Using electricity!

Chapter 19

Quick Bites

• Do you know who discovered electricity?– Thomas Edison– Benjamin Franklin– President Bush– Arnold Schwazenagger

http://www.pbs.org/benfranklin/exp_shocking.html

Importance of electricity?

• Power your house!• Power your handphones• Power TV, computers!• Try identify some appliances in the classroom

that uses electricity.

What is power?

• What is meant by more power?• What are the words that comes to your mind

when you talk about power?• Let us play a power game!

Arm wrestling!

Power

• Power = Energy/Time

P =Et

Power units is Watts (W)Energy is Joules (J)Time is Seconds (s)

P = Et

•What is meant by 1W?

P = Et

1w = Et

1W

•1W is when 1 J of energy is released in 1 second

What is 1kW?

1. 1 kW is also written as 1 kilo Watts

2. What does the word kilo means?

3. What does 50 kilo means to you?

4. 1kW = 1000W5. 1Kg = 1000g

How to increase power? Which value should I decrease or increase?

P =Et

To lower the power?

P =Et

Calculate the power for the following

• A hair dryer uses 8000 J of energy in 8s, what power is it operating at?

Calculate

• A radio has a power of 1500W, how much energy is used in 10s?

Electric Safety

• What you have seen is a person dying from electrocution!

• Under what situations do you think you will get electrocuted?

Household wires

Electric shocks

Electrical Hazards

• Damaged Insulation

Insulating rubber covering the wires will deteriorate with time and use. When user touches the exposed wires, they can get a severe electric shock.

Overheating of cables

The large amount of heat produced, due to overloading or short circuit, can melt the insulation and start a fire.

• Damp conditionsWater or wet conditions provides a conducting path for large current to flow. The dry human skin has a resistance of 100,000 but when wet it is reduced to only a few hundred ohms.

Electrical Hazards

Human electrical resistance

• The electric current passing through the human body is usually determined by the skin resistance

• Ranges from about 1000 ohms for wet skin to about 500,000 ohms for dry skin.

• The internal resistance of the body is however, very small, being between 100 and 500 ohms.

Electrocution

• A human body has resistance ranging 300-500 Ω.

• When exposed to a large voltage (>50V), current flowing through can kill.

• But essentially it is not the voltage that kills but the current that passes through our body.

• As long as our hands are dry, the current passing through our body will be very small.

Effect of Current on Human

Current Effects

1 mA (0.001A) Start to feel electric shock

5 mA (0.005A) Harmless shock

10-20 mA (0.01 – 0.02 A)

Uncontrolled muscular contractions

50 mA (0.05 A) Pain felt; Breathing difficulties

100 – 300 mA (0.1 – 0.3 A)

Fatal

Quick Check .

• Playful Dan placed his finders into the live electrical outlet of his home. If the voltage is 230 V, and if Dan’s resistance is 460,000 Ω, find the current flowing through him. Will Dan play again?

I = V/R

= 230/460,000

= 0.0005 A = 0.5 mA

Dan will experience an electric shock only.

Quick Check .• Silly Sal came out from the pool and without drying

herself, stepped onto a live electrical wire of 230 V. If given that her resistance is greatly reduced by the presence of water to only 2000Ω, what is the current flowing through her? Will Silly Sal be silly again?

I = V/R

= 230 / 2000

= 0.115 A = 115 mA

Silly Sal will be electrocuted. Sadly she will not be able to act silly again.

Knowing your wires and plugs

• What is inside a plug? Do you know?• What is found inside a wire?

Mains Electricity

• Refers to the circuits in appliances.• There are usually three wires to be connected

to an appliance from the main socket.

(i) Live wire(ii) Neutral wire(iii) Earth wire

Wiring in a Plug

Neutral wire (blue)

Earth wire (yellow and green)

Live wire (red / brown)

Fuse

Cord gripCord

Wiring in a Plug

• Live wire – the “supply” wire Electricity flows from this wire to the appliance

• Neutral wire Electricity from the appliance back into the socket through this wire.

• Earth wireSafety wire that removes excess charges that may build up on the surface of the appliance

How they are connected in an appliance• Appliances that makes use of

high current to operate at high voltage, needs to be earthed.

• Purpose is to reduce risk of electrocution

• Wear and tear may cause wire to come loose, causing electric current to flow through metal casings of the appliance

Earth wire

• An earth wire connects an appliance directly to the earth.

• When fault occur, a large current flows into the earth through the earth wire instead of passing through a person.

• The large current 'blows' the fuse in the circuit and cuts off the electricity supply

Fuse

• A piece of wire that acts as a safety device• Controls amount of current in the circuit• Melts when current gets too high thus

disconnecting the circuit• Must be connected in the live wire.• Fuse ratings are written as the maximum

amount of current they allow to pass before they melt. E.g. “3A”

Investigate your home

• Pick 3 appliances from your home• Check out how much power they are

consuming!