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Current: It is the rate of flow of electric charge.
I = Q / t
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Unit: A
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Current is measured using an ammeter or galvanometer.
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A short-circuit happens when load (resistance) is bypassed and excess current flows due to lack of resistance.
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A short-circuit may cause heating and start a fire.
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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
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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
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A comparison between e.m.f. and potential difference:
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Similarities
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Both involve energy and charge
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Both are measured in volts.
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Both are measured by voltmeter.
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Differences
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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.
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e.m.f. is property of source. p.d. is property of part of circuit.
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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.
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A voltmeter is connected in parallel.
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Voltmeters have a high resistance.
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An ammeter is connected in series.
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Ammeters have a low resistance.
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Resistance: It is the ratio of potential difference across a material to the current flowing in it.
R = V / I
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Unit: Ω
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Resistors are used in circuits to limit the flow of current. Their types are:
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Fixed resistors
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Variable resistors (rheostats)
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Ohm’s Law: It states that current is directly proportional to potential difference provided that temperature does not change.
V ∝ I
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Limitations: Ohm’s Law is applicable to
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Pure metals
- at constant temperature
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- 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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