Physics Power

From High School Online Collaborative Writing

[edit] What is Power?

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

[edit] James P. Joule

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.

[edit] Electrical power

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

[edit] Practice Problems

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 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 4 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 5 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.

PROBELM 6 Calculare the rate at which energy is supplied by a 240V source to a circuit if the current in the circuit is 3.5A.

[edit] Solutions to Practice Problems

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 Since the Power equation is derived from the Work equation, in this problem you must use both Work and Power. You would start with W=mgh: W=(1000kg)(9.8m/s)(100m) = 9.8×105 Joules. Now, you need to calculate the time in order to find out the Power. t=d/v --> t=(100m)/(4 m/s) t=25s. Now you're ready to calculare the POWER! P=W/t --> P=(9.8×105J)/(25s)--> 39200W or 39.2kW

PROBLEM 4 P=W/t --> P=(6000J)/(30.0s) --> P=200W

PROBLEM 5 P=Fv --> P=(500N)(20m/s) --> P= 10,000W OR 10kW

PROBLEM 6 P=VI --> P=(240V0(3.5A) --> P=840W