51 Outcome 6: Electrical Power
Outcome/Competency: You will be able to apply the laws of voltage, current, and resistance to solve electrical power problems.
Rationale:
Why is it important for you to learn this skill?
Electrical power is the commodity that is transported by power lines. This power is consumed to do work by anything that gets plugged in. You will understand the relationship between the three properties of electricity (voltage, resistance, and current) and the power it delivers. People pay for power; understanding units of electrical power allows you to understand how it is charged for.
Objectives:
To be competent in this area, the individual must be able to:
- Define power in an electrical circuit
- Solve problems of electrical power
Learning Goals
- Draw comparison between mechanical and electrical energy and power
- Calculate electric power in a circuit by identifying the correct formula
- Solve problems using the energy cost formula
Introduction:
- In this module, we will review comparisons between mechanical and electrical energy and power, how to calculate electrical power within a circuit, and how to apply the energy cost formula. This chapter will include class discussions, articles to be reviewed, review questions, and a cumulative test.
Topic 1: Electric Power
Prerequisite Skills:
- Solving equations and combining two equations by substitution will be discussed. It may be a good opportunity to review points of algebra.
We know that horsepower is a rate of doing work. Electrical power is also referred to as the rate of doing work. A more correct definition of power is the rate at which electrical energy can be converted to heat, light, acoustic, or mechanical energy. The energy is this case must be thought of as work not as output. The metric unit for energy is newtons – meter.
One Newton – metre was called a “joule” for ease of use. Therefore, if power is a rate doing work we can see that:
Watt’s Law
One watt of power is produced when one volt forces one ampere of current through a resistance of one ohm.
Topic 2: Watt’s Law
Since electrical power is measured in watts and it has a direct relationship with voltage and amperage, we should be able to use it in a mathematical formula. The formula is:
This formula is similar in format to Ohm’s law ( E = I X R ) so we can use the same tradesperson’s triangle method of transposing the equation:
we also know ohms law as:
we can combine the two to include resistance in a formula:
another formula is:
We can now solve for the power in a circuit using any two variables.
Example 1
Use all three power equations to find power in this circuit and see if they confirm each other.
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Example 2
A radio is rated 36 watts. How much force is needed to push 3 amps through it?
Example 3
Power ratings for heating and lighting (including toasters ovens, hair dryers, etc.) are usually rated or measured in watts. If this power rating was in mechanical terms, what would the appliance be rated in