Define discharging (of a capacitor).

The process in which a capacitor loses its stored charge by releasing electrons through a resistor , with no power supply connected, until there is no p.d. between the plates.

How does the current vary with time as a capacitor charges?

The current starts at a maximum value and decreases exponentially to zero.

How do the potential difference and charge vary with time as a capacitor charges?

Both start at zero and increase exponentially up to a maximum value; the p.d. -time and charge-time graphs have the same shape.

What effect does a low resistance have on the rate at which a capacitor discharges?

A low resistance means the current is larger, so charge flows from the plates quickly and the capacitor discharges faster .

The area under a current-time graph for a charging or discharging capacitor represents the .......... stored in that time interval.

The area under a current-time graph for a charging or discharging capacitor represents the charge stored in that time interval.

What does the gradient of a charge-time graph represent?

The current at that point in time.

True or False? Conventional current flows in the same direction as electron flow.

False. Conventional current flows in the opposite direction to electron flow.

Define the time constant , τ, of a discharging capacitor.

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

What is the definition of the time constant for a charging capacitor?

The time taken for the charge or voltage of a charging capacitor to rise to 63% of its maximum value.

What is the definition of the half-life, t 1/2 , of a discharging capacitor?

The time taken for the charge, current, or voltage of a discharging capacitor to reach half of its initial value.

The time constant, τ, is measured in units of .......... .

The time constant, τ, is measured in units of seconds .

True or False? The time constant of a charging capacitor is the time taken to reach 37% of its maximum value.

False. For a charging capacitor, the time constant is the time taken to reach 63% of its maximum value; 37% applies to a discharging capacitor.

Define e , the exponential constant.

A number approximately equal to 2.718 ; its inverse function is the natural logarithm, ln .

What effect does a smaller time constant have on the rate of discharge of a capacitor?

A smaller time constant, τ, causes a quicker exponential decay of the current (or charge or p.d.) during discharge.

No matter how much charge is initially on the plates of a discharging capacitor, the time taken for that charge to .......... is always the same.

No matter how much charge is initially on the plates of a discharging capacitor, the time taken for that charge to halve is always the same.

In the charging equations, what do Q 0 and V 0 represent?

The maximum (final) charge and potential difference stored on the capacitor when it is fully charged, rather than the initial values.

What are the independent and dependent variables in the capacitor charge/discharge required practical?

Independent variable: time , t Dependent variable: potential difference , V

What are the control variables in the capacitor charge/discharge required practical?

Resistance of the resistor Current in the circuit

A resistor with a .......... resistance should be used so the capacitor discharges slowly enough for accurate timing.

A resistor with a large resistance should be used so the capacitor discharges slowly enough for accurate timing.

How can parallax error be reduced when reading an analogue voltmeter in this experiment?

Read the p.d. at eye level to the meter.

Why should a capacitor be fully discharged and left for a few minutes before being removed from the circuit?

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

True or False? Using a datalogger instead of a stopwatch to record p.d. reduces random error in this experiment.

True. A datalogger removes the error caused by human reaction time when trying to stop a stopwatch at a specific p.d., giving more accurate results.

A LevelPhysicsAQAFlashcardsFields & Their ConsequencesCapacitor Charge & Discharge

Capacitor Charge & Discharge (AQA A Level Physics): Flashcards

Exam code: 7408

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FrontCharge & Discharge Graphs
Define discharging (of a capacitor).

