IGCSE Coordinated Science: Current, Electromotive forces and Potential Difference

IGCSE Coordinated Science: Resistance

Resistance

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

Since resistance = potential difference / current:

↑ in Voltage = ↑ in resistance

↑ in Current = ↓ in resistance

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

When you need to calculate resistance, Ohm’s law states that:

R = V/I

Resistance (Ω) = Voltage (V) / Current (I)

For example, a light bulb has a potential resistance of 3 volts. If a current of 0.6 amps is flowing through the lightbulb, what is the resistance?

R = V/I

R = 3 / 0.6

Resistance = 5 Ω

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

A simple experiment can be performed to find out the resistance across an object:

Set up an ammeter somewhere in the series circuit: this will give you the amount of current flowing in the circuit.

Now, set up a voltmeter in parallel to the object, in this case a light bulb, to find the potential difference across it.

Using theequation R = V/I , we can find the resistance.

If the light bulb has a potential difference of 4V, and the circuit has a current of 2A, then the resistance is: 4/2 = 2 Ohms (Ω)

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

Factors Affecting the Resistance of a Conductor:

• Material:
• Some are more conductive than others; this is dependent on a factor known as resistivity.

• Length:
• Length is directly proportional to resistance (i.e. constantan):
  As such when the length of a conductor is doubled, its resistance would also double.
• In this case, a water pipe can serve as a suitable analogy; the longer the pipe, the more energy one must put into delivering water along its full length.

• Cross-sectional area:
• Is inversely proportional to resistance: As such when the cross-sectional area of a conductor is doubled, its resistance is halved.
• A water pipe can again provide a suitable analogy. In the case of a water pipe, the water it is, the wider the pipe is, the easier it is to pump water through it and vice versa.

• Temperature:
• Is proportional to resistance in many cases: For instance in a lamp, resistance increases when temperature rises

5. Relate (without calculation) the resistance of a wire to its length and to its diameter. (Same as point 4, but for core)

• Length:
• Length is directly proportional to resistance.

• Cross-sectional area:
• Is inversely proportional to resistance.
• Remember that for a wire with a circular cross-section, area is determined by the formula