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Power, electrical power and fuses Objectives: • To know that power is the rate of energy transfer • To know that power is measure in Watts (and kilowatts) • To know how to calculate electrical power • To be able to convert to SI units when necessary • To evaluate the correct fuse for an appliance In this mixture of light bulbs, which do you think would be the brightest? Power and energy Power is a measure of the rate of energy transfer. It is calculated using the formula: power = energy time Power is measured in watts (W). Energy is measured in joules (J). Time is measured in seconds (s). The higher the power rating of a device, the more energy is transferred every second. This is why a 100 W light bulb is brighter than a 40 W light bulb: more electrical energy is transferred into light energy every second. 4 of 28 © Boardworks Ltd 2011 1 Watt = 1Joule/second 1 Kilowatt = 1000 Joules/ second A 100Watt light bulb is transferring 100 Joules of energy every second 5 of 28 © Boardworks Ltd 2011 1. Calculate the power of a light bulb that uses 36J of electrical energy in 6 seconds. Write down the formula Substitute the numbers for the letters-careful about units! Solve the equation Don’t forget units in your answer! Calculate the power of a girl who performs 3000J of work in 6 seconds. Calculate the power of a car that uses 120kJ of chemical energy in 5 seconds. Calculate the power of a person who does 24000J of work in 2 minutes. Calculate the power of a clock that uses 14.4kJ of electrical energy in 2 hours. Calculate the energy used by a light bulb of power 40W in 2 minutes. Which appliance costs more to run? Electrical power Electrical power is the rate at which an appliance uses electrical energy. All appliances have a power rating. Power is measured in watts (W): 1 watt means 1 joule of energy is used every second. 1000 watts = 1 kilowatt (kW). Appliances that need to create heat, such as washing machines, cookers, hairdryers and kettles, usually use the most power. TVs, radios and computers usually use the least amount of power. 13 of 15 © Boardworks Ltd 2011 What is the formula for electrical power? In electrical devices, power can be calculated using the formula: power = current × voltage P = IV What are the units of power, current and voltage? Power is measured in watts (W). Current is measured in amps (A). Voltage is measured in volts (V). 14 of 15 © Boardworks Ltd 2011 Using 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… I = × 15 of 15 P V © Boardworks Ltd 2011 What two measurements do we need to take in order to find out the power of a motor? The voltage across the motor The current through the motor You have 10 minutes to set up an appropriate circuit, take the correct readings and calculate the power of your motor Electrical power calculations 18 of 15 © Boardworks Ltd 2011 Calculating the correct value for a fuse What is a fuse? A fuse is a safety device containing a thin wire that melts if the current is too high, breaking the circuit and protecting the cable from overheating and catching fire. fuse Fuses act as an early warning system, preventing appliances from being damaged by surges in electricity and warning owners of faults. All UK plugs are fitted with fuses. Fuses are available in a range of amperes to suit different current values. 20 of 25 © Boardworks Ltd 2011 Fuses come in different ratings. The more usual ones are 3A, 5A and 13A. The fuse melts when the current gets too high. Using the mains voltage as 230V, work out the current flowing and the fuse required to protect it 21 of 25 Appliance Power rating Microwave 1200W Kettle 2.5kW Lamp 100W Current flowing 5.2A 11A 0.4A fuse 13A 13A 3A © Boardworks Ltd 2011