Calculating Resistance

Describing Resistance  How much energy does it take to push a 50 kg box across a room?  Depends on…  Floor surface  Box material  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  Ohm’s 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?????  Resistance = ___________ Voltage Amperes

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

Let’s 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 Ω

One more  Let’s 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  = 7.33V/A  = 7.33Ω

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

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

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

Here’s 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.