plate connected to the negative terminal (which becomes negatively charged).

Why does the charging current decrease as a capacitor charges?

As negative charge builds up on the plate, electrostatic repulsion between the electrons already there and incoming electrons increases, so fewer electrons can be pushed on; charging stops once no more electrons can be added.

During discharging, electrons flow back from the negative plate to the .......... plate until there is no potential difference between them.

During discharging, electrons flow back from the negative plate to the positive plate until there is no potential difference between them.

How is a capacitor discharged?

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

True or False? At the start of charging, the current is small and gradually increases to a maximum.

False. At the start of charging, the current is large and gradually falls to zero as the capacitor becomes fully charged.

How do the potential difference and charge on the plates change as a capacitor charges?

Both increase until the potential difference equals that of the power supply, and the charge reaches its maximum value determined by the capacitance.

Capacitors connected in series store the same charge. What differs across each one?

The potential difference across each capacitor differs; the total p.d. is shared between them, so V = V 1 + V 2 .

Capacitors connected in parallel have the same potential difference. What differs across each one?

The charge stored on each capacitor differs, since the current is split at each junction; the total charge is the sum of the individual charges, Q = Q 1 + Q 2 .

Capacitors connected in series have different p.d. across them but the same .......... .

Capacitors connected in series have different p.d. across them but the same charge .

True or False? The equations for combining capacitors in series and parallel are the same form as the equivalent equations for resistors.

False. They are the opposite way around : capacitors in series combine like resistors in parallel (reciprocal sum), and capacitors in parallel combine like resistors in series (simple sum).

In the capacitor-charging investigation, what are the independent and dependent variables?

Independent variable: potential difference , V . Dependent variable: charge , Q .

How is the charging current kept constant in the test circuit?

The variable resistor is continuously adjusted to keep the charging current constant, although this becomes impossible once the capacitor is close to fully charged.

A graph of charge stored against potential difference for a capacitor is a straight line through the .......... .

A graph of charge stored against potential difference for a capacitor is a straight line through the origin .

Why must capacitors be fully discharged before being removed from a circuit after this experiment?

Capacitors can retain charge even after the power supply is removed, which could cause an electric shock .

True or False? The gradient of the charge-potential difference graph for a parallel combination of capacitors is smaller than for a series combination.

False. The gradient equals the total capacitance , and since a parallel combination has a greater total capacitance, its gradient is greater than that of the series combination.

How can the energy stored in a capacitor be found from a potential difference-charge graph?

It is equal to the area under the graph , which forms a right-angled triangle: Area = 0.5 × base × height.

As a capacitor charges, the potential difference across it .......... as the charge increases, because it becomes progressively harder to push more charge onto the negatively charged plate.

As a capacitor charges, the potential difference across it increases as the charge increases, because it becomes progressively harder to push more charge onto the negatively charged plate.

A LevelPhysicsOCRFlashcards6. Particles & Medical PhysicsCapacitors in Circuits

Capacitors in Circuits (OCR A Level Physics): Flashcards

Exam code: H556

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Define capacitance.

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Cards in this collection (30)

