Transcript Using Electricity PPT

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 =
E
Pt
t
Power units is Watts (W)
Energy is Joules (J)
Time is Seconds (s)
= E
•What is meant by 1W?
E
P =
t
E
1w =
t
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 = 1000W
5. 1Kg = 1000g
How to increase power? Which value
should I decrease or increase?
E
P =
t
To lower the power?
E
P =
t
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.
Electrical Hazards
• Damp conditions
Water 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.
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 – Uncontrolled muscular contractions
0.02 A)
50 mA (0.05 A)
100 – 300 mA
0.3 A)
Pain felt; Breathing difficulties
(0.1 – 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
Earth wire (yellow and green)
Live wire (red /
brown)
Neutral wire
(blue)
Fuse
Cord
Cord grip
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 wire
Safety 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!