Download Power, electrical power and fuses

Power, electrical power and
fuses
Objectives:
• To know that power is the rate of energy transfer
• To know that power is measure in Watts (and kilowatts)
• To know how to calculate electrical power
• To be able to convert to SI units when necessary
• To evaluate the correct fuse for an appliance
In this mixture of light bulbs, which do you think
would be the brightest?
Power and energy
Power is a measure of the rate of energy transfer. It is
calculated using the formula:
power =
energy
time
 Power is measured in watts (W).
 Energy is measured in joules (J).
 Time is measured in seconds (s).
The higher the power rating of a device, the more energy is
transferred every second. This is why a 100 W light bulb is
brighter than a 40 W light bulb: more electrical energy is
transferred into light energy every second.
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1 Watt = 1Joule/second
1 Kilowatt = 1000 Joules/ second
A 100Watt light bulb is transferring 100 Joules
of energy every second
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1. Calculate the power of a light bulb that uses 36J
of electrical energy in 6 seconds.
Write down the formula
Substitute the numbers for the letters-careful about units!
Solve the equation
Don’t forget units in your answer!
Calculate the power of a girl who performs 3000J of work in 6 seconds.
Calculate the power of a car that uses 120kJ of chemical energy
in 5 seconds.
Calculate the power of a person who does 24000J of work in 2 minutes.
Calculate the power of a clock that uses 14.4kJ of
electrical energy in 2 hours.
Calculate the energy used by a light bulb of power 40W in 2 minutes.
Which appliance costs more to run?
Electrical power
Electrical power is the rate at which an appliance uses
electrical energy. All appliances have a power rating.
Power is measured in watts (W): 1 watt means 1 joule of
energy is used every second. 1000 watts = 1 kilowatt (kW).
Appliances that need to
create heat, such as
washing machines, cookers,
hairdryers and kettles,
usually use the most power.
TVs, radios and computers
usually use the least
amount of power.
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What is the formula for electrical power?
In electrical devices, power can be
calculated using the formula:
power = current × voltage
P = IV
What are the units of power,
current and voltage?
 Power is measured in watts (W).
 Current is measured in amps (A).
 Voltage is measured in volts (V).
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Using a formula triangle
A formula triangle helps you to rearrange a formula.
The formula triangle for P = IV is shown below.
Cover up whatever quantity you are trying to find to leave
the calculation required.
So to find current (I),
cover up I…


…which gives
the formula…
I =
×
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P
V
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What two measurements do we need to take in order
to find out the power of a motor?
The voltage across the motor
The current through the motor
You have 10 minutes to set up an appropriate circuit,
take the correct readings and calculate the power of
your motor
Electrical power calculations
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Calculating the correct value for a fuse
What is a fuse?
A fuse is a safety device
containing a thin wire that
melts if the current
is too high, breaking the
circuit and protecting the
cable from overheating
and catching fire.
fuse
Fuses act as an early warning
system, preventing appliances from
being damaged by surges in electricity
and warning owners of faults.
All UK plugs are fitted with fuses. Fuses are available in a
range of amperes to suit different current values.
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Fuses come in different ratings. The more usual ones are
3A, 5A and 13A.
The fuse melts when the current gets too high.
Using the mains voltage as 230V, work out the current
flowing and the fuse required to protect it
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Appliance
Power rating
Microwave
1200W
Kettle
2.5kW
Lamp
100W
Current
flowing
5.2A
11A
0.4A
fuse
13A
13A
3A
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