[PHY] Chapter 19 - Practical Electricity

DARRELL ER (COPYRIGHTED) © TOPIC 19: PRACTICAL ELECTRICITY DARRELL ER (COPYRIGHTED) ©

CHAPTER ANALYSIS THE ABOUT TIME EXAM WEIGHTAGE DARRELL ER (COPYRIGHTED) ©

HEATING EFFECT OF ELECTRICITY APPLICATIONS KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

HEATING EFFECT When current flows through a circuit, it may create a heating effect due to the resistance of the components. Heating element are usually made of nichrome wire coiled around an insulating fireproof material. Nichrome is the most commonly used due to: - high resistance, therefore heats up quickly - high melting point - does not oxidise or rust easily APPLICATION Electric Kettle The heating element is enclosed in a metal tube and when a current flows through the heating element, the water around the element is heated by conduction and thermal energy is then transferred throughout the fluid by convection. Electric Iron Thermal energy produced is spread evenly over a large metal base which conducts thermal energy well. Electric irons usually contain a thermostat which switches the current off when it gets too hot and switches it back on when the iron cools below selected temperature. HEATING EFFECT OF ELECTRICITY Nichrome is commonly used in ovens as well! DARRELL ER (COPYRIGHTED) ©

ENERGY SOURCES ELECTRICAL POWER ELECTRICITY CONSUMPTION KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

ENERGY SOURCES Renewable Energy Non-Renewable Energy Pros Cons Pros Cons Solar Energy Harnessing free energy. Does not produce greenhouse gas. Solar panels are still rather expensive and inefficient. Wind Energy Harnessing free energy. Does not produce greenhouse gas. Wind farms take up a lot of land and is inefficient. Hydroelectric Energy Harnessing free energy. Does not produce greenhouse gas. Affects aquatic life. May cause flooding downstream. Fossil Fuels Cheaper. Fossil fuels produce large amount of greenhouse gas. Air pollution due to ash. Nuclear Energy Does not produce greenhouse gas. Nuclear waste is hard to store. SUMMARY TABLE DARRELL ER (COPYRIGHTED) ©

ELECTRICAL POWER Electrical power is defined as the rate at which electrical energy is converted to other forms of energy due to the circuit components. Units: Watts (W) P = E / t Where, P = Power E = Energy converted t = time 2R P = V2 / R ELECTRICAL POWER When we buy light bulbs, there’s usually a rating like ‘230V, 40W’. This means that the light bulb has a potential difference of 230V. Each charge is required to ‘deposit’ 230 J of electrical energy to go across the component. The 40W means that for the light bulb to produce light, 40 J of electrical energy is used per second. If we apply P = VI, we can then derive that for the light bulb to operate, it needs to draw a current of 0.17A. (Since each charge brings 230J while only 40J is consumed each second, only 0.17 charge needs to flow per second.) (Recall rate of flow of charges is known as current.) DARRELL ER (COPYRIGHTED) ©

ELECTRICITY CONSUMPTION Joule or even kilo-Joules are too small units to measure electricity consumption. Hence the electricity consumption is measured in terms of kilowatt-hour (kWh). Energy used = Power x Time 1kWh = 1kW x 1 hour = 1000W x 3600s = 3 600 000 J = 3.6 MJ So technically, 1kWh is 3.6 MJ. ELECTRICITY CONSUMPTION How much will the cost be if you uses a 3500W air-con for 30 days straight every night for 8 hours? The cost is 25 cents per kWh. Total electricity consumed = 3.5kW x 8 x 30 = 840kWh Total cost = 840 x 0.25 = $210 (You are an expensive kid. RIP electricity bills) (1) Energy Efficiency Rating (2) Ticks (3) Annual Energy Cost (4) Annual Energy Consumption (kWh) (5) Brand name (6) Model (7) Type (8) Size (9) Test Standards (10) Disclaimer (11) Registration Number DARRELL ER (COPYRIGHTED) ©

DANGERS OF ELECTRICITY SAFE USE OF ELECTRICITY EARTH WIRE KEY CONCEPT DARRELL ER (COPYRIGHTED) ©

DANGERS OF ELECTRICITY SUMMARY TABLE Dangers Effects Damaged insulation ○ due to wear and tear ○ metal casing may become live ○ exposed live wire can cause an electric shock if someone touches it ○ may result in short circuit between the live and neutral wires, causing a surge in current and a fire Overheating of cables ○ thinner wires may be cheaper but have a higher resistance, producing more heat → can catch fire if current flow is high (i.e. when used for appliances which require high power) ○ overloading outlets (i.e. usage of extension cords and adaptors) increases current drawn from the mains, heating up the wires, causing a fire Damp conditions ○ minerals in skin dissolve in water, becoming an electrical conductor ○ water lowers resistance of human skin, causing electrocution ○ dry skin has a resistance of 100 000Ω or more, while wet skin can have a resistance of as low as a few hundred ohms DARRELL ER (COPYRIGHTED) ©