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 = I2R• P = V2/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-1K-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 = 2 200 000 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 Electricity Pros Cons

Fossil Fuels Cheap Not EfficientBad for Environment

Nuclear Efficient ExpensiveBad for Environment

Solar Clean Not EfficientExpensive

Wind Clean Not EfficientQuite Expensive

Hydroelectric Most EfficientClean

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 FeaturesSafety Feature How it WorksSwitch on Live Wire Live wire is disconnected when appliance

is not in useFuse on Live Wire Blows if current exceeds fuse rating,

preventing large current from flowingEarth Wire connected to outer metal surface of appliance

Prevents humans from being exposed to high voltage should live wire touch casing by accident

Double Insulation Humans not exposed to high voltage, even if live wire touches outer casing

Circuit Breaker Box Cuts 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

3 Pin Plug

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

Summary

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