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Cards in this collection (29)

Define discharging (of a capacitor).
The process in which a capacitor loses its stored charge by releasing electrons through a resistor, with no power supply connected, until there is no p.d. between the plates.
How does the current vary with time as a capacitor charges?
The current starts at a maximum value and decreases exponentially to zero.
How do the potential difference and charge vary with time as a capacitor charges?
Both start at zero and increase exponentially up to a maximum value; the p.d.-time and charge-time graphs have the same shape.
What effect does a low resistance have on the rate at which a capacitor discharges?
A low resistance means the current is larger, so charge flows from the plates quickly and the capacitor discharges faster.
The area under a current-time graph for a charging or discharging capacitor represents the .......... stored in that time interval.
The area under a current-time graph for a charging or discharging capacitor represents the charge stored in that time interval.
What does the gradient of a charge-time graph represent?
The current at that point in time.
True or False?
Conventional current flows in the same direction as electron flow.
False.
Conventional current flows in the opposite direction to electron flow.
Define the time constant, τ, of a discharging capacitor.
The time taken for the charge, current, or voltage of a discharging capacitor to decrease to 37% of its original value.
What is the definition of the time constant for a charging capacitor?
The time taken for the charge or voltage of a charging capacitor to rise to 63% of its maximum value.
State the equation for the time constant, τ, in terms of resistance and capacitance.
$τ = R C$
What is the definition of the half-life, t_1/2, of a discharging capacitor?
The time taken for the charge, current, or voltage of a discharging capacitor to reach half of its initial value.
How is the half-life of a discharging capacitor related to the time constant?
$t_{1 / 2} = 0.69 τ = 0.69 R C$
The time constant, τ, is measured in units of ...........
The time constant, τ, is measured in units of seconds.
True or False?
The time constant of a charging capacitor is the time taken to reach 37% of its maximum value.
False.
For a charging capacitor, the time constant is the time taken to reach 63% of its maximum value; 37% applies to a discharging capacitor.
Define e, the exponential constant.
A number approximately equal to 2.718; its inverse function is the natural logarithm, ln.
State the equation for the exponential decay of current for a discharging capacitor.
$I = I_{0} e^{- \frac{t}{R C}}$
State the equations for charge and potential difference during capacitor discharge.
- $Q = Q_{0} e^{- \frac{t}{R C}}$
- $V = V_{0} e^{- \frac{t}{R C}}$
What effect does a smaller time constant have on the rate of discharge of a capacitor?
A smaller time constant, τ, causes a quicker exponential decay of the current (or charge or p.d.) during discharge.
No matter how much charge is initially on the plates of a discharging capacitor, the time taken for that charge to .......... is always the same.
No matter how much charge is initially on the plates of a discharging capacitor, the time taken for that charge to halve is always the same.
State the equation for the charge on a capacitor as it charges.
$Q = Q_{0} (1 - e^{- \frac{t}{R C}})$
In the charging equations, what do Q₀ and V₀ represent?
The maximum (final) charge and potential difference stored on the capacitor when it is fully charged, rather than the initial values.
True or False?
The equation for current during charging is different from the equation for current during discharging.
False.
The charging equation for current is the same as the discharging equation, $I = I_{0} e^{- \frac{t}{R C}}$ , since the current always decreases exponentially.
What are the independent and dependent variables in the capacitor charge/discharge required practical?
- Independent variable: time, t
- Dependent variable: potential difference, V
What are the control variables in the capacitor charge/discharge required practical?
- Resistance of the resistor
- Current in the circuit
What quantities should be graphed, and what does the gradient represent, to determine capacitance in this experiment?
Plot ln(V) against time; the gradient of the line of best fit is equal to $- \frac{1}{R C}$ , from which the capacitance C can be calculated.
A resistor with a .......... resistance should be used so the capacitor discharges slowly enough for accurate timing.
A resistor with a large resistance should be used so the capacitor discharges slowly enough for accurate timing.
How can parallax error be reduced when reading an analogue voltmeter in this experiment?
Read the p.d. at eye level to the meter.
Why should a capacitor be fully discharged and left for a few minutes before being removed from the circuit?
Capacitors can still retain charge after the power is removed, which could cause an electric shock.
True or False?
Using a datalogger instead of a stopwatch to record p.d. reduces random error in this experiment.
True.
A datalogger removes the error caused by human reaction time when trying to stop a stopwatch at a specific p.d., giving more accurate results.

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