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Resistance

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Resistance is a property of a substance to hinder the movement of electricity and turn it into another form of energy (like heat).

Resistance

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  • Resistors are electrical devices used in circuits to impede the flow of electrons. The greater the resistance, the more the potential drops (that is the potential difference gets bigger), as more energy is required to push a current through a wire.

Electronics often have resistors to control how much electricity is used.

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Resistance

All loads and connecters have at least a small amount of resistance.

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An ohmmeter is used to measure the resistance of a circuit.

Resistance

Symbol for ohmmeter

Ohmmeter

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Symbol for Resistance: R

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Unit of Measurement: Ohm (Ω).

Resistance

Resistance is measured in ohms. The symbol is (omega) Ω

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Resistance

For example, if we say the resistance in 10 metres of a certain wire is 100 Ohms, then

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R = 100 Ω

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Resistance

The German scientist Georg Ohm was the first to record his observation which relates voltage and current:

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Potential Difference = Resistance x Current

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V = R x I

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Calculating Resistance

Name

Unit

Symbol

Potential Difference

Volt

V

Current

Ampere

I

Resistance

Ohm

Ω

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Ohm’s Law

  • V= R I
  • y = mx + b

The origin should be (0, 0) therefore b disappears.

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Resistance

Ohm's Law:

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The voltage between two points on a conductor is directly related to the electric current flowing through the conductor.

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This means the graph of V vs I is a straight line for an ohmic load.

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Resistance

If the graph of V vs I does not turn out to be a straight line, the load is said to be "non-ohmic".

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Complete the table

Variable Name

Formula

Unit (& symbol)

Symbol

Current

​

​

​

​

​

​

V

​

R = V/I

Ohms (Ω)

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Complete the table

Variable Name

Formula

Unit (& symbol)

Symbol

Current

I = V/R

Ampere (A)

I

Potential Diff.

V = I x R

Volts (V)

V

Resistance

R = V/I

Ohms (Ω)

R

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(Problem Set Resistance Question #1)

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What is the potential difference across a tungsten filament in a 100 W light bulb if the resistance in the filament is 144 Ω and a current of 0.833 A is flowing through it?

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Try This.

V = I x R

V = (0.833 A) x (144 Ω)

V = 119.952 V

V = 120 V

.: the potential difference is 120 V

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TVO Video

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Questions

  • Some conductors have a very low resistance, like copper wiring in our homes. Why is this useful?

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  • Some conductors have high resistance, like tungsten wire in a light bulb filament. Why is this useful?

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The type of material obviously affects resistance. Can you think of any other factors that may affect Resistance?