Measuring Energy YEAR 8 PHYSICAL SCIENCE

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

Be prepared to share your ideas 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

Energy is the ability to do work and is used in order to perform work. 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 0.948Btu 1 calorie = 4.19kJ or 0.0039Btu 1 British thermal unit (Btu) = 1.055kJ or 252 calories.

Calculating Energy YEAR 8 PHYSICAL SCIENCE

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

BE PREPARED TO SHARE YOU ANSWERS 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

To calculate the amount of work being done we use this equation: 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)

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

To calculate the amount of power being used we use this equation: 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)

Power = work (J) / time (seconds) Example Power = work (J) / time (seconds) If I pushed the chair for 3 seconds and it took 1500 joules (1.5kJ) of work to move the chair how much power did I use? Power = ______J (work) /___ seconds (time) Power = _____ watts