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POWER Noadswood Science, 2012

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POWER To understand, and be able to calculate power Monday, August 10, 2015

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POWER When an electrical appliance is used it transforms electrical energy into other forms of energy The power of the appliance (in watts (W)) is the energy it transforms in joules per second (J sec -1 ) power (W) = energy transformed (J) time (s)

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POWER The power rating of an appliance is simply how much energy it uses every second (1 Watt = 1 Joule per second) Energy (J) Power (W) Time (s)

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POWER QUESTION A light bulb transforms 30’000 J of electrical energy when it is on for 5 minutes – calculate the power power (W) = energy transformed (J) time (s) Energy transformed = 30’000 J Time = 300s (5 x 60) power (W) = 30’000 J 300s power = 100W

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POWER QUESTION A human heart transforms 30’000 J of energy in a school day of 8 hours – calculate its power power (W) = energy transformed (J) time (s) Energy transformed = 30’000 J Time = 28’800s (8 x 60 x 60) power (W) = 30’000 J 28’800s power = 1.04W

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POWER II For any electrical appliance: - Current through it is a measure of the number of electrons passing through it each second (charge flow per second) Potential difference across it is a measure of how much energy each electron transfers Power supplied is the energy transferred to it each second related by the following equation: - The power supplied (in watts (W)) is the current (I) multiplied by the potential difference (V) power (W) = current (I) x potential difference (V)

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POWER II Power (W) Voltage (V) Current (I) Power = Voltage x CurrentVoltage = Power ÷ CurrentCurrent = Power ÷ Voltage

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POWER QUESTION II What is the power on an electric motor if it supplied a current of 4A and a potential difference of 12V? power (W) = voltage (V) x current (I) power (W) = 12V x 4A power = 48W

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POWER QUESTION II A torch has a power of 0.3W. If its potential difference is 3V what is the current supplied to it? current (I) = power (W) ÷ potential difference (V) current (A) = 0.3W ÷ 3V current = 0.1A

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CHARGE When an electrical appliance is on, electrons are forced through the appliance by the potential difference of the voltage supply unit The potential difference causes a flow of charge through the appliance, carried by the electrons (-ve charge) The rate of flow if charge is the electric current through the appliance Charge (measured in coulombs) flowing through a component in a certain time depends on the current and the time charge (Q) = current (I) x time (s)

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CHARGE Charge (Q) = current (I) x time (s) Charge (Q) Current (I) Time (s)

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CHARGE QUESTION – HIGHER Calculate the charge flow when the current is 8A for 80 seconds charge (Q) = current (I) x time (s) charge (C) = 8A x 80s charge = 640C

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ENERGY TRANSFORMED – HIGHER The amount of energy that flows in a circuit will depend on the amount of charge carried by the electrons and the voltage pushing the charge around When a resistor is connected to a battery electrons are made to pass through the resistor by the battery Each electron repeatedly collides with the vibrating atoms of the resistor, transferring energy to them (heating the resistor) The energy transformed to the resistor depends on the amount of charge passing through it and the potential difference across it

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ENERGY TRANSFORMED – HIGHER Energy transformed (J) = potential difference (V) x charge (Q) Energy transformed (J) Voltage (V) Charge (Q)

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ENERGY QUESTION – HIGHER Calculate the energy transformed in a component when the charge flow is 30C and the potential difference is 20V energy transformed (J) = potential difference (V) x charge (Q) energy transformed (J) = 20V x 30C energy transformed = 600J

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E XPERIMENT Look at the standard filament bulb against an energy efficient bulb… Using the equation P = I x V use the ammeter and voltmeter in circuits with the two types of bulb to see the difference in power being drawn…

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