# Electricity: Circuit Devices

## Understanding Circuit Devices

• Circuit devices are components in an electrical circuit that have a specific function.
• Resistors limit the flow of current through a circuit. They convert electrical energy into heat energy.
• A fixed resistor has a constant resistance. This means that the current through it is directly proportional to the voltage across it.
• A variable resistor, also known as a rheostat, can be adjusted to change the resistance. This allows current to be controlled.
• The symbol for a resistor is a jagged line whilst a variable resistor is depicted by a jagged line with an arrow across it.

## Light Emitting Diodes (LEDs) and Diodes

• Light Emitting Diodes (LEDs) emit light when current flows through them. They’re used in various equipment, from remote controls to traffic lights.
• Diodes allow current to flow in one direction only. This makes them useful in circuits where direction of current is critical.
• LEDs and diodes are represented in diagrams by a triangle into a line. For an LED, two arrows are drawn away from the triangle to signify light emission.

## Fuses and Circuit Breakers

• A fuse is a safety device meant to protect an electrical circuit from excessive current.
• When a too high current flows through the fuse, it melts and breaks the circuit. The excessive current will then not reach the rest of the components in the circuit.
• A circuit breaker is another safety mechanism. It automatically switches off the circuit if the current gets too high.
• Fuses and circuit breakers are crucial for preventing potential electrical fires and equipment damage.

## Thermistors and Light Dependent Resistors (LDRs)

• Thermistors are type of variable resistor that changes its resistance with temperature. In Negative Temperature Coefficient (NTC) thermistors, resistance decreases as temperature increases.
• A Light-Dependent Resistor (LDR) is a resistor that changes its resistance with the amount of light it is exposed to. More light will cause the resistance to decrease.
• Both thermistors and LDRs are valuable in responding to changes in the environment, aiding in the functions of devices like thermostats and automatic lighting systems.

## Understanding Relays

• A relay uses a small current to control a much larger current. This allows circuits to be switched on and off without the risk of a dangerous electric shock.
• Relays are useful in situations where a circuit needs to be controlled by another circuit that might not be able to handle the first circuit’s power demands.

Understanding these circuit devices is vital for appreciating the complexity and functionality of electrical circuits.