Ministry of Education Secondary Engagement Programme Grade 11 Physics WEEK 6 Lesson 2 Topic: Electricity Sub-topic: Electricity in Home Objective: Given information and with the aid of diagrams, students will: i. discuss the reasons for using parallel connections of domestic appliances; ii. explain the purpose of a fuse or circuit breaker and the earth wire; iii. select a fuse or circuit breaker of suitable current rating for a given appliance; iv. state the adverse effects of connecting electrical appliances to an incorrect or fluctuating voltage supply. Content: Distributing electricity Electrical energy is most efficiently generated and distributed using alternating current. Most Caribbean countries, and also the USA, utilize an a.c. frequency of 60 Hz. A few territories use a frequency of 50 Hz. Minimizing Power Loss When current travels along a wire, the wire becomes hot and heat energy is lost to the atmosphere. Power is lost according to the formula P = I2 R where P is the power lost, I is the current in the wire and R is the resistance of the wire To minimize power loss, two approaches are used in distribution systems. i. Keep the resistance (R) of the conducting wires as small as possible. The transmission wires, therefore, have larger diameters, since thick wires have less resistance per unit length than thin wires. ii. A second approach is to use small currents in the wires. To achieve this, the power is sent at very high voltages. Household electric circuits Parallel wiring 68 Ministry of Education Secondary Engagement Programme Grade 11 Physics Parallel wiring is used in household circuits rather than series connections so that appliances can be controlled individually without affecting other devices. If anyone switches in a series circuit are open, there will be no current in the circuit. Further, if anyone of the devices in a series circuit ‘blows’, and creates an open circuit, again, there will be no current and the other appliances in the circuit will not work. For this reason, series connections are only used in special circuits, for example, fairy lights, where lamps are connected in series so that one switch can turn off and on an entire set of lamps. Electrical wiring in a house Live (L), earth (E) and neutral (N) wires Two wires enter the house from the electricity company’s supply. One wire is earthed at the local substation and is called the neutral. Because this wire is connected to the earth, its voltage is the Earth’s voltage, that is, zero. The other wire is called the live wire. The voltage on this wire is alternately positive and negative. Colour-coding and plugs The plastic insulation on electrical wires used in household wiring or attached to plugs or equipment is often colour-coded. In the international system, the live wire is coloured brown, the neutral is blue, and the earth is yellow and green Earthing Outlets usually carry a third wire, the earth wire, whose insulation is coloured yellow/green. This wire is connected to earth outside the home, usually utilizing a metal pole driven into the ground. The earth wire is connected electrically to the casing (called the ‘chassis’) of electrical devices such as refrigerators, cookers, or computers. Fuses and circuit-breakers These devices in electrical circuits protect against short circuits and overloads. 69 Ministry of Education Secondary Engagement Programme Grade 11 Physics Fuses are safety devices that disconnect an appliance if a fault in the appliance causes the current to become too large. It contains a very thin piece of wire that melts or “blows” when too much current flows through it, thereby breaking the circuit. It would then have to be replaced. Colour-coding of wires attached to 3-pin plugs Circuit breakers – These devices disconnect the supply when a rated current is exceeded. They can be reset when a fault has been investigated and repaired. Unlike fuses, they do not have to be replaced. Electrical outlets are usually wired in a ring circuit, which allows twice the power (compared with a simple parallel arrangement) to be delivered safely using the same size of wire. Fuses, circuit-breakers and switches are placed in the live line in series with loads. Parallel, ring and high-power circuits Electricity comes into a home through one mains cable. This Passes through a mains fuse, then an electricity meter, then into a distribution box, then an electricity meter and finally a distribution unit. Two circuit systems are generally used: a simple parallel circuit a ring parallel circuit. Lamps (which use small currents) are usually wired in a simple parallel arrangement. Electrical outlets are also connected in parallel but they are usually wired on the ring circuit. These outlets are suitable for devices that use moderate power. Appliances with a very high-power requirement, such as stoves, are usually connected in parallel directly to the main power distribution box. Lighting circuits Lights are wired in simple parallel arrangements. Switches are placed on the live lines (L) leading to the lights. When a switch is turned off, there will be no live connection to a lamp, so the lamp can be safely handled. 70 Ministry of Education Secondary Engagement Programme Grade 11 Physics Ring circuits Electrical outlets are usually wired on a ring circuit. The ring circuit is also a parallel circuit but, unlike the lighting circuit, it allows current to flow to an outlet via two routes. Thus, each live wire in the ring carries only half the current required by the devices. The ring circuit can therefore safely have more outlets than would be possible with the simple parallel arrangement used for lighting circuits. Notice the ring main consists of several sockets which appliances can be plugged in, and the lighting circuit consists of the distribution of lamps in a room or around the home. Physical layout of electrical wiring in a house showing lighting, ring and high-power circuits. Inside each of the orange and green cables are the live, earth and neutral wires. Inside the blue cable, only live and neutral wires are enclosed. Paying Electricity Bill We pay for electrical energy converted by appliances in our homes. We calculate the energy expended using the formula energy = power × time Most power companies measure power in kilowatts (kW) and time of usage in hours (h). Hence power companies measure energy in kW × h or kWh (kilowatt-hours). 71 Ministry of Education Secondary Engagement Programme Grade 11 Physics A kilowatt-hour is called the ‘unit’ of electricity used. One kilowatt-hour of energy is used when a 1 kW device converts energy for 1 hour. For example, a 3000 W stove, operating for 4 hours converts: 3 kW × 4 h, or 12 units (12 kWh), of electrical energy. Cost of electricity 1. A company’s fixed rate is 17 ¢ per unit, and the fuel adjustment rate is 2.3 ¢ per unit. A consumer uses a 200 W floodlight for 36 hours (e.g. 4 hours every night for nine nights). How much would the energy used cost the consumer? Solution The energy used = 200 W × 36 h = 7200 Wh = 7.2 kWh The subtotal for the fixed rate = 7.2 × 17¢ = $1.22 The fuel adjustment charge = 7.2 × 2.3¢ = 17¢ The total charge = $1.22 + $0.17 = $1.39. 2. 72 Ministry of Education Secondary Engagement Programme Grade 11 Physics The electricity bill shown above states that 270 kilowatt-hours (kW h) of energy have been used. (usage column) The fuel charge is based on the number of kilowatt-hours used. The customer has been charged as follows: Additional charges, including a further charge of $26.54 per unit, increase the total bill to $10 877.50. Task: Discuss ways in which energy can be conserved in the home. References Avison, J., & Petheram, L. (2014). Physics for CSEC (2nd ed.). Cheltenham: Nelson Thornes Ltd. DeFreitas, P. (2015). Concise Revision Course CSEC Physics. London: Collins. Duncan, T. (2013). Physics for CSEC examination (5th ed.). London: Hodder Education. 73 Ministry of Education Secondary Engagement Programme Grade 11 Physics Farley, A., & Trotz, C. (2014). Physics for CSEC Examinations (3rd ed.). (M. Taylor, Ed.) London: Macmillan. Lambert, N., & Santos, N. L. (2000). Physics for CXC (2nd ed.). Essex: Heinemann. 74 .