Define capacitance.
Capacitance is the charge stored per unit potential difference between the plates of a capacitor.
What equation defines capacitance in terms of charge and potential difference?
$C = \frac{Q}{V}$
Where C = capacitance (F), Q = charge stored (C), V = potential difference (V).
A capacitor is made from two conductive metal plates, usually separated by a .........., which prevents charge from flowing between them.
A capacitor is made from two conductive metal plates, usually separated by a dielectric, which prevents charge from flowing between them.
What is the SI unit of capacitance?
The farad (F). In practice, capacitance is often quoted in microfarads (μF), nanofarads (nF) or picofarads (pF), since 1 F is a very large unit.
Name three practical uses of capacitors.
Any three from: producing a bright flash in cameras, smoothing currents, electronic timing circuits, providing backup power during power cuts, and storing information in devices with memory (e.g. calculators).
True or False?
The charge stored by a capacitor is the total charge on both plates added together.
False.
The charge stored refers to the magnitude of charge on each plate individually, not the sum of both plates.
When a capacitor charges, from which plate are electrons pulled and onto which plate are they pushed?
Electrons are pulled from the plate connected to the positive terminal (which becomes positively charged), and pushed onto the plate connected to the negative terminal (which becomes negatively charged).
Why does the charging current decrease as a capacitor charges?
As negative charge builds up on the plate, electrostatic repulsion between the electrons already there and incoming electrons increases, so fewer electrons can be pushed on; charging stops once no more electrons can be added.
During discharging, electrons flow back from the negative plate to the .......... plate until there is no potential difference between them.
During discharging, electrons flow back from the negative plate to the positive plate until there is no potential difference between them.
How is a capacitor discharged?
A capacitor is discharged through a resistor, with no power supply present.
True or False?
At the start of charging, the current is small and gradually increases to a maximum.
False.
At the start of charging, the current is large and gradually falls to zero as the capacitor becomes fully charged.
How do the potential difference and charge on the plates change as a capacitor charges?
Both increase until the potential difference equals that of the power supply, and the charge reaches its maximum value determined by the capacitance.
What is the equation for the total capacitance of capacitors connected in series?
$\frac{1}{C_{t o t a l}} = \frac{1}{C_{1}} + \frac{1}{C_{2}} + \frac{1}{C_{3}} . . .$
What is the equation for the total capacitance of capacitors connected in parallel?
$C_{t o t a l} = C_{1} + C_{2} + C_{3} . . .$
Capacitors connected in series store the same charge. What differs across each one?
The potential difference across each capacitor differs; the total p.d. is shared between them, so V = V₁ + V₂.
Capacitors connected in parallel have the same potential difference. What differs across each one?
The charge stored on each capacitor differs, since the current is split at each junction; the total charge is the sum of the individual charges, Q = Q₁ + Q₂.
Capacitors connected in series have different p.d. across them but the same ...........
Capacitors connected in series have different p.d. across them but the same charge.
True or False?
The equations for combining capacitors in series and parallel are the same form as the equivalent equations for resistors.
False.
They are the opposite way around: capacitors in series combine like resistors in parallel (reciprocal sum), and capacitors in parallel combine like resistors in series (simple sum).
In the capacitor-charging investigation, what are the independent and dependent variables?
Independent variable: potential difference, V. Dependent variable: charge, Q.
How is the charging current kept constant in the test circuit?
The variable resistor is continuously adjusted to keep the charging current constant, although this becomes impossible once the capacitor is close to fully charged.
How is the charge stored on the capacitor calculated during the investigation?
Using $Q = I t$ , since the current is kept constant, allowing the charge at each time interval to be calculated.
A graph of charge stored against potential difference for a capacitor is a straight line through the ...........
A graph of charge stored against potential difference for a capacitor is a straight line through the origin.
Why must capacitors be fully discharged before being removed from a circuit after this experiment?
Capacitors can retain charge even after the power supply is removed, which could cause an electric shock.
True or False?
The gradient of the charge-potential difference graph for a parallel combination of capacitors is smaller than for a series combination.
False.
The gradient equals the total capacitance, and since a parallel combination has a greater total capacitance, its gradient is greater than that of the series combination.
How can the energy stored in a capacitor be found from a potential difference-charge graph?
It is equal to the area under the graph, which forms a right-angled triangle: Area = 0.5 × base × height.
State the equation for energy stored by a capacitor in terms of charge and potential difference.
$E = \frac{1}{2} Q V$
State the equation for energy stored by a capacitor in terms of capacitance and potential difference.
$E = \frac{1}{2} C V^{2}$
State the equation for energy stored by a capacitor in terms of charge and capacitance.
$E = \frac{Q^{2}}{2 C}$
As a capacitor charges, the potential difference across it .......... as the charge increases, because it becomes progressively harder to push more charge onto the negatively charged plate.
As a capacitor charges, the potential difference across it increases as the charge increases, because it becomes progressively harder to push more charge onto the negatively charged plate.
True or False?
Doubling the potential difference across a capacitor doubles the energy stored in it.
False.
Since $E = \frac{1}{2} C V^{2}$ , energy is proportional to V², so doubling the potential difference quadruples the energy stored.

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