P12.4 – Resistance

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.

 

2. 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

 

3. 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.

 

4. 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.

 

5. 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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