Chapter 5
Remember from Grade 9……
Work (joules) = Force (newtons) x Distance (metres)
Energy is the ability to do work.
(also measured in joules)
Challenge:
A student whose weight is 500 N walks up some
stairs (each step is 0.2 m)
a) How much work is done when the student goes
up 1 step?
W = 500 N x 0.2 m = 100 J
b) How much work is done when the student goes
up 10 steps? W = 500 N x 2 m = 1000 J
c) How much potential energy is gained by the
student when they go up the 10 steps?
Energy = Work = 1000 J
Power is the rate at which energy is produced,
absorbed, or transferred
Power (watts) = Energy (joules)
Time (seconds)
These same principles apply to electricity as well.
A battery applies a force to electrons to make them move.
Because a force is applied over a distance, work is done on
the electrons
The more electrons that are moved the more work is done
The more quickly the electrons are moved the more power
is used.
Voltage of a battery indicates how much work is done per
Coulomb of charge transferred.
Voltage (volts) =
Energy (joules)
Charge transferred (coulombs)
We know:
Voltage of a battery indicates how much work is done per
Coulomb of charge transferred.
Voltage (volts) =
Energy (joules)
Charge transferred (coulombs)
Energy = Voltage x Charge Transferred
We also know:
Current (amps) = Charge transferred (coulombs)
Time (seconds)
Charge Transferred = Current x Time
Therefore:
Energy = Voltage x Current
Charge Transferred
x Time
Energy (J) = Voltage (V) x Current (A) x Time (s)
Challenge:
A drill has a voltage of 120 V and draws a current of 13 A.
If it is used for 10 sec. How much energy will be used?
E =
V
x
I
x
t
E = 120V x 13A x 10s
= 15600 J
If the drill is used for 1 min, how much energy will be used?
E = 120V x 13A x 60s
= 93600 J
To calculate the power of an appliance you do not need to
know how long it is used:
Power = Energy = Voltage x Current x Time
Time
Time
Therefore:
Power (W) = Voltage (V) x Current (A)
For the drill used previously:
Power (W) = 120V x 13A = 1560 W
The drill uses power at this rate the entire time it is run.
Therefore the energy it uses can be calculated using:
Energy = Power x Time
When measuring electrical energy in your home, a much
larger unit than the Joule is required.
We use the kilowatt hour (kW·h)
1 kW·h = 3 600 000 J
Electric Companies use electric meters to measure and record
the energy used in your home.
Each meter contains an electric motor that turns as long as
power is being used.
How to read an electric meter.
If it is digital, you read it just like a digital watch
This one reads: 46372 kW·h
If it is a dial meter, it is more complicated
choose the lower number that the dial is between
6
6
6
4
9
Classwork:
– Page 95
 “Review
5.1” Q. # 1, 3 – 6
Homework:
– Find the electric meter in your home
and take two readings one day apart
– Finish Household Electricity Worksheet
(to be handed in for marks)