YEAR 8 PHYSICAL SCIENCE Measuring Energy Student Objectives

YEAR 8 PHYSICAL SCIENCE Measuring Energy

Student Objectives By the end of this lesson you will be able to recall the units of measurement for energy, work, and power.

1 MINUTE REVIEW Last lesson we looked at the definitions of energy, work and power. Do you remember their definitions? Be prepared to share your ideas

Definitions Energy is the ability to do work and is used in order to perform work. Work is a force over a distance acting to move or change an object Power is how fast work is done (or the rate at which work is done)

Measuring Energy - JOULES Both energy and work can be measured in a unit called the joule (j). Joule is another name for newton·metre. If you push an object along the floor with a force of 1 newton for 1 metre, you have done 1 joule of work.

Comparing units of energy 1 kilojoule = 0. 2388 calories or 0. 948 Btu 1 calorie = 4. 19 k. J or 0. 0039 Btu 1 British thermal unit (Btu) = 1. 055 k. J or 252 calories.

YEAR 8 PHYSICAL SCIENCE Calculating Energy

Student Objectives By the end of this lesson you will be able to use formulae to calculate work and power.

1 MINUTE REVIEW What are the units used to measure work and energy? What is the unit used to measure power? What is the unit used to measure force? BE PREPARED TO SHARE YOU ANSWERS

MEASURING WORK To calculate the amount of work being done we use this equation: W = F×d Where W = work (joules) F = force (newtons) d = distance (metres)

Example • Work = magnitude of Force (N) × distance (m) • If I pushed a chair 5 metres with 20 N of force how much work would I have done? • Work= _____N (force) x ___metres (distance) • Work = _______ J or _____ k. J

Measuring Power To calculate the amount of power being used we use this equation: P = W/ t Where P = power (watts) W = work (joules) t = time (seconds)

Example • Power = work (J) / time (seconds) • If I pushed the chair for 3 seconds and it took 1500 joules (1. 5 k. J) of work to move the chair how much power did I use? • Power = ______J (work) /___ seconds (time) • Power = _____ watts