# Physics - Recall for Chapter 21: Capacitance

> Source: https://ollybritton.com/notes/a-level/physics/recall-questions/chapter-21-capacitance/ · Updated: 2022-04-18 · Tags: physics, school, a-level, electricity

## Flashcards
### 2022-04-18
##### What is capacitance??
The charge stored per unit potential difference.

##### What happens to the two plates of an uncharged capacitor when a p.d. is applied across it??
Charge flows off of one plate and flows onto the other, causing one to become positive and one to become negative.

##### What is the Farad??
$$
CV^{-1},\ \text{Coulombs per Volt}
$$

##### How do you calculate the total capacitance $C_T$ for capacitors $C_1, C_2, C_3$ in parallel??
$$
C_T = C_1 + C_2 + C_3
$$

##### How do you calculate the total capacitance $C_T$ for capacitors $C_1, C_2,\ \ldots$ in series??
$$
\frac{1}{C_T} = \frac{1}{C_1} + \frac{1}{C_2} + \frac{1}{C_3}
$$

##### If capacitors are connected in series, what quantity is the same for all the capacitors??
Charge stored.

##### If capacitors are connected in parallel, what quantity is the same for all capacitors??
Potential difference.

##### What is represented by the gradient of a charge-pd graph for a capacitor??
Capacitance.

##### What is represented by the area under a charge-pd graph for a capacitor??
Energy stored.

##### What is the relationship between energy stored $E$ and potential difference $V$ for a capacitor??
$$
E \propto V^2
$$

##### What is the relationship between energy stored $E$ and the charge on a capacitor $Q$??
$$
E = Q^2
$$

##### Out of $Q, V, I$, which variables follow the relationship $x = x_0 e^{-t/CR} when discharging a capacitor??
All of them, $Q, V, I$

##### Out of $Q, V, I$, which variables follow the relationship $x = x_0 e^{-t/CR} when charging a capacitor??
$$
Q, V
$$

##### How do you calculate the time constant in a capacitor circuit??
$$
CR
$$

##### What is meant by the constant-ratio nature of exponential decay??
In equal time intervals, the quantity decreases by equal factors.

##### How much charge remains after a duration of one time constant when discharging a capacitor??
$$
e^{-1} \approx 0.37
$$

##### How much charge remains after a duration of $N$ time constants when discharging a capacitor??
$$
e^{-N}
$$

##### What is represented by the gradient of a charge-time graph when charging a capacitor??
The current.

##### Can you give two uses of a capacitor??
* Energy storage for camera flashes
* Energy storage for spot welding
* Timing circuits
* Voltage smoothing
* Audio filters

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