During charging, why does the current decrease and eventually stop?

As negative charge builds up on the plate, electrostatic repulsion pushes back against arriving electrons, so fewer are pushed onto the plate. Charging stops when no more electrons can be pushed onto the negative plate.

How does the direction of conventional current relate to the direction of electron flow in a charging or discharging capacitor?

Conventional current flows in the opposite direction to electron flow.

As a capacitor discharges, the current, potential difference and charge all decrease .......... .

As a capacitor discharges, the current, potential difference and charge all decrease exponentially .

True or False? The rate at which a capacitor discharges is independent of the resistance in the circuit.

False. A higher resistance makes the capacitor discharge more slowly , while a lower resistance causes it to discharge faster .

What is the aim of the required practical investigating the charging and discharging of a capacitor?

To calculate the capacitance of the capacitor.

State two control variables in the capacitor charge and discharge investigation.

Resistance of the resistor Current in the circuit

Why must a resistor with a large resistance be used when investigating the discharge of a capacitor?

So the capacitor discharges slowly enough for the time to be measured accurately at each potential difference interval.

Why must a capacitor be fully discharged before removal from a circuit, even after the power supply is disconnected?

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

Define time constant (for a discharging capacitor).

The time constant is the time taken for the charge, current or voltage of a discharging capacitor to decrease to 37% of its original value.

For a charging capacitor, the time constant is the time taken for the charge or voltage to rise to .......... % of its maximum value.

For a charging capacitor, the time constant is the time taken for the charge or voltage to rise to 63% of its maximum value.

True or False? The time constant of a discharging capacitor is the same quantity as its half-life.

False. The time constant is the time to fall to 37% of the original value, whereas the half-life is the time to fall to 50% — they are different quantities.

For the charging equations, what do Q 0 and V 0 represent, compared with the discharging equations?

In the charging equations, Q 0 and V 0 are the final (maximum) charge and p.d. reached, rather than the initial values used in the discharging equations.

Name the two methods used to verify that the p.d. or charge on a capacitor decreases exponentially.

Constant ratio method – check the time constant (or time to halve) is constant Logarithmic graph method – plot ln V against t and check for a straight line

In the logarithmic graph method, what does the y -intercept of a ln( V )- t graph equal?

ln( V 0 ), the natural log of the initial potential difference .

In the constant ratio method, the .......... of the potential difference must remain constant over equal time intervals to validate exponential decay.

In the constant ratio method, the time constant of the potential difference must remain constant over equal time intervals to validate exponential decay.

True or False? The area under a current-time graph for a discharging capacitor represents the potential difference at that time.

False. The area under a current-time graph represents the charge stored (Δ Q = I Δ t ), not the potential difference.

How is the current at a particular time found from a charge-time graph?

By drawing a tangent to the curve at that point and calculating the gradient of the tangent.

A LevelPhysicsOCRFlashcards6. Particles & Medical PhysicsCharging & Discharging Capacitors

Charging & Discharging Capacitors (OCR A Level Physics): Flashcards

Exam code: H556

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During charging, why does the current decrease and eventually stop?

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During charging, why does the current decrease and eventually stop?
As negative charge builds up on the plate, electrostatic repulsion pushes back against arriving electrons, so fewer are pushed onto the plate. Charging stops when no more electrons can be pushed onto the negative plate.
How does the direction of conventional current relate to the direction of electron flow in a charging or discharging capacitor?
Conventional current flows in the opposite direction to electron flow.
As a capacitor discharges, the current, potential difference and charge all decrease ...........
As a capacitor discharges, the current, potential difference and charge all decrease exponentially.
True or False?
The rate at which a capacitor discharges is independent of the resistance in the circuit.
False.
A higher resistance makes the capacitor discharge more slowly, while a lower resistance causes it to discharge faster.
What is the aim of the required practical investigating the charging and discharging of a capacitor?
To calculate the capacitance of the capacitor.
State two control variables in the capacitor charge and discharge investigation.
- Resistance of the resistor
- Current in the circuit
Why must a resistor with a large resistance be used when investigating the discharge of a capacitor?
So the capacitor discharges slowly enough for the time to be measured accurately at each potential difference interval.
Why must a capacitor be fully discharged before removal from a circuit, even after the power supply is disconnected?
Capacitors can retain charge after power is removed, which could cause an electric shock.
Define time constant (for a discharging capacitor).
The time constant is the time taken for the charge, current or voltage of a discharging capacitor to decrease to 37% of its original value.
State the equation for the time constant, τ, in terms of resistance and capacitance.
$τ = R C$
State the equation for the half-life, t_1/2, of a discharging capacitor in terms of the time constant.
$t_{1 / 2} = \ln (2) τ \approx 0.69 R C$
For a charging capacitor, the time constant is the time taken for the charge or voltage to rise to ..........% of its maximum value.
For a charging capacitor, the time constant is the time taken for the charge or voltage to rise to 63% of its maximum value.
True or False?
The time constant of a discharging capacitor is the same quantity as its half-life.
False.
The time constant is the time to fall to 37% of the original value, whereas the half-life is the time to fall to 50% — they are different quantities.
Write the equation for the current, I, during capacitor discharge in terms of the initial current, I₀.
$I = I_{0} e^{- \frac{t}{R C}}$
Write the equation for the charge, Q, on a charging capacitor in terms of its maximum charge, Q₀.
$Q = Q_{0} (1 - e^{- \frac{t}{R C}})$
For the charging equations, what do Q₀ and V₀ represent, compared with the discharging equations?
In the charging equations, Q₀ and V₀ are the final (maximum) charge and p.d. reached, rather than the initial values used in the discharging equations.
Name the two methods used to verify that the p.d. or charge on a capacitor decreases exponentially.
- Constant ratio method – check the time constant (or time to halve) is constant
- Logarithmic graph method – plot ln V against t and check for a straight line
In the logarithmic graph method, what does the gradient of a ln(V)-t graph equal?
$- \frac{1}{R C}$
In the logarithmic graph method, what does the y-intercept of a ln(V)-t graph equal?
ln(V₀), the natural log of the initial potential difference.
In the constant ratio method, the .......... of the potential difference must remain constant over equal time intervals to validate exponential decay.
In the constant ratio method, the time constant of the potential difference must remain constant over equal time intervals to validate exponential decay.
True or False?
The area under a current-time graph for a discharging capacitor represents the potential difference at that time.
False.
The area under a current-time graph represents the charge stored (ΔQ = IΔt), not the potential difference.
What physical quantity does the gradient of a charge-time graph represent for a charging or discharging capacitor?
The current at that time.
Write the equation relating the rate of change of charge to charge, resistance and capacitance, used for modelling a discharging capacitor.
$\frac{Δ Q}{Δ t} = - \frac{Q}{R C}$
How is the current at a particular time found from a charge-time graph?
By drawing a tangent to the curve at that point and calculating the gradient of the tangent.

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