Calculating ResistanceCalculating Resistance

Ohm's LawOhm's Law

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

VRI

Describing ResistanceDescribing Resistance

How much energy does it take to push a How much energy does it take to push a 50 kg box across a room?50 kg box across a room?

Depends on…Depends on… Floor surfaceFloor surface Box materialBox material Size of openingSize of opening

What about electrical What about electrical resistance?resistance?

How much energy does it take to push a How much energy does it take to push a coulomb of charge through a light bulb coulomb of charge through a light bulb filament?filament? Depends on the resistance of the filamentDepends on the resistance of the filament

Ohm’s law was developed by Greg Ohm - a Ohm’s law was developed by Greg Ohm - a school teacher in 1828school teacher in 1828Electrical Resistance = ratio of potential Electrical Resistance = ratio of potential difference across the load to the current difference across the load to the current through the loadthrough the loadWhat the heck does that mean?????What the heck does that mean?????

VoltageAmperes

Resistance =Resistance =

Calculating ResistanceCalculating Resistance

Light bulb circuitLight bulb circuit 1 volt battery1 volt battery 1 amp is measured going across the light bulb 1 amp is measured going across the light bulb

filamentfilament

What is the resistance?What is the resistance? R = V/I = 1v / 1a = 1 ΩR = V/I = 1v / 1a = 1 Ω

Let’s try this oneLet’s try this one

A heater is connected to a wall socketA heater is connected to a wall socketI = 12.5 A (you can find this on the back of every I = 12.5 A (you can find this on the back of every appliance)appliance)We want to find out the resistance ® in ohmsWe want to find out the resistance ® in ohmsHere we go…Here we go…A wall outlet has a potential difference of …A wall outlet has a potential difference of …110 V110 VFind the resistance of the coil in the heater..Find the resistance of the coil in the heater..

The heaterThe heater

R = V / IR = V / I

= 110 V / 12.5 A= 110 V / 12.5 A

= 8.8 V/A= 8.8 V/A

= = 8.8 Ω8.8 Ω

One moreOne more

Let’s do one more…Let’s do one more…

Your stove uses 2 times as much voltage Your stove uses 2 times as much voltage as a heater…as a heater…

220 Volts of potential difference220 Volts of potential difference

The current used by the stove is 30 A The current used by the stove is 30 A (check the fuses)(check the fuses)

Stove…Stove…

R = V/AR = V/A

= 220 V / 30 A= 220 V / 30 A

= 7.33V/A= 7.33V/A

= = 7.33Ω7.33Ω

One more…One more…

A light bulb has 110 V passing through it A light bulb has 110 V passing through it (plugged into the house circuit)(plugged into the house circuit)

The ampere’s passing through the bulb The ampere’s passing through the bulb filament is 0.25 Afilament is 0.25 A

What is the resistance across the What is the resistance across the filament?filament?

Use Ohm’s Law…Use Ohm’s Law…

Light bulbLight bulb

R = V / IR = V / I

R = 120 V / 0.25 AR = 120 V / 0.25 A

R = 480 V/AR = 480 V/A

R = R = 480 Ω480 Ω

But are all resisters the same?But are all resisters the same?

Only some materials resist perfectly.Only some materials resist perfectly.

Some change their resistance when Some change their resistance when temperature changestemperature changes

Here’s the rub…Here’s the rub…

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

Ohmic MaterialsOhmic Materials

Materials that have constant resistance Materials that have constant resistance regardless of temperatures are called regardless of temperatures are called Ohmic resisters.Ohmic resisters.

No matter how much voltage is pushed No matter how much voltage is pushed through the resistance stays the same.through the resistance stays the same.