Presentation on theme: "Calculating Resistance. Ohm's Law Ohm's law is an assertion that the current through a device is always directly proportional to the potential difference."— Presentation transcript:
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.
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
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 =
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
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..
The heater R = V / I = 110 V / 12.5 A = 8.8 V/A = 8.8
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)
Stove… R = V/A = 220 V / 30 A = 220 V / 30 A = 7.33V/A = 7.33V/A = 7.33 = 7.33
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…
Light bulb R = V / I R = 120 V / 0.25 A R = 480 V/A R = 480
But are all resisters the same? Only some materials resist perfectly. Some change their resistance when temperature changes
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
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.