Electrical Power and Energy

Electrical Energy can drive motors, give us light, cook our food, heat our homes, and lots more. How do we measure that energy and that power?  Recall that Voltage is a potential difference, and that the units are Joules per Coulomb, or Volts. Also, recall that Current measures the charge flowing through a conductor in a circuit. The units are Coulombs per second, or Amps. The product of Voltage and Current (V x I) gives us units of Joules/Second or Watts. This is the unit of power we learned in mechanics as well. So electrically,

Power = Voltage x Current

or

P = VI

Electrical energy used in a given time (seconds) is the product of Power and time.

Energy = Power x Time 

or

E = Pt

If we recall Ohm's Law (V = IR), then we can write energy in a few different ways:

E = Pt = VIt = I2Rt

Let's keep the units straight. Energy can be in Joules or Kilojoules (1000 Joules). Power will be in Watts, Kilowatts (1000 watts), or Megawatts (1,000,000 watts). When we are relating Heat Energy to a transfer of heat in some mass, keep mass in Kilograms.

  • As an aside: Electric Power is the rate at which electrical potential energy is converted by charge carriers into non-electrical forms of energy.
  • One kilowatt-hour (unit of energy used by power companies) is the energy delivered in one hour at a constant rate of 1 kilowatt. 1 kw-hr = 3.6 x 10Joules.

Heat lost or generated in a circuit means that power is not available to do work. We often use the formula P = I2R and call these heat losses "I2R losses" or "joule heating". This is one reason that commercial power transmission lines operate at such high voltages, often on the order of 115,000 Volts. For a constant power, P = VI. As V goes up, I goes down. Thus, heat lost (I2R) also drops.

 

Try the following websites for extras on current, resistors, power, and energy:

 

http://micro.magnet.fsu.edu/electromag/java/ohmslaw/index.html

http://micro.magnet.fsu.edu/electromag/java/varcapacitor/index.html

http://micro.magnet.fsu.edu/electromag/java/resistor/index.html

http://science.howstuffworks.com/power2.htm
(A great website on power generation)

http://www.saburchill.com/physics/chapters/0082.html

 

Some coding tips and interesting facts about resistors at: http://www.williamson-labs.com/resistors.htm

More about Ohm’s law some sample problems, and
a simulated experiment at:
http://www.physics.uoguelph.ca/tutorials/ohm/Q.ohm.intro.html

 

For Practice Problems, Try:

Giancoli Multiple Choice Practice Questions (Go ahead - try a few.)