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In addition to the revision notes for Electric Power and Efficiency on this page, you can also access the following Electrodynamics learning resources for Electric Power and Efficiency
| Tutorial ID | Title | Tutorial | Video Tutorial | Revision Notes | Revision Questions | |
|---|---|---|---|---|---|---|
| 15.6 | Electric Power and Efficiency |
In these revision notes for Electric Power and Efficiency, we cover the following key points:
Electric power is the work done by an electric source in the unit time.
There are two types of electric power: (1) the total (or input) power, i.e. the power generated by the source; it is calculated by
where ε is the electromotive force of the source and I is the current in the circuit, and (2) the useful (or output) power, i.e. the power generated by an operating appliance; it is calculated by
where ΔV is the potential difference between the terminals of the appliance.
The unit of power is Watt. 1 Watt = 1 Joule/second. In terms of fundamental SI units, the unit of power is:
Since Useful (output) Power = Work/Time and Total (input) Power = Total Energy/Time, we obtain
and
The last formula is known as Joule's Law. It calculates the total energy delivered by an electric source during a given time.
Not all the electric energy produced by the source goes for doing work. Some of this electricity is lost on the way to reach the appliance because of the existence of internal resistance of the source and conducting wire, which decreases the efficiency of circuit. An ideal circuit would have an efficiency of 100%, i.e. all the energy produced by the source was used by the appliance to do work. This is not possible, so we must account this element when calculating the work done by any circuit component.
Therefore, the efficiency of all electrical appliances is smaller than 100% as expected. We have
Joule's Law for electric heaters is written as
where Q is the heat energy delivered by the heater's resistor.
Joule is not suitable to use as a unit of electrical energy s it is too small. To make the work of the employees who deal with electricity bill easier but also to make the consumer to easily understand how much he has to pay, another unit of electrical energy known as kilowatt - hour (kW-h) is used. Kilowatt- hour means kilowatts multiplied by hour. Given that 1 kW = 1000 W and 1 hour = 60 ∙ 60 = 3600 s, the conversion factor between kW-h and Joule is
Other forms of writing the electric power formula written when combining Ohm's Law and the meaning of electric power include:
Multiplying the above expressions by the time t, we obtain the work done (the useful energy consumed) by the appliance and the total energy delivered by the source in terms of resistance R:
and
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