Electric Power

Review of Power

Recall that power describes the rate at which energy is used or work is done. As a rate, the unit for power includes time. Power is oficially measured in Watts, where a Watt is equal to a Joule/second.

Electrical energy is often measured in a unit call the kilowatt-hour. Although this unit contains the word watt, it is a unit of energy, not power (based on the idea that energy = power × time).

Electrical Units and New Formulas for Power

The Volt

The Ampere

Putting them together to get Watts

By multiplying voltage by current, the combination of units becomes a Joule/second or a Watt.

Since both voltage and current can be expressed in terms of Ohm's Law, we can write the formula for electrical power in the following three ways:

Example 1 - Is your light bulb on?

• Question
• Answer

What current flows through a 100W light bulb operating at a voltage of 120V?

What current flows through a 100W light bulb operating at a voltage of 120V?

Example 2 - Blown fuse

• Question
• Answer

Three 100W lamps are placed a fused circuit as shown in the diagram. The lamps are designed to operate at 110V. A fuse is a device that will break if the current passing through it gets too high. Fuses are rated in Amps. What size fuse is required to make sure the current through the lamps is not too great?

Three 100W lamps are placed a fused circuit as shown in the diagram. The lamps are designed to operate at 110V. A fuse is a device that will break if the current passing through it gets too high. Fuses are rated in Amps. What size fuse is required to make sure the current through the lamps is not too great?

Example - Power to the Resistance!

• Question
• Answer

The circuit shown below is run with 50V. If R₁=200Ω, R₂=65Ω, R₃=85Ω, and R₄=75Ω, find the power rating of the circuit.

The circuit shown below is run with 50V. If R₁=200Ω, R₂=65Ω, R₃=85Ω, and R₄=75Ω, find the power rating of the circuit.