Science notes, chapter 6

Electricity and magnetism

Textbook Science Form 3 (KSSM, Dual Language Programme), pages 160–207

What this chapter covers

Describe renewable and non-renewable energy sources, explain how an induced current is produced and how power stations generate electricity (6.1)
Differentiate direct current from alternating current and use a C.R.O. to show their traces and read voltage (6.1)
Compare step-up and step-down transformers and solve problems with Vp/Vs = Np/Ns (6.2)
Explain the components of the transmission and distribution system, home wiring, plugs and safety components such as fuses, earth wires, MCB and ELCB (6.3)
Define energy efficiency, calculate power, current, electrical energy and its cost in kWh, and suggest ways to save electricity (6.4)

6.1Generation of electricity

Energy sources

Renewable energy sources
Can be replaced continually and will never deplete.
Hydro, wave, solar, tidal, wind, biomass and geothermal energy.

Non-renewable energy sources
Cannot be replaced and will deplete.
Nuclear energy, coal, natural gas and petroleum (diesel).

Malaysia uses both groups. Examples from the textbook: Bakun (hydro, Sarawak), Sultan Azlan Shah, Manjung, Perak (coal), Tuanku Jaafar, Negeri Sembilan (natural gas), Gelugor, Pulau Pinang (diesel) and a biomass station in Sabah.

How a generator makes current

A generator is a device that generates electricity. When the magnetic field lines are cut by a wire or a coil (a solenoid), an induced current is produced. Michael Faraday investigated this in 1831.

The magnetic field lines can be cut in two ways, and both give a deflection on the galvanometer:

The wire or coil moves through the space between the magnetic poles.
The magnet moves past a stationary wire or coil.

Four set-ups with a galvanometer: a wire moved between the poles of a U-shaped magnet, a solenoid moved over a bar magnet, a U-shaped magnet moved over a wire, and a bar magnet moved in a solenoid — Figures 6.3 to 6.6: the pointer deflects whenever the wire, coil or magnet moves so that the field lines are cut.

Textbook Figures 6.3–6.6, p. 164.

Thinking a stationary magnet and coil still make a current. If the magnet and the coil are both at rest, no field lines are cut, so there is no induced current and the galvanometer pointer stays at zero.

A magnet is held still inside a coil connected to a galvanometer. Does the pointer deflect? Explain.

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No. The magnetic field lines are not being cut, so no current is induced. The pointer deflects only while the magnet or the coil is moving.

Power stations

Most power stations drive a turbine that turns a generator. What differs is how the turbine is turned.

Power station	What turns the turbine	Energy changes
Thermal (diesel, natural gas, coal)	Steam from boiling water	Chemical → heat → kinetic → electrical
Nuclear	Steam from a nuclear reaction	Nuclear → heat → kinetic → electrical
Biomass	Steam from burning methane produced by biomass	Chemical → heat → kinetic → electrical
Hydroelectric	Water flowing from a high dam to a lower level	Gravitational potential → kinetic → electrical
Wind	Moving air turns the blades	Kinetic → electrical
Solar	No turbine: solar panels change light directly to electricity	Solar (light) → electrical

Thermal power station: fuel heats the boiler, steam turns the turbine joined to a generator, the steam is cooled in a condenser by seawater, and a transmission tower carries the electricity away — A thermal power station: boiler, turbine, generator and condenser.

Textbook Figure 6.8, p. 167.

Steam, wind and falling water all do the same job: they give the turbine kinetic energy. The generator turns that into electrical energy.

Direct current and alternating current

	Direct current (d.c.)	Alternating current (a.c.)
Direction	Flows in one direction only	Constantly reverses direction
Sources	Solar cells, accumulators, batteries (dry cells)	Generators in power stations, the 240 V mains
Devices	Torchlight, calculator, toy car	Bread toaster, hair dryer, air conditioner
Trace on a C.R.O.	A straight horizontal line (fixed voltage)	A wave that rises and falls above and below zero

Cathode ray oscilloscope (C.R.O.)

A C.R.O. is an electronic device that shows the shape of the graph, the direction of the current and the change in voltage for d.c. and a.c. The voltage is the displacement of the trace multiplied by the Y-gain setting.

Y-gain changes the height of the light spot; Y-shift and X-shift move the spot vertically and horizontally.
Time-base makes the light spot sweep across the screen horizontally.
Intensity controls brightness; focus controls sharpness.

Three C.R.O. screens: a straight line above the centre for a dry cell, a straight line below the centre when the cell is reversed, and a wave for an a.c. supply — Traces with the time-base on. Reversing a d.c. cell moves the line to the other side of zero; a.c. gives a wave.

