A circuit is a complete loop that allows an electrical current to flow. Electrons flow around the circuit from the negative electrode of the power source to the positive electrode. But this was only discovered recently so circuit diagrams are drawn showing the current flowing from the positive to the negative electrode.
Fixed and Variable Resistors
Resistance is a measure of how hard it is to get a current through a component in a circuit at a particular voltage (potential difference). Resistance is measured in ohms.
The current through a circuit can be controlled by varying the resistence. There are two types of resistor:
- A fixed resistor has a constant resistance. The bigger the resistance, the smaller the current that flows for a particular voltage.
- A variable resistor (or rheostat) has a changeable resistance.
The resistancr of the rheostat can be changed by moving the slider, changing the length of the wire between the contacts.
- Long wire = high resistance, low current.
- Shirt wire = low resistance, large current.
Current, Voltage and Resistance
For a given resistor, current increases as voltage increases (and vice versa). For a fixed volage, curresent decreases as resistance increases (and visa versa).
Current, voltage and resistance are related by this formula:
Live, Neutral and Earth Wires
Electrical appliances to mains electricity by a cable 30-pin plug. Most cables and plugs contain three wires:
- Live wire (brown) - carries current to the appiances at a high volatge (230V).
- Neutral wire (blue) - completes the circuit and carries current away the appliance.
- Earth Wire (green and yellow) - safety wire that stops the appliance becoming live.
All appliances with outer metal cases (conductors) have earth wire, so they are earthed. An earthed conductor can't become live. Appliances with cases made of insulators don't have an earth wire (although they still have a fuse). They are double insulated so they can;t become live.
Electrical appliances with outer metal case are earthed in order to protect the appliance and the user. The earth wire and fuse work together.
1. The fault in the appliance causes the casing to become live.
2. The circuit short-circuits (i.e. the path of the flow of charge changes) because the earth wire offers less resistance.
3. The fuse wire melts.
4. The circuit is broken.
5. The application and the user are protected.
Fuses and Circuit Breakers
Fuses and circuit breakers are safety devices designed to break a circuit if a fault occurs. This can prevent fires, injuries and deaths.
A fuse is a short thin piece of wire with a low melting point. It's used to prevent cables or appliances from overheating. To work properly, the current rating of the fuse must be just above the normal current that flows through the appliance.
1. A fault causes the current in the appliance to exceed the current rating of the fuse.
2. The fuse wire get hot and melts or breaks.
3. The circuit is broken so the current is unable to flow.
4. The appliance and user are protected.
A circuit breaker acts in a similar way to a fuse, but it can be easily restet rather than replaced.
The power rating of a device tells you how quickly electrical energy is being changed (transferred) into another form within that device. Power is measured in Watts (W). For example, a 2400W hairdryer changes electrical energy into heat and kinetic energy (as well as sound) at a rate of 2400 joules (J) per second.
Power (W) = Current (A) x Voltage (V)
Example of a Fuse in Action
1. If the current flowing through an appliance is below the current rating of the fuse, the appliance will work properly.
2. But, if a fault occurs es inside the appliance, the live wire will make contact with the neutral wire. The current flowing would then be higher than the current rating of the fuse due to lower resistance,
3. This causes the fuse wire to get hotter and hotter until it melts and brakes the circuit. The current is unable to flow so there is no danger of the flex overheating (resulting in a fire). Further damage to the appliance, or injury to the user is prevented.
Fuses and circuit breakers prevent:
- Injury and death as they stop appliances from becoming 'live'.
- Fires as they stop cables and flexes from overheating.
- Damage to the components of an appliance because they break the circuit if a higher than normal current flows through the appliance.