(h) Practical Circuitry

Learning Objectives

• describe the use of the heating effect of electricity in appliances such as kettles, ovens and heaters

• recall and apply the relationships P = VI and  E = VIt to new situations or to solve related problems.

•  calculate the cost of using electrical appliances where energy unit is the kW h

•  compare the use non-renewable and renewable energy sources such as fossil fuels, nuclear energy, solar energy, wind energy and hydroelectric generation to generate electricity in terms of energy conversion efficiency, cost per kW h produced and environmental impact

• state the hazards of (i)         damaged insulation   (ii)        overheating of cables     (iii)       damp conditions

• explain the use of  fuses and circuit breakers in electrical circuits and of fuse ratings

• explain the need for earthing metal cases and for double insulation

• state the meaning of the terms:  live, neutral and earth

• describe the wiring in a mains plug

• explain why switches (placed after fuses), fuses and circuit breakers are wired into the live conductor

Uses of electricity

Electrical energy is used to power many appliances around the home. It can be supplied by batteries or from the mains supply and is easily converted to other forms of energy.

One of the main uses for electricity is to generate heat. Ovens, coffee makers, irons, water heaters, and hairdryers all convert electrical energy into thermal energy.

All heaters contain a heating element. This converts electrical energy into heat energy. Heating elements are often made of tight coils of Nichrome wire.

Nichrome is a poor electrical conductor that gives off heat energy when a current is passed through it. For this reason, nichrome wires are often used to make heating elements in electrical applicances.

Nichrome is used for two reasons:

• It has a high resistance so is good at converting electrical energy into heat energy.

• It does not oxidize or rust when exposed to the air.

Electrical power and energy

Some microwave ovens can heat up food more quickly than others. This is because they have different power ratings, and use up electrical energy at different rates. For example, a microwave oven with a 500 watt power rating will take twice as long as a 1000 watt microwave oven to heat up the same meal.

Power is used to measure how quickly an electrical component uses up. The power rating of an appliance shows how many joules of electrical energy the appliance uses per second when the woking voltage is supplied to it.

Power rating of an electric oven is much larger than that of a light bulb.

Power is the rate of work done or at which energy is changed from one form to another.

Power = energy/time

P = E/t

SI unit : watts, W

Since V = W/Q and Q = It

Hence electical power is given by P = IV and electrical energy is given by W= VIt

Energy Efficiency

http://app.e2singapore.gov.sg/

http://energy.gov/public-services/homes/saving-electricity/appliances-electronics

http://tlc.howstuffworks.com/home/energy-efficient-appliances.htm

Dangers of electicity

Electric shock and electric fire are two major dangers when using electricity.

An electric shock may be caused by touching wet electrical appliances or faulty ones with current leakage.

http://www.straitstimes.com/singapore/circuit-breaker-could-be-a-lifesaver

Electric fires are mainly caused by over-heating of cables when there is an over loading or a short circuit.

http://www.ema.gov.sg/Electrical_Safety.aspx

Safe Use of Electricity at Home

The domestic electricity supply has a high voltage of 240 V. If a fault developed, it could potentially be very dangerous. Houses and electrical appliances contain several safety features that prevent injury to people and also limit the possibility of damage to the appliances and to the house. These safety features include fuses, earth wires, and insulation.

Switches

A switch is used to turn an electrical appliance on or off by connecting or breaking the circuit respectively.

Faulty switches have poor contacts which possess high resistance. Current flowing through them would generate a lost of thermal energy, which may lead to electric fires.

Switches are all fitted onto the 'live' wire so that switching off the circuit will disconnect the high voltage from the appliance.

It is absolutely important that a three pin plug is wired with the correct coloured wires correctly.

Earth wire

The earth wire is the third wire in a power circuit which functions as a safety measure for electrical appliances having metal casings.

If a live wire touches the metal casing, a large current flows from the live wire to the earth through the earth wire and melts the fuse. Thus the circuit is broken and electric shocks are avoided.

Some electrical devices do not have a metal case. In fact, many have no metal parts on the outside at all. Instead the casing is made of two layers of a strong non-conducting material. A hairdryer is a good example of this.

An appliance such as this does not need to have an earth wire, as there is no way to get an electrical connection between yourself and the mains by touching the case. Appliances such as this are called double insulated and have a symbol as shown below.

Circuit Breakers

Circuit breaker can save you from electrocution

Circuit breakers are safety devices that can switch off electrical supply in a circuit when faults are detected in home electrical circuitry. The consumer unit circuit breaker can normally be found inside the house near the main door. It consists of two types of circuit breakers, namely the miniature circuit breaker (MCB) and the earth leakage circuit breaker (ELCB).

The Miniature Circuit Breaker will 'trip' or break the circuit when there is a short circuit or when there is too large a current through the circuit. The MCB can be reset by switching it back to the 'ON' position to get the electricity supply again. This should be done only after the faulty circuit has been rectified.

The Earth Leakage Circuit Breaker (ELCB) or Residual current circuit Breaker (RCCB) detects small leakage from the live wire to the earth wire. When a small leakage current, about 10 - 30 mA is detected in the earth wire, the ELCB will 'trip' and switch off the mains electricity supply.

Electrical circuits at home

Electricity is supplied to your home. It is different in several ways from the electricity supplied by a battery. The domestic electricity supply is much more dangerous because it is a high voltage. It is supplied by a general purpose circuit to all sockets in your house. The domestic electricity does not flow constantly in one direction. Instead, it is an alternating current.

The domestic electricity supply is generally the same anywhere within one country, but it can vary between countries. For example, in North America and some countries in Central America the domestic supply has a voltage of around 120 V and a frequency of 60 Hz. In Europe, Australia, and much of Asia it has a voltage of around 230 V and a frequency of 50 Hz.

The voltage of the domestic electric supply is much higher than the potential difference of most batteries. This means that the domestic electricity can be used to powermuch larger appliances, such as washing machines and water heaters. However, the higher voltage is dangerous, and can injure you if you touch it.

Lighting Circuit

The lamps are always connected in parallel so that each lamp receives the main voltage of 240 V. In addition, if one lamp should fail, the rest of the lamps will not be affected since they are connected in parallel.

Ring main circuit

This circuit supplies electricity to all the wall sockets in the house. Since the current can flow to any particular socket by two different ways, thinner wires can be used in the whole ring. Besides both the live and neutral wires running a complete ring round the house, an earth wire is also added for safety reasons.