# Capacitors

Capacitors

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- Created by: Laura Charles
- Created on: 09-05-12 21:13

## What is Capacitance?

Definition of Capacitance

- Capacitance is defined as the amount of
**charge stored**per**volt** **C=Q/V**- Capacitance is measured in Farads(F)
- Farads are a big unit- therefore usually in micro/nano/pico farads

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## Practicals involving Capacitors

You can investigate the Charge stored by a Capacitor Experimentally

- Set up a test circuit to measure current and potential difference
- Constantly adjust the
**variable resistor**to keep charging current**constant**for as long as possible - NB-it is impossible to keep charging current constant when capicitor is nearly fully charged
- At regualr intervals record
**p.d**until it**equals**the**battery p.d** - It is then possible to plot graphs of Current(I)/Amps vs Time(t)/seconds and Charge(Q)/Coulombs vs Potential Difference(V)/Volts
**Gradient**of**Q/p.d**is**Capacitance**

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## Capacitance and Energy

Capacitors store Energy

- When
**charge**builds up on plates of the**capacitor**,**electrical energy**is**stored** **Area**under**graph**of**p.d**vs**charge stored**=**energy stored**in the capacitor**p.d**across the capacitor is**proportional to charge stored**on it- therefore the graph will be a straight line through the origin
- Area under graph(triangle) -
**E=1/2xQV**

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## Charging a Capacitor

A capacitor can be charged by connecting it to a battery

- when connected to a
**battery**, a**current**flows in circuit**until**capacitor is**fully charged**, then**stops** **Electrons**flow**onto negative terminal**plate- (-ve) charge builds- Build up of (-ve) charge
**repels**electrons off plate connected to**positive terminal**of battery, making that plate positive - These electrons are attracted to the positive terminal of the battery
- An
**equal**and**opposite**charge builds plates. Causes**Potential Difference**between them (NB-no charge can flow between plates-Insulator) **Current**starts**high**. As more charge builds up**electrostatic repulsion**makes it more difficult for electrons to be deposited- When
**p.d across capacitor=p.d across battery**, current is 0 - Capacitor is now
**fully charged**

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## Discharging a Capacitor

To Discharge a Capacitor, Take out the Battery and Reconnect the Current

- When a
**charged capacitor**is connected across a**resistor**, the p.d drives a**current**through the circuit - This
**current**flows in**opposite direction**from**charging current** - If a
**voltage sensor**attached to a**datalogger**is connected across the capacitor , a**discharge curve**can be plotted - Capacitor is
**fully discharged**when**p.d**across plates and**current**in circuit are both**zero**.

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## Charging and Discharging

The **Time Taken** to **charge** or **discharge** depends on:

- Capacitance of Capacitor (C). This affects the amount of charge that can be transferred at a given voltage.
- Resistance of circuit (R). This affects the current in the circuit.

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## Charging and Discharging

The **Charge** on a Capacitor **Decreases Exponentially**

- When the capacitor
**discharges**, the charge left on the plates falls**exponentially with time** - Therefore it always takes the same length of time for the charge to
**halve**, no matter**how much charge**you start with

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## Time Constant = RC

**Time constant**, is time taken for charge on a discharging capacitor to**fall**to**37%**of**Qo**- Therefore its also time taken for charge of a charging capacitor to
**rise**to**63%**of**Qo** - The
**larger**the**resistance**in series with the capacitor the**longer it takes**to charge/discharge - In practice, time taken for capacitor to charge/discharge
**fully**is about**5RC**

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