Topic Seven:
P1.3.1 Transferring electrical energy
Electrical Devices
Electrical energy is very versatile. It can be transformed into most other kinds of energy:
Light
bulb
Electric heater
Electric motor
Hair dryer
useful energy
wasted energy
Light from glowing filament
Thermal energy supplied to the surroundings
Thermal energy supplied to the surroundings
light from the glowing heating element
Kinetic energy supplied to objects
Thermal and sound energy supplied to the surroundings
Sound energy released. Kinetic and thermal Thermal energy in the energy given to the air
dryer itself
Energy (in joules, J)
is the ability to cause change:
Energy and power
Work (also in joules, J)
is how much energy
changes form.
eg, chemical potential energy
each transformation is work
kinetic energy
sound energy
thermal energy
Power (in watts, W)
is the rate of doing work
OR
Power is how much energy
changes form per second.
Calculating power
If power (in W) is the amount of energy transformed per second...
...then power =
energy
time
If power (in watts, W) = energy (in joules, J)
time (in seconds, s)
...then 1W = 1J/s
Measuring power
lava
lamp
Power = energy
time
= 4,000J
100s
= 40W
This mains joulemeter
measures electrical energy flowing through three­pin mains sockets
It can be set for:
one flash = 100J
OR, one flash = 1000J
Power = energy
time
= 120,000J
100s
vacuum
cleaner
= 1200W
Measuring human power
• run up the stairs one at a time, timing how long it takes
Energy used
= the gain in gravitational potential energy
= weight (in newtons, N) x gain in height (m)
40
500
= ______N X ______ m
20,000
= ______J
then, Power = energy = 20,000J = W
500
40s
time
Time taken to run up the stairs
running
walking
110W
166W
145W
139W
183W
139W
149W
393W
407W
428W
507W
322W
600W
528W
600w
A power calculation
(1) Which is more powerful ­ a 3kW kettle or a 10,000W cooker?
10kW
3000W
The 10,000W/10kW cooker is more powerful
(2)There are 20 million homes in Britain.
If a 3kW kettle is switched on in 1 in 10 homes, how much power would need to be supplied? 3kW = 3000W
20million/10 = 2million, so power = 2million x 3000W = 6billion W
(3) A machine has a power rating of 100kW*
If the machine runs for 2 minutes*, how much
energy does it transfer? (higher tier)
Power = energy
10kW = 100,000W, and 2 minutes = 2x60s = 120s
time
Energy = Power x Time
= 100,000W x 120s
*traps for the unwary
= 12,000,000W = 12MW
­ be careful
Paying for electrical energy
The Electricity Board measures the energy it transfers to your house in "kilowatt­hours" (kWh).
Energy transferred = Power x time
(in kWh) (in kilowatts, kW) (in hours, h)
The proper unit of energy is joules!!!!!!
The proper unit
of power
is watts!!!!!!
The proper unit
of time
is seconds!!!!!!
The Electricity Board can do this
­ it has the power!!
• attempt to convert one kWh into joules.
One kilowatt is 1000W
One watt = 1J/s
One kW = 1,000W = 1000J in one second
One hour = 60 minutes x 60s = 3,600s
So, number of joules in one hour = 1,000J x 3,600s
= 3,600,000J
= 3.6MJ
• (extension: suggest a reason why the electricity board uses kWh/units instead of joules)
3,600,000J is cumbersome and intimidating.
1kWh also known as "one unit" is much more accessible.
Energy transferred = Power x time
(in kWh) (in kilowatts, kW) (in hours, h)
How many kWh does a 1kW heater use in 2 hours?
1kW x 2h = 2kWh
How many kWh does a 120kW digger use in 30 minutes?
30 minutes = 0.5 hours
so, 120kW x 0.5h = 60kWh
How many kWh does a 100W bulb use in 1 day?
100W = 0.1kW, and 1 day = 24 hours
so, 0.1kW x 24h = 2.4kWh
The electricity board calls each kWh a "Unit"
1kWh = 1 "Unit"
To get the cost we multiply the number of units by the cost per unit.
eg, if one unit costs 3p, how much would 50kWh cost?
50kWh = 50 units
50 units x 3p = 150p = £1.50
old style spinney disk electricity meter
digital electricity meter
A 4000W heater is left on for 3 hours.
(1) How many kilowatt­hours of energy does it use?
4,000W = 4kW
4kW x 3h = 12kWh
(2) How many units of electricity does it use?
12kWh = 12 units
(3) How much does it cost to run if one unit costs 3p
12 units x 3p = 36p
Energy (kWh) = Power (kW) x time (h)
Energy transferred = Power x time
(in kWh) (in kilowatts, kW) (in hours, h)
One unit = 13p
1) Calculate how 70W = 0.07kW
much it costs to run refrigerator
the fridge for one average power consumption = 70W
year
Cost = 0.07kW x (365days x 24hours) x 13p = £79.72
2) Calculate how much it costs to run the freezer for one year
Power is 10x greater,
so cost is 10x greater = £797.20
freezer
average power consumption = 700W