1. State that resistance = p.d./current and understand qualitatively how changes in p.d. or resistance affect current.

The electrical resistance is a measure of the difficulty to pass an electric current through a conductor.

Resistance is calculated by dividing the voltage between two points by the current flowing through the points.

R = V  ÷ I

From this formula, we can conclude that an increase in voltage will increase resistance. Reducing voltage will reduce the resistance.


  1. Recall and use the equation R = V / I

If the voltage is 6 volts and the current is 2 amps, the resistance = 6 ÷ 2 = 3 ohms

This formulas is known as Ohm’s law and rearranged to find each of the values

I = V ÷ R

V = I * R


  1. Describe an experiment to determine resistance using a voltmeter and an ammeter.

Voltmeter measured voltage/p.d. and ammeter measures current, so you can use these devices to determine the resistance between two points in a circuit.

  • Set up an ammeter somewhere in the series circuit; this will give you the amount of current flowing in the circuit.
  • Next set up a voltmeter in parallel to the object, in this case a light bulb, to find the potential difference across it.
  • Using the equation R = V/I , we can find the resistance.


  1. Relate (without calculation) the resistance of a wire to its length and to its diameter.

As the length of the conducting wire increases, the resistance of the current flowing increases. Resistance and length of a wire are directly proportional

The greater the diameter of the wire, the smaller the resistance. Resistance and cross-sectional area of a wire are inversely proportional.


  1. Recall and use quantitatively the proportionality between resistance and length, and the inverse proportionality between resistance and cross-sectional area of a wire.

We now know that resistance is directly proportional to length, denoted as R α length.
Resistance is also inversely proportional to cross-sectional area, denoted as
R α (1 ÷ cross-section area)

Combining these two we get:
R α length ÷ cross-section area OR   R α l ÷ A

we can add a constant ρ (pronounced as rho, in greek alphabet) to rewrite it as:
R = ρ * (l ÷ A)

In order to find the constant (by what proportion resistance changes with respect to length and cross-section area), we can rearrange the formula to:
ρ = R * (A ÷ l)



Notes submitted by Lintha

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4 thoughts on “P12.4 – Resistance

  1. “From this formula, we can conclude that an increase in voltage will increase resistance. Reducing voltage will reduce the resistance.”

    isn’t it the other way around?


        1. Sure, no problem. I understand that you are busy as students as well. It also shows me that I am aware of what I am learning, not just skimming through them. 🙂


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