Home » Handbook of O Level Physics » UNIT 18: CURRENT ELECTRICITY


  • Current: It is the rate of flow of electric charge.

    I = Q / t

    • Unit: A

    • Current is measured using an ammeter or galvanometer.

  • A short-circuit happens when load (resistance) is bypassed and excess current flows due to lack of resistance.

    • A short-circuit may cause heating and start a fire.

  • Electromotive force (e.m.f.) of a source: It is the energy converted by a source from non-electrical form to electrical form in driving a unit charge round a complete circuit.

    e.m.f. = E / Q

  • Potential difference between two points: It is the energy converted from electrical to other forms when a unit charge passes between the two points.

    V = E / Q

  • A comparison between e.m.f. and potential difference:

    • Similarities

      • Both involve energy and charge

      • Both are measured in volts.

      • Both are measured by voltmeter.

    • Differences

      • In e.m.f. the energy change is from non-electrical to electrical. In p.d. the energy change is from electrical to non-electrical.

      • e.m.f. is property of source. p.d. is property of part of circuit.

  • Volt: The potential difference between two points in a conductor is one volt if one joule of energy is converted from electrical to other forms when one coulomb of positive charge flows through it.

  • A voltmeter is connected in parallel.

    • Voltmeters have a high resistance.

  • An ammeter is connected in series.

    • Ammeters have a low resistance.

  • Resistance: It is the ratio of potential difference across a material to the current flowing in it.

    R = V / I

    • Unit: Ω

  • Resistors are used in circuits to limit the flow of current. Their types are:

    • Fixed resistors

    • Variable resistors (rheostats)

  • Ohm’s Law: It states that current is directly proportional to potential difference provided that temperature does not change.

    V ∝ I

    • Limitations: Ohm’s Law is applicable to

      • Pure metals

      • at constant temperature

  • Resistance and resistivity

    • The constant called resistivity is denoted by ρ.
    • Every material has its own value of resistivity.
    • Resistivity can be used to tell good conductors from bad.
    • It is better to say that copper has low resistivity than to say copper has low resistance.

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