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 Since Energy = Power x time Energy of an electric component E = IVt

Electrical Energy & Power You may come across variants of P = IV by substituting R = V/I (definition of resistance) P = I 2 R P = V 2 /R

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.

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

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?

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)

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

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.

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)

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)

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 -1 K -1 ) (ii) State an assumption you made in your calculations

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 = Jkg -1 )

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

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.

ELECTRICITY GENERATION

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

Forms of Electricity Generation (pg 371) Source of ElectricityProsCons Fossil FuelsCheapNot Efficient Bad for Environment NuclearEfficientExpensive Bad for Environment SolarCleanNot Efficient Expensive WindCleanNot Efficient Quite Expensive HydroelectricMost Efficient Clean Quite Expensive

ELECTRIC SAFETY

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

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.

Electrical Mains Should you touch the live wire and your feet are not insulated, current will flow through you to/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.

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

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.

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

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.

Summary of Safety Features Safety FeatureHow it Works Switch on Live WireLive wire is disconnected when appliance is not in use Fuse on Live WireBlows if current exceeds fuse rating, preventing large current from flowing Earth Wire connected to outer metal surface of appliance Prevents humans from being exposed to high voltage should live wire touch casing by accident Double InsulationHumans not exposed to high voltage, even if live wire touches outer casing Circuit Breaker BoxCuts current off should current flow be too large

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.

3 Pin Plug

You are required to memorize which the colour of each wire, and which wire goes where.

Electrical Hazards 1) Damaged Insulation 2) Overheating of cables 3) Damp Conditions

Damaged Insulation

Person may accidentally touch an exposed live wire, giving him electric shock Live wire may touch the neutral wire, causing a short circuit (which in turn may cause fires)

Overheating of Cables Can occur when (i)power cables are too thin or (ii)sockets are overloaded

Overheating of Cables Each appliance draws current when a socket is overloaded, a large total current is drawn from one socket Which may overheat the main cable and thus cause a fire

Damp Conditions Don’t use electrical appliances with wet hands or in damp conditions Electrical resistance of human skin is high when dry (around Ω) When wet, it drops to just a few hundred ohms Should an electric shock occur (due to faulty insulation etc), the current is increased if conditions are damp Water is also a conductor of electricity (but not pure distilled water)

Summary Electrical Power & Energy Sources of Electricity Generation Electrical Safety & 3 Pin Plug 3 Electrical Hazards