Worked example: voltage from a trace

A dry cell is connected to the Y-input with the Y-gain at 0.5 V per division. The trace is 3 divisions above the zero line.
Voltage = displacement × Y-gain = 3 × 0.5 = 1.5 V.

The trace for a supply is a straight line 2 divisions below zero with the Y-gain at 1 V per division. What type of current is it and what is its voltage?

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A straight line means direct current. Voltage = 2 × 1 = 2 V. Below zero means the terminals are connected the other way round.

6.2Transformer

A transformer is a device that changes the voltage of an alternating current. It has a laminated soft iron core wound with two insulated coils: the primary coil (input) and the secondary coil (output).

A step-up transformer with few primary turns and many secondary turns, and a step-down transformer with many primary turns and few secondary turns, each with its circuit symbol — Step-up and step-down transformers.

Textbook Table 6.1, p. 178.

	Step-up transformer	Step-down transformer
Voltage	V_s is higher than V_p	V_s is lower than V_p
Turns	N_p is less than N_s	N_p is more than N_s
Example	Before transmission at the power station	Mobile phone charger, ceiling fan regulator

A transformer only works with alternating current. A laminated core (layers of soft iron separated by insulators) reduces the eddy current and makes the transformer more efficient.

The transformer equation

V_p / V_s = N_p / N_s
V_p = primary (input) voltage, V_s = secondary (output) voltage, N_p and N_s = number of turns of the primary and secondary coils.

Worked example (textbook p. 182)

A 40 V bulb is connected to a 240 V supply through a transformer. The primary coil has 120 turns. Find N_s.
240 / 40 = 120 / N_s → N_s = 120 × 40 / 240 = 20 turns. Since V_s is less than V_p this is a step-down transformer.

Worked example: finding the output voltage

N_p = 500, N_s = 2 000 and V_p = 12 V.
V_s = V_p × N_s / N_p = 12 × 2 000 / 500 = 48 V (step-up).

Put the unknown on the left of the line if it helps: V_s = V_p × (N_s / N_p). More secondary turns than primary turns means the voltage goes up.

Using a transformer with a battery (d.c.). A d.c. supply does not give a changing magnetic field in the core, so the secondary coil has no output.

A transformer has 100 turns in the primary coil and 20 turns in the secondary coil. The primary voltage is 10 V. Find the secondary voltage.

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V_s = 10 × 20 / 100 = 2 V. It is a step-down transformer.

6.3Transmission and distribution of electricity

From the power station to your home

Flow chart: power station at 11 or 25 kV, step-up transformer station to 132, 275 or 500 kV, National Grid Network, main substation and switch zone at 33 kV, step-down transformer stations to 11 kV, 415 V and 240 V — Transmission and distribution of electricity.

Based on textbook Figure 6.28, p. 184.

Component	Function
Power station	Generators produce a.c. at 11 kV or 25 kV
Step-up transformer station	Increases the voltage to 132 kV, 275 kV or 500 kV
National Grid Network	Network of transmission cables carrying high voltage a.c. over long distances
Switch zone (at the main substation)	Sends electricity to the branch substation when needed; lets power stations and grids be closed for maintenance without cutting the supply
Step-down transformer station (main and branch substations)	Reduces the voltage in stages for the consumer: 33 kV for heavy industry, 11 kV for light industry, 415 V for offices and 240 V for homes

Strong electromagnetic fields are produced around high voltage cables on the National Grid pylons. A compass needle near the pylons is affected, and residents have raised health concerns.

Wiring in Malaysia

Single-phase wiring: suitable for use up to 10 kW or 50 A, such as rural homes.
Three-phase wiring: more stable and reliable; used in commercial and industrial areas above 10 kW or 50 A.

In a home, the live wire (240 V) and neutral wire (0 V) come from the main cable, through the electric meter (measures the units used) and the consumer unit with the main switch, the ELCB (cuts the circuit when too much current flows through it) and MCBs (separate the final circuits for the lighting, power, heating and air-conditioning).

Plugs and wire colours

A 3-pin plug with the earth wire (yellow and green stripes), neutral wire (blue), live wire (brown) and 13 A fuse labelled — International colour code for wiring a 3-pin plug.

Textbook Figure 6.32, p. 190.

Wire	Colour
Live	Brown
Neutral	Blue
Earth	Green and yellow stripes

A 3-pin plug (kettle, iron) has an earth pin. A 2-pin plug (hair dryer, electric toothbrush) has no earth wire.

