# Physics - Capacitance

## Flashcards

### 2022-03-22

What is the formula for capacitance?

What does a capacitor consist of?

Two metallic plates separated by an insulator.

What’s the name for the insulator that separates a capacitor?

A dielectric.

How can a current still flow when a capacitor effectively creates a gap in the circuit?

The electric field generated by the bunched up electrons push the electrons on the other side away.

Why does the current drop to zero after a capacitor is charged?

Because there are too many electrons on the capacitor, so no more can be forced on (and therefore forced off on the other side).

When does the current in a circuit drop to zero when a capacitor is involved?

When the potential difference at the capacitor is equal to the EMF of the power supply.

What happens to the resistance of a capacitor as it charges?

It increases.

What is true about the area of a capacitor and the capacitance it has?

Directly proportional

\[\text{area} \propto \text{capacitance}\]What is the formula for $C _ T$ given $C _ 1, C _ 2, C _ 3$ etc. in SERIES?

What is the formula for $C _ T$ given $C _ 1, C _ 2, C _ 3$ etc. in PARALLEL?

What is the relationship between the capacitance and resistance formulas for series and parallel circuits?

They are opposites.

## \[\frac{1}{C _ T} = \frac{1}{C _ 1} + \frac{1}{C _ 2} + ...\]
What does this describe; capacitors in series or parallel?

Series.

## \[C _ T = C _ 1 + C _ 2 + ...\]
What does this describe; capacitors in series or parallel?

Parallel.

### 2022-03-29

What does the graph for the potential difference against charge in a capacitor look like?

What does the area under a potential difference-charge graph represent?

The energy stored in a capacitor.

By considering this graph, what is a formula for the energy stored by a capacitor in terms of $Q$ and $V$?

What pattern does the potential difference across a capacitor discharging through a resistor follow?

Exponential decay.

What three things are proportional when a capacitor discharges through a resistor?

What does the time constant $\tau$ in a capacitor-resistor circuit tell you?

If you say “the potential difference across a capacitor discharges with exponential decay”, what assumption are you making?

The capacitance and resistance remains constant.

What is the formula for the time constant $\tau$?

What is the formula for $V$, $I$ or $Q$ after a certain amount of time has elapsed in a capacitor-resistor circuit?

What differential equation are you approximating with Euler’s method in the iterative model of a capacitor discharging?

## \[\frac{\text{d}Q}{\text{d}t} = \frac{V}{R}\]
How do you rewrite $V$ in order to use iterative modelling on this differential equation?

What is the step $\Delta Q$ for modelling a capacitor using iteration?

### 2022-04-06

What is the formula for current $I$ as a capacitor charges?

What is the formula for voltage or charge as a capacitor charges?

Which out of charge $Q$, voltage $V$ and current $I$ do you formulas for charging a capacitor involve $1 - e^{-\frac{t}{CR}}$?

Charge $Q$ and voltage $V$

What does $V _ 0$ and $Q _ 0$ represent in the capacitor charging questions?

The final charge and voltage.

### 2022-06-05

## \[C _ T = C _ 1 + C _ 2 + C _ 3 \ldots\]
How can you prove this for capacitors in parallel?

Consider the fact charge will be shared but voltage is the same, so $C _ T V = C _ 1 V + C _ 2 V$

## \[\frac{1}{C _ T} = \frac{1}{C _ 1} + \frac{1}{C _ 2} + \frac{1}{C _ 3} \ldots\]
How can you prove this for capacitors in series?

Consider the fact that charge will be the same but voltage will be shared, and then use $V = Q/C$

Why will the charge be the same for capacitors in series?

Equal and opposite charges are acquired by opposites sides of capacitors, so capacitors next to each other must have the same charge.