What is Power?[edit | edit source]

In Physics, POWER is the rate of doing work, in other words the rate in which energy is transferred. Power can de defined using the definition P= W/t Power is measured in watts (W). One watt is one joule of energy transferred in one second. Considering the fact that a watt is such a small unit of measurement, power is often measured in kilowatts (kW). 1 kilowatt = 1000 watts

P=W/t = Fd/t = Fv

James P. Joule[edit | edit source]


James P. Joule is an English physicist who found that the power lost in a resistor is given by P = RI^2, a law known as Joule's law. He discovered that the heat produced by various types of energy was proportional to the energy expended, and in established the equivalence between heat and mechanical energy. This began the study of Power.

Electrical power[edit | edit source]

The instantaneous electrical power P delivered to a component is given by

Psource = VI

P is the instantaneous power, measured in watts (joules per second)

V is the potential difference (or voltage drop) across the component, measured in volts

I is the current flowing through it, measured in amperes

Practice Problems[edit | edit source]

PROBLEM 1 A box weights 575N and is lifted a distance of 20.0m straight up by a cable attatched to a motor. The job is done in 15 seconds. What is the power developed by the motor in watts AND kilowatts?


PROBLEM 2 Calculate the rate at which energy is supplied by a 120v source to a circuitif the current in the circuit is 5.5 amperes.

PROBLEM 3 An electric heater raises the temperature of a measured quantity of water. The water absorbs 6000 joules of energy from the heater in 30.0 seconds. What is the minimum power supplied to the heater?

PROBLEM 4 A 500N force is applied to an object. If the object travels with a constant velocity of 20 meters per second, calculare the power expended on the object.

PROBLEM 5 A miniature lamp requires 0.025 A of 130v to light a small room. What is the power rating of the lamp?

PROBLEM 6 An elevator must lift 1000 kg a distance of 100 m at a velocity of 4 m/s. What is the average power the elevator exerts during this trip?

PROBLEM 7 The headlights of a car has a 5.4 A of current and is supplied by 12v source by a car battery, what is the power rating of the headlights?

Solutions to Practice Problems[edit | edit source]

PROBLEM 1 P=Fd/t --> P= (575N)(20.0m)/(15s)--> P=766.67W or .767kW

PROBLEM 2 P=VI --> P=(120V)(5.5A) --> P- 660W

PROBLEM 3 P=e/t--> 6000J/30s--> P=200W

PROBLEM 4 P=Fv --> P=(500N)(20m/s) --> P= 10,000W OR 10kW

PROBLEM 5 P= VI --> P= (130v)(0.025A) --> P= 3.25W

PROBLEM 6 Weight=mgh.-->{1000kg)(9;8m/s)(100m)=9.8x10^5J-->Time=x/v=(100m/4m/s)=25s-->P=w/t-->(9.8x10^5)/25s--> P=3.9x10^4W OR 39kW

PROBLEM 7P= VI --> P= (12v)(5.4) --> P= 64.8 W

Resources[edit | edit source]

1. Sparknotes

2. Encyclopedia

3. Electrical Power

4. Barron's Regents Review Book

5. Barron's SAT II Physics Review Book

6. Famous Physicists

References[edit | edit source]

1. Watts

2. Kilowatts

3. Amperes

4. Volts

5. Potential difference

6. James P. Joule

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