Download 8.3.4 power and energy

8.3.4 POWER AND ENERGY
The amount of power is related to the rate at which energy is transformed
4.1 Explain that power is the rate at which energy is transformed from one form to another
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where P is power (watts), energy is in Joules, and t is time (seconds)
Power – rate at which energy is transformed, measured in watts – joules per second
o kW and mW are kilo/megawatts, = 103 W and 106 W
o E.g. Battery generating 12W transforms/converts 12 J of chemical energy in 1 second
4.2 Identify the relationship between power, potential difference and current
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Potential difference (voltage) – energy per coulomb used, passing through a resistor
Current – coulombs of charge per second, passing through a resistor
Therefore, we can get power by multiplying voltage and current:
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So
4.3 Identify that the total amount of energy used depends on the length of time the current is flowing and can
be calculated using:
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Since power is joules per second, we can find total energy used (W, energy in joules) by multiplying time
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Since
, then
4.4 Explain why the kilowatt-hour is used to measure electrical energy consumption rather than the joule
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Kilowatt-hour – amount of energy used by a 1 kW device for 1 hour
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o Used to measure consumption as large amount of energy is measured, joules is too inconvenient
4.P1 Perform a first-hand investigation, gather information and use available evidence to demonstrate the
relationship between current, voltage and power for a model 6 V to 12 V electric heating coil
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See Experiment 4.P1 in 8.3 Experiment Booklet
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KC Notes
Copyright 2013
Kris Choy
4.P2 Solve problems and analyse information using:
and
There is a voltage drop of 100 V across a 5 Ω resistor. Calculate the power dissipated.
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Two resistors, R1 with resistance of 2 Ω and R2 with resistance of 3 Ω, are connected in series to a 200 V power supply.
Calculate the power generated by the power supply and the power dissipated in each of the resistors.
Since it is in series, current is equal through both resistors
Therefore we can find total power of power supply, by the formula:
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Now, since we know current, we can work out other resistors by finding their separate voltages:
Note that power from power supply = P1 + P2
Calculate the time it takes a 40 W lamp to dissipate 800 J of electrical energy.
Calculate the energy that is converted to heat energy in 10 minutes, from a 2 Ω resistor with a 6 V battery supply.
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Convert this into kilowatt-hours in 150 minutes.
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KC Notes
Copyright 2013
Kris Choy