Energy & Power

Overview

Energy and power are topics more usually associated with physics rather than electronics. However, understanding power allows us to calculate the correct value for a fuse and use components with the correct power rating so that they will function correctly. Power rating is an important consideration for resistors and transducer drivers in particular.

This lesson considers electrical energy and electrical power in terms of voltage, current and resistance and develops the different relationships between these quantities. It is quite mathematical demanding a good grasp of basic algebra.

Learning Objectives

• Analyse circuits in terms of voltage, current, resistance, energy and power and use the equations:
• voltage = current × resistance
• power = voltage × current
• energy transfer = power × time
• power = (current)² × resistance
• power = (voltage)² ÷ resistance


• Be able to calculate and select the correct fuse for domestic appliances and electronic systems
• Be able to select components such as resistors and transistors with the correct power rating to avoid these components being damaged in use

Lesson Content

Reading: Read the webpage about energy and power. Read the introduction and the basic concepts section which develops all the various equations. Make a note of each of the equations as they are all useful.

Video (6 min): The interactive white board video derives each of the power equations and gives an examples of where each equation might be used. Watch the video and makes notes about each of the derivations.

Reading: Continue to read the webpage by working through each of the examples given. Make sure that you can calculate each of the different quantities and use the different equations in all their various combinations. Pay careful attention to the use of units.

Exercise: Complete the exercises and use the answers to check your understanding.

Video (2 min): Watch this short video about how to calculate the correct fuse for use in a domestic appliance.

Lesson Review

Review your learning by working through the presentations or notes which summarise the website content.

Presentation: Powerpoint download with additional questions and examples. Use the powerpoint presentation to review each of the equations for power and examples of how they are used.

Notes: PDF download. The notes are a reformatted copy of the webpage. Read the notes to consolidate your understanding of the website content.

Self Assessment

Complete either the questions (pdf download) OR the on-line quiz. They are the same questions.

Questions: PDF download. Questions about Energy, Power, Voltage, Current and Resistance.

Quiz: On-line quiz about the relationship between Energy, Power, Voltage, Current and Resistance.

Self Evaluation

I can:

• Recall that power is energy transferred per second (P = E / t)
• Use the equation for electrical power (P = V x I) to find power
• Use the modified power equations P = I² R and P = V² / R to find power
• Rearrange the power equations to find V, I or R given the power and one other quantity
• Recall that energy = power x time (E = P x t)
• Calculate energy transferred in Joules or units of kWh
• Determine the correct fuse to use given the power rating of a device
• Calculate the power dissipated by components such as resistors and transistors