PHYS 2212 Module 5.5

Electrical Energy and Power

Recommended Reading

5.5 Electrical Energy and Power

Learning Objectives

By the end of this section, you will be able to:

  • Express electrical power in terms of the voltage and the current
  • Describe the power dissipated by a resistor in an electric circuit
  • Calculate the energy efficiency and cost effectiveness of appliances and equipment

Using Ohm’s Law in Resistor Circuits

Electrical Power in Circuits

In physics 1, you discussed mechanical power which is the rate at which mechanical work is done: .

The units for power are J/s, which are called Watts, symbol: W (but don’t confuse the unit W with the symbol for work, they’re not the same thing). Generally, power is the measure of how quickly energy is transferred into or out of a system. 

Here we will discuss electrical power, which is also measured in J/s = W (watts). But instead of looking at how the kinetic energy or the gravitational potential energy changes, we will focus on how the electric potential energy changes in time.


Practice 5.5.1
Household appliances (in the US) all have = 120 V.

What is the approximate resistance of the filament (at operating temperature) of a 100 W lightbulb?
Check your answer: B. R = 144 Ohms
Practice 5.5.2
US light bulbs are rated for 120 V. 

European bulbs are designed for 240 V. 

If you buy a 100 W light bulb as a souvenir in Paris, and plug it in at home, what happens?
Check your answer: D. It glows 1/4 as bright as usual