Direct current is in one direction only. Alternating current repeatedly reverses its direction.
Cells and batteries supply direct current. The current from the mains supply is alternating current
UK mains electricity
- Frequency is 50 Hz so it alternates direction 50 times per second. The voltage is 230V
- The potential difference between the live wire and 'earth' is usually reffered to as the 'potential' or 'voltage' of the live wire.
- The voltage of the live wire alternates between a positive and a negative potential.
- The voltage of the live wire alternates between +325 volts and -325 volts. In terms of electrical power, this is equivalent to a direct potential difference of 230 volts. The neutral wire stays at zero volts.
- The frequency of an ac supply can be determined by connecting the supply to an oscilloscope. The time taken for one cycle, T, can be found from the oscilloscope trace by measuring the time taken for as many complete cycles as possible
- f (Hz)=1/T where T is the time for one cycle in seconds, s.
Half wave rectification
A diode in series with a resistor may be used to convert alternating current to direct current. An oscilloscope may be used to show how the potential difference across the resistor varies with time.
The diode conducts in the half of each cycle when it is forward biased. The current is said to be half wave rectified.
Cables and plugs
- Sockets and plugs are made of stiff plastic materials, which enclose the electrical connections. The pins in a plug are made of brass which is a good electrical conductor, hard and will not rust or oxidise.
- Mains cable consists of two or three insulated copper wires surrounded by an outer layer of flexible plastic material.
- Cables of different thicknesses are used for different purposes.The more current to be carried, the thicker the cable needs to be, otherwise the cable would overheat.
- In a three-pin plug or a three-core cable: the live wire is brown; the neutral wire is blue; the earth wire is green and yellow.
- Appliances with metal cases must be earthed to (to prevent the case becoming live if the live wire touches it). The case is attached to the earth wire in the cable.
- Appliances with plastic cases do not need to be earthed. They are said to be double insulated and are connected to the supply with two-core cable containing just alive and a neutral wire.
- The cable grip must be fastened tightly over the cable. There should be no bare wires showing inside the plug and the correct wire must be connected firmly to the terminal of the correct pin.
- A fuse contains a thin wire that heats up ad melts if too much current passes through it. This cuts the current off.
- A fuse is always fitted in series with the live wire. This cuts off the appliance from the live wire if the fuse blows.
- Appliances with metal cases need to be earthed. Otherwise if a fault develops, and the live wire touches the metal case, the case becomes live and could give an electric shock to anyone who touches it.
- If a fault develops in an earthed appliance, a large current flows to earth melts the fuse, This disconnects the supply.
- A mains appluance with a plastic case does not need to be earthed because plastic is an insulator and cannot become live.
- The rating of the fuse should be slightly higher than the normal working current of the appliance. If it is much higher, it will not melt soon enough. If it is not higher than the normal current, it well melt as soon as the appliance is switched on.
- A circuit breaker is an electromagnetic switch that opens and cuts off the supply if the current is bigger than a certain value. It can be reset once the fault that caused the current to be cut off is fixed.
- A residual current circuit breaker (RCCB) cuts off the current in the live wire if it is different the current in the neutral wire. It works faster than a fuse or an ordinary circuit breaker and is more sensitive than an ordinary circuit breaker.
Electrical power and potential difference
- The power supplied to a device is the energy transferred to it each second.
- Electric power supplied (watts) = current (amperes) x potential difference (volts).
where P is the powe in watts, W
E is the energy transferred in joules, J
t is the time in seconds, s
- Current rating (in amperes) for a fuse = electric power (watts)/potential difference (volts)
P=I x V
where P is the power in watts, W
I is the current in amperes, A
V is the potential difference in volts, V.
- Electrical appliances have their power rating shown on them. The pd of the mains supply is 230V.
- This equation can be used to calculate the normall current through an appliance and so work out the size of fuse to use. The fuse is chosen so that its value is slightly higher than the calculated current.
Electrical energy and charge
- An electric current is the rate of flow of charge.
The equation relating charge, current and time is:
Q = I x t
where Q is the charge in coulombs, C
I is the current in amperes, A
t is the time in seconds, s.
- When charge flows through an appliance. electrical energy is transferred to other forms. In a resistor. electrical energy is transferred to the resistor so the resistor becomes hotter.
- The amount of energy transferred can be calculated using the equation:
E=V x Q
where E is the energy in joules, J
V is the potential difference in volts, V
Q is the charge in coulombs, C.
Using electrical energy
- For any appliance, the energy supplied to it depends on:
- how long it is switched on for,
- the power supplied to it.
- The kilowatt-hour (kWh) is the energy supplied to a 1KW battery in 1 hours.
- kilowatt-hour = number of kilowatts x number of hours
- We use the kilowatt-hour (kWh) as the unit of energy supplied by mains electricity.
The energy E transferred to a mains appliance is given by the equation:
E = P x t
where E is the energy transferred in kWh
P is the power kilowatts (kW)
t is the time in hours.
- An electricity meter in your home records the total electricity energy supplied to your home in kilowatt-hours.
- The difference between two meter readings gives the number of kilowatt-hours in the time between the two readings.
- The cost of the electrical energy supplied is calculated using the equation:
- Total cost = number of kWh used x cost per kWh
- Electrical faults are dangerous because they can cause electric shocks and fires.
- Electrical faults may occurs as a result of damage to sockets, plugs, cables or appliances.
- Electrical equipments should be checked regularly for wear. Worn or damaged items should be replaced or repaired by a qualified electrician.
- Avoid overloading sockets as this may cause overheating and a risk of fire.
- Electrical appliances should be handled safely and never used in a bathroom or with wet hands.
- Never touch a mains appliance (or plug or socket) with wet hands. Never touch a bare wire or terminal at a potential of more than 30V.
- When choosing an electrical appliance, the power and efficiency rating need to be considered as well as the cost.
- Filament bulbs and halogen lambs are much less efficient than low energy compact fluorescent lamps and do not last as long.
The National Grid
- The National Grid is a network of pylons and cables that connects power stations to homes, schools, factories and other buildings. Since the whole country is connected to the system, power stations can be switched in or out according to demand.
- The cables cover long distances across the countryside supported by overhead pylons. In towns and close to homes the cables are buried underground.
- The National Grid's voltage is 132000 V or more. Power stations produce electricity at a voltage of 25000V.
- In power stations, electricity is generated at a particular voltage. The voltage is increased by step- up transformers before the electricity is transmitted across the National Grid. This is because transmissions at high voltage reduces the current in the cables and therefore reduces energy wasted in the cables, making the system more efficient.
- It would be dangerous to supply electricity to consumers at these very high voltages. So, at local sub-stations, step-down transformers are used to reduce the voltage to 230 volts for use in homes and offices