5.3 - electrical power
TRANSCRIPT
Syllabus points
• Power is the rate at which energy is transformed by a
circuit component; power enables quantitative analysis of
energy transformations in the circuit
This includes applying the relationship
= = W
P V I t
Learning goals
Define:
Power – Watts – Joules/second
Recall that the kilowatt hour (kW h) is an alternative unit for energy and is equal to 3.6 MJ
Be able to use and rearrange the equations for
power 𝑃 =𝑊
𝑡= 𝑉𝐼
Give the SI unit for: power
Power Work = transfer of energy
Power = rate of doing work, or rate of energy transfer/transformation (same as in motion topic)
Units Watts, W
1 W = 1 J/s
𝑃 =𝑊
𝑡
Power Work = transfer of energy
Power = rate of doing work or rate of energy transfer/transformation (same as in motion topic)
Units Watts, W
𝑃 =𝑊
𝑡Power (W)
Work (J)
Time (s)
𝑃 =𝑊
𝑡
𝑃 =𝑞𝑉
𝑡
𝑃 = 𝑉𝐼
We can substitute in
𝑊 = 𝑞𝑉
To get
𝐼 =𝑞
𝑡
So we can replace the q/t with I to get
Remember how
1.
2.
3.
Example 1Revisiting:
A current of 2.0 A flowing through a heater for one hour, converts 1.7 MJ of electric energy into heat energy. Calculate:
a) The power rating of the heater
b) The energy used by the heater in 2 hours
Example 1Revisiting:
A current of 2.0 A flowing through a heater for one hour, converts 1.7 MJ of electric energy into heat energy. Calculate:
a) The power rating of the heater
Example 1Revisiting:
A current of 2.0 A flowing through a heater for one hour, converts 1.7 MJ of electric energy into heat energy. Calculate:
b) The energy used by the heater in 2 hours
From: http://www.originenergy.com.au/4218/How-to-read-your-electricity-bill
This chart is wrong:
Energy is measured in Joules
Power is measured in Watts
However:Energy = Power x Time
So kWh (Kilowatt Hours) can be a unit of Energy, although not technically SI!
If a typical Australian home uses 17 kWh per day, what is the typical POWER drawn at any given moment?
From: http://www.originenergy.com.au/4218/How-to-read-your-electricity-bill
This chart is wrong:
Energy is measured in Joules
Power is measured in Watts
However:Energy = Power x Time
So kWh (Kilowatt Hours) can be a unit of Energy, although not technically SI!
If a typical Australian home uses 17 kWh per day, what is the typical POWER drawn at any given moment?
17kWh / 24h = 708 Watts
Power (con) E.g.
A 50 W light bulb will use 50 J in one second
A 100 W light bulb will use 100 J in one second
In Australia the kilowatt hour (kWh) is used to measure domestic energy use
1 kWh is the energy used by a 1 kWh appliance in one hour
E(kWh) = P(kW) * t(h) = 103 W * (60 min * 60 s)
1 kWh = 3.6 * 106 J
Example 2A TV needs 250 W. It is switched on for 30 minutes. If each kWh costs 20 cents, how much does it cost to run the TV?
Example 2A TV needs 250 W. It is switched on for 30 minutes. If each kWh costs 20 cents, how much does it cost to run the TV?
ResourcesFurther Reading
Origin Energy - How to read your energy bill