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# Circuit components

## Cells, bulbs and switches

An electrochemical cell generates direct current by converting chemical energy to electrical energy.

A battery (often used interchangeably with cell) is a group of two or more cells connected in series.

Left: A single cell. Right: A battery

A switch is a circuit element that makes it easy to interrupt a circuit.

The simplest switch consists of two wires. When the wires touch, the switch is on. If they do not touch, the switch is off.

A switch also allows you to change between different sections of a circuit.

A bulb, light bulb, or lamp lights up when current passes through it. Bulbs are brighter when the current passing through them is larger, but if the current is too large they can blow.

A blown bulb can no longer conduct electricity.

A cell is connected to a switch and a lamp. When the switch is closed, current will flow and the lamp will begin to shine.

## Measuring devices

An ammeter is an instrument used to measure electric current. It has a very small resistance and so can be connected in series without affecting the current flowing through the circuit.

A galvanometer is a type of ammeter that shows the direction of current (either positive or negative) in addition to the magnitude.

A voltmeter measures the potential difference or voltage.

A voltmeter has a very high resistance and almost no current passes through it. As a result, a voltmeter must be connected in parallel to a component.

## Light dependent resistors and thermistors

The resistance of a light dependent resistor (LDR) decreases as light intensity increases.

LDRs are used in alarm devices and street lights. The street light turns on or the alarm rings when the light reaches a certain level.

The resistance of a thermistor decreases as temperature increases.

Thermistors are mainly used in electric thermometers.

Left: A light dependent resistor. Right: A thermistor

## Semiconductor diodes - forward and reverse bias

A diode is an electrical component that allows current to flow in only one direction.

Diodes are like a one-way valve, such as those found in plumbing, or in the heart.

Forward biased diode

This diode is forward biased (the positive end of the diode is connected to the positive terminal of the battery).

When the arrow from the circuit symbol for a diode points in the direction of conventional current, it is forward biased and conducts electricity.

The diode allows current to flow, and the lamp lights up.

Reverse biased diode

This diode is reverse biased (the positive end of the diode is connected to the negative terminal of the battery).

The diode does not allow current to flow, and the lamp does not light up.

## Resistors and rheostats

A resistor is a component which introduces resistance into a circuit.

Resistors can protect other circuit components within a circuit by limiting currents.

The resistance of a resistor is measured in ohms ($\Omega$).

Fixed resistors have a resistance that remains approximately constant over a wide range of temperatures. This means that they are ohmic conductors.

The symbol for a fixed resistor.

A rheostat is a variable resistor. Rheostats are used to change the current in a circuit.

The symbol for a rheostat

Rheostats are used to vary the brightness of lights and in radio controls.

A rheostat contains a length of wire with a high resistance. A slider is used to change the length of wire that the current flows through. A long wire gives a high resistance while a short wire gives a low resistance.