Peoples Physics Concepts

http://www.ck12.org Chapter 13. Electric Circuits: Batteries and Resistors

13.6 Energy Efficiency

• Define and calculate the efficiency of electrical devices.

Students will learn how to properly think about efficiency and how to calculate the efficiency of electrical devices.

Key Equations

P=V·I; Power in electricity is the voltage multiplied by the current

E=P·∆t ; the electrical energy used is equal to the power dissipated multiplied by the time the circuit is running

E f f=PPoutin ; Efficiency is the Power out divided by the Power input

Assuming same time periods:E f f=EEoutin=WorkEin

Guidance

Conservation of Energy Electrical Efficiency:

Electrical energy is useful to us mostly because it is easy to transport and can be easily converted to or from other

forms of energy. Of course, conversion involves waste, typically as heat.

Electrical energy consumed can be determined by multiplying power by time(E=P∆t). Recall the equations for

mechanical and thermal energy/work(PE=mgh,KE= 1 / 2 mv^2 ,Q=mc∆T). An important idea is theefficiency

of an electrical device: the fraction of electrical energy consumed that goes into doing useful work(Eout/Ein),

expressed as a percentage.

Example 1

You use a 100 W electric motor to lift a 10 kg mass 5 m, and it takes 20 s.

Theelectricalenergyconsumed isEelec=P·t= ( 100 W)( 20 s) = 2000 J

Theworkdone is against gravity, so we usePE=mgh= ( 10 kg)( 10 m/s^2 )( 5 m) = 500 J

The efficiency isEout/Ein= ( 500 J)/( 2000 J) = 0. 25 =25% efficient

Watch this Explanation

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