(O Level Phy) Practical Electricity Teaching & Learning Notes

1. Power stations supply electricity for households, factories and office buildings. Fuel power stations burn fossil fuels (gas, oil, coal) while nuclear power stations make use of nuclear fission of uranium-235 to produce electrical energy. Fossil fuels and uranium-235 are non-renewable because they cannot be replaced. On the other hand, the hydroelectric, solar and wind power stations make use of renewable sources of energy. Hydroelectric power stations, for example, change kinetic energy of running water into electrical energy. Different solar power stations use different ways of changing solar energy to electrical energy. Wind power stations change kinetic energy of wind to electrical energy. One disadvantage of fuel power stations is that they produce air pollutants. Power stations that use renewable sources of energy do not produce air pollutants. 

2. Different power stations have different energy conversion efficiencies. Power plant efficiency is the ratio between the rate of useful electricity output and the rate of energy supplied by the source. For example, a hydroelectric power station may have an efficiency of 95% while an oil power station may have an efficiency of 45%. Why are their efficiencies so different? The hydroelectric power station uses running water to spin its turbine. The oil power station, on the other hand, uses steam to spin its turbine. Much energy has been lost during energy conversions from the oil to the steam and to the turbine. This makes the oil power station less efficient than the hydroelectric power station. Which power station, the hydroelectric station or the oil power station, do you think, would charge more per unit kWh of energy?

3. Electricity can be converted to other forms of energy by using different electrical components. These electrical components make use of different effects of electricity. A heating element uses the heating effect of electricity to convert electrical energy to heat. Heating elements can be found in electric kettles, electric irons and water heaters. A filament lamp uses the heating and lighting effects of electricity to convert electrical energy to light and heat. An electric motor uses the magnetic effect of electricity to convert electrical energy to mechanical energy. What electrical appliances make use of motors? 

4. The power of an electrical component is the rate of conversion of electrical energy into another form of energy. Its SI unit is the Watt which is equivalent to J/s. For example, an electric lamp that is rated as “60 W” converts 60 J of electrical energy to heat and light in 1 s. 

5. Using the definition of potential difference (V = W/Q), we can show that power (P = E/t) is the product of current and voltage i.e. P = IV. We can further derive P = I²R and P = V²/R by inserting the Ohm's Law relationship. Let's apply these formulas in this question. If a small heater operates at 12 V, 2 A, how much energy will the heater use if it is run for 5 minutes?

6. The SI unit of energy is the Joule. The commercial unit of energy, however, is the kilowatt hour (kWh). The kilowatt is the unit of power while the hour is the unit of time. Since energy = power x time, the kWh is a unit of energy. Knowing that 1 kW = 1000 J/s and 1 h = 3600 s, we can easily convert between J and kWh. Now, let's suppose you have an electric cooker that has a hotplate rated at 1500 W and an electric oven rated at 2000 W. During a day, the hotplate is switched on for a total of 1.0 h and the oven is switched on for 3.0 h hours. What is the cost of using the electric cooker during one day if the cost of electricity is 24 cents per kWh?

7. Electrical hazards include electric shocks and electric fires. Electric shocks are caused by damaged insulation and damp or wet conditions. Short circuits between the live and neutral wire occurs when there is damaged insulation. If you happen to touch the metal casing of an appliance that is “live” due to damaged insulation, you would get an electric shock. Damp conditions make it worst because water lowers the electrical resistance of our body from 500 000 Ω to 1000 Ω, making our body a better conductor of electricity. Damaged insulation can also cause electric fires. The large current that results from short circuits produce enough heat to start a fire. Another cause of electric fires is overloading. A power outlet that is overloaded tends to overheat because it draws in current greater than what the electrical wires can safely carry. Note that thinner wires produce heat faster (P = I²R) than thicker wires because thinner wires have greater resistances (R  = ρl/A).

8. The mains three-pin plug contains the brown Live wire, the yellow-green Earth wire and the blue Neutral wire. It also has a fuse that is connected to the Live wire which is at a high voltage, typically at 240 V. The neutral wire which is at 0 V completes the circuit and enables current to flow through the appliance. The Earth wire is the safety wire that conducts current to the earth in case the metal casing becomes “live”. While the Live and Neutral wire should never be in contact with the metal casing of appliances, the Earth wire is always connected to the metal casing. If the appliance has a double insulation, however, there would not be a need for earthing. That is why you see electric irons have a three-pin plug while the hair dryer which has a plastic casing has a two-pin plug.

9. Fuses and circuit breakers are safety devices used to protect appliances from damage caused by excessively large currents. Inside the fuse, there is a short wire that heats up and melts if current exceeds its fuse rating. Once the fuse melts, it has to be replaced. We should choose a fuse rating which is slightly larger than the current that normally flows through the appliance. Common fuse ratings include 3 A, 5 A and 13 A. Let's suppose you have an appliance that uses 3 A. What is the most suitable fuse rating and in which wire should the fuse be connected?

10. One common question is “How do the fuse and Earth wire work together as a safety precaution to prevent electric shocks when using electrical appliances?” If the metal casing becomes “live” due to damaged insulation, the low resistance Earth wire draws a very large current from the Live wire causing the fuse to melt and the circuit to be switched off.

11. Unlike the fuse, the circuit breaker, on the other hand, can just be reset and so does not need to be replaced. Inside the circuit breaker, there is an iron lever next to an electromagnet. The electromagnet is a coil of insulated wire that is wound around a soft iron core. The core magnetizes when a current flows through the coil of wire. If the current is large enough, the electromagnet will be strong enough to attract the iron lever. As the iron lever is pulled away from a contact, the circuit is broken. Note that fuses and circuit breakers are wired into the Live conductor that is at a high voltage. Now, design your own circuit breaker. You should include an  electromagnet, an iron lever that can rotate about a pivot and that makes or breaks a contact, and a spring that is attached to the lever.

12. The household electric circuit consists of the lighting circuit and a ring socket circuit that are both parallel circuits and are both connected to the electricity meter and consumer unit. The electricity meter records the electrical energy used in kWh. The consumer unit contains the main switch and some fuses and circuit breakers. The current in the lighting circuit is much lower than the current in the ring circuit. That is why the lighting circuit uses a lower fuse rating (5 A fuse) than the ring circuit (30 A fuse). That is also why thicker wires (2.5 mm²) are used in ring circuits than in lighting circuits (1.5 mm²). In the ring circuit, the Live wire, Neutral wire and Earth wire goes round the household in the loop without connection to one another. The Live and Neutral wires are connected only when an appliance is plugged to a socket and switched on. Now, draw a table to compare between the lighting circuit and the ring socket circuit.