Safety components

Component	What it does
Fuse	A short, fine wire that heats up and melts when the current is more than its value, cutting off the supply
Earth wire	Connects the metal casing to Earth; if the casing becomes live, a large current flows to Earth and melts the fuse
MCB (miniature circuit breaker)	Cuts off the circuit when the current is too large; can be reset
ELCB (earth leakage circuit breaker)	Breaks the circuit when too much current flows through it, such as a leakage of current to Earth
Lightning conductor	Carries lightning safely to Earth from a building
Switch	Opens and closes the circuit; fitted in the live wire

A cartridge fuse in a glass casing with metal contact caps, a fuse wire inside and its value printed on it — A cartridge fuse.

Textbook Figure 6.33, p. 191.

The value of a fuse is the maximum current that can flow without melting the fuse wire. Choose a fuse slightly higher than the normal maximum current of the appliance. Common values: 1, 2, 3, 5, 10, 13, 15 and 30 A.

Worked example: choosing a fuse

An electric kettle takes 11.34 A normally.
A 10 A fuse is too low (it would melt every time). A 13 A fuse is slightly higher, so it is the 13 A fuse.

Choosing a fuse much larger than needed, or replacing it with a wire. A fuse that is too large will not melt when the current is dangerous, so the appliance and the wires can overheat.

A short circuit happens when the live and neutral wires touch, so the resistance is very small and the current is very large; the fuse melts. Many appliances plugged into one socket give an overloaded circuit, which is also dangerous.

What is the colour of the wire connected to the pin with the fuse in a 3-pin plug, and why is the fuse put there?

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The live wire (brown). If too much current flows, the fuse melts and cuts off the live supply to the appliance.

6.4Calculating the cost of electricity consumption

Energy efficiency

Energy efficiency is the percentage of energy input that is changed to useful energy output.
Energy efficiency = (useful energy output ÷ energy input) × 100%

Worked example (textbook p. 195)

A filament bulb is supplied with 100 J of electrical energy and gives 8 J as light (the other 92 J is wasted as heat).
Efficiency = 8 / 100 × 100% = 8%.

Lighting device	Maximum energy changed to light	Lasts about
Filament lamp	about 10%	1 000 hours
Energy saving lamp (CFL)	about 50%	8 000 hours
LED lamp	about 90%	20 000 to 50 000 hours

The Energy Commission (EC) puts an energy efficiency label with 1 to 5 stars on appliances. More stars means more energy saving; choose appliances with more stars.

Power, current, voltage and energy

Quantity	Equation	S.I. unit
Electric power, P	P = E ÷ t (energy ÷ time)	watt, W (1 W = 1 J s⁻¹)
Electric current, I	I = Q ÷ t (charge ÷ time)	ampere, A (charge in coulomb, C)
Voltage, V	V = E ÷ Q (energy ÷ charge)	volt, V
Power, voltage and current	P = V × I	W = V × A

Worked example: current in an appliance (textbook p. 199)

An electric jug is rated 2.2 kW, 240 V.
I = P ÷ V = 2 200 ÷ 240 = 9.17 A.

Cost of electrical energy

The kilowatt-hour (kWh) is the energy used at the rate of 1 kW for 1 hour. 1 kWh is one unit of electricity.
Electrical energy (kWh) = power (kW) × time (h)
Cost = number of units × rate per unit

Worked example (textbook p. 200)

A 2 kW kettle takes 10 minutes to boil water. The rate is 21 sen per unit.
Energy = 2 × (10 ÷ 60) = 1/3 kWh. Cost = 1/3 × 21 = 7 sen.

Not changing the units. Power must be in kW and time in hours for kWh: 800 W is 0.8 kW and 30 minutes is 0.5 h.

Ways to save electrical energy

Use appliances with more stars on the energy efficiency label, and LED or CFL lamps instead of filament lamps.
Switch off lights and appliances that are not in use.
Use natural lighting and good ventilation. A green building uses efficient ventilation, natural lighting and solar panels to cut its energy cost.

A 100 W lamp is used for 5 hours. The rate is 30 sen per unit. What is the cost?

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100 W = 0.1 kW. Energy = 0.1 × 5 = 0.5 kWh. Cost = 0.5 × 30 = 15 sen.

Summary

Remember	Key fact
Induced current	Produced when magnetic field lines are cut by a wire or coil, by moving the wire or the magnet
d.c. and a.c.	d.c. flows one way (cells, batteries); a.c. keeps reversing (power station generators)
C.R.O.	Shows the shape, direction and voltage change; voltage = displacement × Y-gain
Transformer	Works with a.c. only; V_p/V_s = N_p/N_s; step-up has N_s more than N_p
Transmission	Power station → step-up station → National Grid → switch zone → step-down stations → consumers
Wires	Live brown, neutral blue, earth green and yellow
Fuse	Value slightly higher than the normal current; melts when current is too large
Power and energy	P = VI; energy (kWh) = power (kW) × time (h); cost = units × rate
Efficiency	Useful output ÷ input × 100%