Today4/30

Questions? Power (Joules/second)

Power

Energy moves from PE to KE, is lost to friction, is lost to heat and light, --In general can move from form to form.

The rate energy moves is called “Power” Units of J/s called Watts (W)

Power example

A 12V battery is connected to a 24 resistor. How much power is supplied by the battery? Energy conservation says the same power will be dissipated by the resistor as heat.

Think V means J/C and current means C/s

P = IV V = IR

I = 12/24 = 0.5A

P = (0.5)12 = 6W

Power example

A hydroelectric plant makes power where water drops 50m through the turbines. If 106kg of water per second move through the turbines, how many 100W light bulbs can the power plant light?

Think PE of the water per second

P = PE/s and Energy conservation says this must be the same as the power output of the plant.

PE/kg = mgh = 1(10)50 = 500J/kgP = PE/s = (PE/kg)(#kg/s) = 500 x 106W

or 5 million 100W light bulbs

Power example

A rope pulls a brick across a rough table at a constant speed of 3m/s. The friction force is 40N. What power is supplied by the person pulling the rope?

Think Work = Force x Distance

P = Work/s and Energy conservation says this must be the same as the power output of the person.

Work/m = 40(1)P = Work/s = (Work/m)(#m/s) = (40)(3)

= 120W