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Practical Electricity

Practical Electricity. Topics. Electrical Energy & Power Electricity Generation Electrical Safety 3 Pin Plug. Electric Power & Energy. Recall. Power = Energy / time Units of Power is Watts Units of Energy is Joules. Electrical Energy & Power. Power of an electric component: P = IV

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Practical Electricity

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  1. Practical Electricity

  2. Topics • Electrical Energy & Power • Electricity Generation • Electrical Safety • 3 Pin Plug

  3. Electric Power & Energy

  4. Recall • Power = Energy / time • Units of Power is Watts • Units of Energy is Joules

  5. Electrical Energy & Power • Power of an electric component: • P = IV • Since Energy = Power x time • Energy of an electric component • E = IVt

  6. Electrical Energy & Power • You may come across variants of P = IV by substituting R = V/I (definition of resistance) • P = I2R • P = V2/R

  7. Worked Example 1 • A light bulb has a current of 0.1 A and a p.d. of 1.5 V. • (i) Determine the Power of the light bulb. • (ii) Determine the energy consumed by the bulb if it was left on for one minute.

  8. Power Rating • On some electrical appliances there is a power rating: e.g. 230 V, 950 W [take note of the units] • What does this power rating mean? • In different countries around the world, there is different mains voltage • Singapore’s main voltage is 230 V. • USA 120 V, China 220 V

  9. Power Rating • An appliance with a power rating 230 V 950 W means that IF it is plugged into a mains of 230 V, THEN it will generate a power of 950 W • You should expect the power to change if it is plugged into the mains of a different voltage • Q: what happens when you take an appliance designed for USA and plug it in Singapore? • Q: what happens when you take an appliance designed for Singapore and plug it in USA?

  10. Power Rating • You encounter power rating questions where the appliance is plugged into a mains voltage which is different from the rating’s • Remember that the resistance of the appliance is always constant (but the p.d. or the power can change)

  11. Worked Example 2 • A lamp has a rating of 60 W, 240 V, and is connected to a 240 V power supply. • (i) Determine current flowing through the lamp • (ii) Determine resistance of the lamp

  12. Worked Example 3 • The same lamp (rating of 60 W, 240 V), and is now connected to a 100 V power supply. • (i) Determine the power generated by the lamp • (ii) Determine the current flowing through the lamp.

  13. Heating Appliances • Some electrical appliances intentionally generate heat (kettle, iron, electric hotplate, electric radiator, etc.) • These appliances work by using a heating element • A heating element is a usually a length of high resistance wire/conductor • One way resistance is increased is to make a coil of long and small filament wire (e.g. light bulb)

  14. Heating Element • No other work is done at the heating element other than to generate heat • Hence, the energy conversion is • Electrical Energy → Thermal Energy • Since Electrical Energy = IVt, and thermal energy is = Q • IVt = Q • And you may be subsequently asked thermal questions (e.g. Q = mcθ, Q = ml)

  15. Worked Example 4 • A kettle has power 2000 W. • (i) Determine how long it takes for it to heat 1 kg of water from 25 °C to 100 °C. (specific heat capacity of water 4200 JKg-1K-1) • (ii) State an assumption you made in your calculations

  16. Worked Example 5 • A heating element of power 1000 W is inserted into a beaker containing 200g of water at 100 °C. Determine how long it takes to completely convert all the water to steam (specific latent heat of vaporization of water = 2 200 000 Jkg-1)

  17. Electricity Consumption • In real life electricity bills, we get charged for the amount of electricity we used in our households • However, the units of energy used is not S.I. Units (Joules) but rather kilo-watt hour (kWh) • 1 kWh = 1000 x (60 x 60) J • Another question you may be asked is to determine electricity costs

  18. Worked Example 6 • The price of electricity is 27 cents per kWh. Determine how much it costs in total to use a 3kW kettle for 20 minutes and a 100 W bulb for 5 hours.

  19. Electricity generation

  20. Forms of Electricity Generation • Fossil Fuels • Nuclear Power • Solar Energy • Wind Energy • Hydroelectric Power

  21. Forms of Electricity Generation (pg 371)

  22. Electric Safety

  23. Electrical Mains • Recall: in order for current to flow through a component, you need TWO connections • A light bulb will not work if only one side is connected to a battery – that’s still an open circuit • Your electrical mains has 3 connections, the live, neutral & Earth

  24. Electrical Mains • 1) Live – this wire is at high potential (“high voltage”). The wire is brown in colour. The Fuse is also attached to the Live Wire • 2) Neutral – this wire is maintained zero potential. The wire is blue in colour. • 3) Earth – this wire is connected to the Earth. It is yellow/green in colour.

  25. Electrical Mains • Should you touch the live wire and your feet are not insulated, current will flow through youto/from the Earth (through your feet), this may result in electric shocks/electrocution • It is safe to touch the Neutral or Earth wires, no current will flow • This is why the fuse is attached to the live wire, should a short circuit happen a large current will flow, and the fuse will blow, disconnecting the live wire.

  26. Electrical Mains • This also explains why the switch is attached to the live wire – so that the live wire is disconnected when the appliance is not in use. • Every household also has a circuit breaker, which is designed to cut the circuit when a large current flows (works using electromagnetic means

  27. Electrical Mains • When an appliance is connected to the mains, it is connected to the live and neutral connections. • If the appliance is has a metal exterior, the metal exterior is connected to the Earth.

  28. Electrical Mains • What happens when the live wire accidentally makes contact with the metal surface of an appliance? • If it there is no fuse & no Earth wire, a human touching the appliance may get electrocuted (current flows through the human to/from the ground) • In reality, a large current will momentarily flow from live wire to Earth, blowing the fuse in the process

  29. Electrical Mains • Some appliances have non-metallic exteriors (e.g. made of plastic). This is called double insulation. These appliances do not need an Earth wire, and they may use only 2 pin plugs.

  30. Summary of Safety Features

  31. 3 Pin Plug • Each pin of the three pin plug is connected to one of the following: • Brown Wire – to live • Blue Wire – to neutral • Yellow/Green Wire – to Earth • The brown wire is also connected to a fuse. This fuse is meant to protect the appliance (not humans) should current flow be too large.

  32. 3 Pin Plug

  33. 3 Pin Plug • You are required to memorize which the colour of each wire, and which wire goes where.

  34. Summary • Electrical Power & Energy • Sources of Electricity Generation • Electrical Safety & 3 Pin Plug

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