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Calculating Resistance

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Ohm's Law Ohm's law is an assertion that the current through a device is always directly proportional to the potential difference applied to the device. Ohm's law is an assertion that the current through a device is always directly proportional to the potential difference applied to the device.

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Describing Resistance How much energy does it take to push a 50 kg box across a room? Depends on… Floor surface Floor surface Box material Box material Size of opening Size of opening

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What about electrical resistance? How much energy does it take to push a coulomb of charge through a light bulb filament? Depends on the resistance of the filament Depends on the resistance of the filament Ohms law was developed by Greg Ohm - a school teacher in 1828 Electrical Resistance = ratio of potential difference across the load to the current through the load What the heck does that mean????? Voltage Amperes Resistance =

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Calculating Resistance Light bulb circuit 1 volt battery 1 volt battery 1 amp is measured going across the light bulb filament 1 amp is measured going across the light bulb filament What is the resistance? What is the resistance? R = V/I = 1v / 1a = 1 R = V/I = 1v / 1a = 1

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Lets try this one A heater is connected to a wall socket I = 12.5 A (you can find this on the back of every appliance) We want to find out the resistance ® in ohms Here we go… A wall outlet has a potential difference of … 110 V Find the resistance of the coil in the heater..

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The heater R = V / I = 110 V / 12.5 A = 8.8 V/A = 8.8

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One more Lets do one more… Your stove uses 2 times as much voltage as a heater… 220 Volts of potential difference The current used by the stove is 30 A (check the fuses)

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Stove… R = V/A = 220 V / 30 A = 220 V / 30 A = 7.33V/A = 7.33V/A = 7.33 = 7.33

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One more… A light bulb has 110 V passing through it (plugged into the house circuit) The amperes passing through the bulb filament is 0.25 A What is the resistance across the filament? Use Ohms Law…

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Light bulb R = V / I R = 120 V / 0.25 A R = 480 V/A R = 480

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But are all resisters the same? Only some materials resist perfectly. Some change their resistance when temperature changes

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Heres the rub… Metals (copper, aluminium...) - resistance usually increases with temperature. Non metals (carbon, silicon, germanium...) - resistance usually decreases with temperature. This has big implications for light bulbs and other things that heat up when electricity passes through

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Ohmic Materials Materials that have constant resistance regardless of temperatures are called Ohmic resisters. No matter how much voltage is pushed through the resistance stays the same.

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