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Wiring Fundamentals: Power

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00:00 - The definition of power is the rate at which energy is transformed.
00:04 Usually in our automotive electrical world this is the rate at which we're transforming electrical energy into mechanical energy, moving something.
00:12 Or thermal energy, heating something up.
00:14 An in depth understanding of power isn't necessary for this course.
00:17 But a brief explanation can be helpful as it leads us to another mathematical equation which is very handy to know.
00:24 Earlier when we discussed the term load we were actually talking about power.
00:28 A device that places a high load on the electrical system means it is converting electrical energy into another form very quickly.
00:35 This in turn means it is drawing a large current.
00:38 There is an equation which relates the power of a device to the voltage we're applying to it and the current passing through it.
00:45 Power is equal to the voltage times the current.
00:48 Or P=VI Much like with ohm's law we can rearrange this equation to find any of the three elements if we know the other two.
00:58 Our most common use for this equation once again is to calculate the current a device will draw so we can correctly size the wires going to it.
01:06 A common example from the automotive world is a halogen headlight bulb.
01:09 It's typical for these bulbs to convert electrical energy into light and heat at a rate of 55 watts.
01:15 We know the charging system of the vehicle will apply 13.8 volts of electrical pressure across the bulb, and from this we can work out the current that will pass through the bulb.
01:25 Rearranging our power equation, the current passing through the bulb is equal to the bulb power divided by the electrical pressure.
01:32 55 watts divided by 13.8 volts results in approximately four amps of current passing through the bulb.
01:40 With this information we can size the wiring and fuse that supplies the lighting circuit of the vehicle.
01:45 When you are reading or writing documentation, power has the unit of watts and the symbol of a capital W.

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