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Electrical Appliances and Power

Electrical Appliances and Power. Learning Objectives. To be able to calculate power, and cost of electricity To be able to apply these calculations to help explain the efficiency of devices To be able to apply this knowledge to complete exam style questions. Bell Work.

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Electrical Appliances and Power

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  1. Electrical Appliances and Power

  2. Learning Objectives • To be able to calculate power, and cost of electricity • To be able to apply these calculations to help explain the efficiency of devices • To be able to apply this knowledge to complete exam style questions

  3. Bell Work Fill in your table with the useful and wasted types of energy • SUCCESS CRITERIA • A completed table showing the type of energy which is useful and the type of energy which is wasted • A comment about where the energy is used or wasted

  4. What is electrical power? Electrical power is the rate at which an electrical appliance uses electrical energy. All appliances have a power rating. Power is measured in watts (W). 1000 watts = 1 kilowatt (kW).1 watt of power means that 1 joule of energy is used every second. Appliances that need to create heat, such as washing machines, cookers, hair dryers and kettles, usually use the most power. TVs, radios and computers usually use the least amount of power.

  5. power = current x voltage What is the formula for electrical power? In electrical devices, power can be calculated using the formula: P =I x V What are the units of power, current and voltage? • Power is measured in watts (W) or kilowatts (kW). • Current is measured in amps (A). • Voltage is measured in volts (V).

  6. P V I = Can I use a formula triangle? A formula triangle helps you to rearrange a formula. The formula triangle for P = IV is shown below. Cover up whatever quantity you are trying to find to leave the calculation required. So to find current (I), cover up I… …which gives the formula…   x

  7. How is power calculated? A filament bulb has a potential difference of 200V across it and a current of 0.2A running through it. At what power is the filament bulb operating? P = IV = 0.2A x 200V = 40W

  8. Mini Review

  9. electrical energy = power x time 1 unit of electricity = 1 unit of electrical energy Calculating the units of electricity The amount of electrical energy (i.e. the amount of electricity) used by an appliance depends on its power and how long the electricity is used for. Power is measured in kilowatts (kW) and the time is measured in hours (h), so what are the units of electricity measured in? = 1 kilowatt hour (kWh) Example: How many units of electricity is 17.6 kWh? 17.6 units

  10. How is electricity paid for? Electricity costs money, which is why every home has an electricity meter. The meter records how much electricity is used in a house in units of electrical energy. The units of electrical energy are called kilowatt hours (kWh). The cost of an electricity bill is calculated from the number of units used.

  11. cost = number of units x cost per unit How much does electricity cost? The cost of electricity is the number of units of electrical energy multiplied by the cost per unit. Example: How much would 10 units of electricity cost at a price of 9p per unit? cost = 10units x 9p/unit = 90p

  12. Buying electricity – example A kettle uses 45.2 kWh of energy.If electricity costs 10p per unit, how much does it cost to use the kettle? Number of units: number of units of electricity = number of kilowatt hours =45.2 units Cost of electricity: cost = number of units x cost per unit = 45.2units x 10p/unit = 452 p or £4.52

  13. Buying electricity – example An iron that operates at a power of 3 kW for 4 hours uses electricity that costs 8p per unit. How much does it cost for the electricity used by the iron in that time? Number of units: number of units of electricity = number of kilowatt hours = 3kW x 4h = 12kWh = 12units Cost of electricity: cost = number of unitsx cost per unit = 12 units x 8p/unit = 96p

  14. Mini Review

  15. Construct your Knowledge – Power of a Motor • You are going to be watching a simulation of an electric motor. Firstly you need to calculate the power of the motor when lifting 100gm. • What values do you need to write down from the simulation to calculate this? Next we will be recapping efficiency in order to calculate the efficiency of a motor!

  16. useful output energy total input energy energy efficiency = How is energy efficiency calculated? The energy efficiency of a device can be calculated using this formula: • Useful energy is measured in joules (J). • Total energy is measured in joules (J). • Energy efficiency does not have any units. • It is a number between 0 and 1 which can be converted into a percentage by multiplying by 100.

  17. Now calculating efficiency Copy this Table (but not the bits in red!)

  18. Mini Review – Think Pair Share • How does the efficiency of the motor change with different amounts of weight? • Try and explain this in terms of energy

  19. Apply your knowledge • Complete exam style questions

  20. Review • To be able to calculate power, and cost of electricity • To be able to apply these calculations to help explain the efficiency of devices • To be able to apply this knowledge to complete exam style questions

  21. Review • Create your own question with answers (make it as difficult as you can but remember you have to give the correct answers with the correct calculations) These will be used for revision next week so make them good!

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