Learning Objective Describe and calculate efficiency To be able to: Key Words: Input, output, useful, wasted

Exam question Thursday, 23 May 2019 Efficiency Date and title in books. You need a pen, pencil & ruler. Starter Activity – 5 minutes Exam question

Learning Outcomes Recall the efficiency equations Target 1-9 Recall the efficiency equations (F/H) Target 4-9 Use the equations to calculate efficiency in decimals and percentages (H) Target 5-9 Rearrange the equations to calculate useful and total input energy/power Extension: Application of Knowledge Target 8/9 Describe ways to increase efficiency (HT only) Learning Outcomes

The more energy that is used usefully, the better the efficiency. Wasted Energy “ooh its freezing in here, put the light on” said no person, ever! When we put the light on, we want light energy. The thermal energy produced is waste energy. The more energy that is used usefully, the better the efficiency.

Calculating Efficiency Efficiency = Useful Energy ÷ Total Energy (x 100) Efficiency = Useful Power ÷ Total Power (x 100) Efficiency is given as a decimal or as a percentage. The higher the efficiency, the more energy the appliance is using usefully.

Calculating Efficiency Efficiency = (Useful Energy ÷ Total Energy) x 100 Efficiency is given as a percentage. Example: A motor converts 2000J of energy into 400J of useful energy. Calculate its efficiency. Efficiency = (400/2000) x100 = 20% So 80% of its energy is wasted.

Your Task You have been given some questions. Answer them in your book Remember to label them properly and show your working out.

Extension: Calculating Efficiency Two light bulbs use 90J of energy. Light bulb A gives out 9J of light Light bulb B gives out 45J of light. Calculate the efficiency of both bulbs and identify which you think is an energy saving light bulb.

HT Only Question On cold days, some of the energy transferred from a hot car engine is used to warm the air inside the car. This is a useful energy transfer. What effect, if any, does this energy transfer have on the overall efficiency of the car engine? Select the correct answer. decreases the efficiency does not change the efficiency increases the efficiency Give a reason for your answer.

Answer (d) increases the efficiency less (input) energy is wasted or accept some of the energy that would have been wasted is (usefully) used or more (input) energy is usefully used accept heat for energy