## What is required for electrical charge to flow through a closed circuit?

The circuit must include a source of potential difference.

## What is electric current?

Electric current is a flow of electrical charge.

## What does the *size* of an electric current mean?

The rate of flow of electrical charge (how fast the charge is flowing)

## How are Charge flow, current and time linked?

Charge Flow = current × time

*[Q = I × t]*

## What is the unit for Charge?

Coulomb ( C )

## What symbol is normally used to represent charge in equations?

Q

## What is the unit of current

Amperes ( A )

## What is the symbol normally used to represent current in equations?

I

## How does current change around a closed loop?

It doesn't: a current has the same value at any point in a single closed loop.

## What is the symbol for an open switch?

[TBC]

## What is the symbol for a closed switch?

[TBC]

## What is the symbol for a lamp?

[TBC]

## What is the symbol for a fuse?

[TBC]

## What is the symbol for a cell?

[TBC]

## What is the symbol for a voltmeter?

[TBC]

## What is the symbol for a battery?

[TBC]

## What is the symbol for an ammeter?

[TBC]

## What is the symbol for a diode?

[TBC]

## What is the symbol for a thermistor?

[TBC]

## What is the symbol for a resistor?

[TBC]

## What is the symbol for a variable resistor?

[TBC]

## What is the symbol for an LDR?

[TBC]

## What is the symbol for an LED?

[TBC]

## What two properties of a component affect the current passing through it?

Resistance of the component and potential difference across it

## How are current, resistance and potential difference related for a component?

potential difference = current × resistance

[V = I R]

## What is the unit of potential difference?

Volts ( V )

## What is the symbol normally used to represent potential difference in equations?

V

## What is the unit of resistance?

Ohms ( Ω )

## What is the symbol normally used to represent resistance in equations?

R

## In what conditions the current passing through a component *directly proportional* to the potential difference across it?

###
- If the component is ohmic
- If the component is at constant temperature

## What is an ohmic conductor

A component for which the resistance stays constant for a given current and potential difference.

(A component that obeys Ohm's law [V = IR])

## How are current and potential difference related for ohmic components?

Resistance is constant, so current is directly proportional to potential difference (V = IR) [Graph TBC]

## Do all components have a fixed resistance?

No, the resistance of components such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component.

## How does the potential difference of a filament lamp change as current increases

###
- Current increase leads to temperature increase (light turns on)
- Temperature increase leads to resistance increase
- Resistance increase leads to potential difference decrease (V = IR)

[Graph TBC]

## How does the potential difference of a diode change as current varies?

###
- Diode only let current flow in one direction
- When current is negative, resistance is very high
- Very high resistance, so potential difference is very low (V=IR)

- When current is positive, resistance is very low
- Very low resistance, so potential difference is very high (V=IR)

- Very high resistance, so potential difference is very low (V=IR)

- Very low resistance, so potential difference is very high (V=IR)

[Graph TBC]

## What is a thermistor?

A component where the resistance *decreases* as the temperature increases

## What might a thermistor be useful for?

Electrical circuits which need to vary based on temperature (e.g. a thermostat)

## What is an LDR

Light Dependent Resistor

Resistance *decreases* as light intensity increases

## What might an LDR be useful for?

Electrical circuits which need to vary based on light (e.g. streetlamps which turn on when it gets dark)

## How can you measure the resistance of a component?

###
- Create a circuit with the component
- unbroken circuit
- has a power source

- Measure the current passing through the component
- Ammeter
- In series (because current is constant in a single loop)

- Measure the potential difference passing through the component
- Voltmeter
- In parallel with the component (measuring the difference between
*before* and *after* the component)

- Resistance is the potential difference divided by the current (V = I R )

- unbroken circuit
- has a power source

- Ammeter
- In series (because current is constant in a single loop)

- Voltmeter
- In parallel with the component (measuring the difference between
*before*and*after*the component)