Current electricity
Electric current, e.m.f., potential difference, resistance and the I–V graphs of a metal, a filament lamp and a diode, for G3 Pure and O-Level Physics.
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Before you begin
Current electricity is about charge flowing round a circuit and the energy it carries. The lessons start with current as a flow of charge, add e.m.f. and p.d. as energy per coulomb, then use resistance to link the two and compare the I–V graphs of a metal, a filament lamp and a diode.
Be comfortable with: static electricity, because current is charge in motion.
Learning goals
- State current as rate of charge flow measured in amperes
- Distinguish conventional current from electron flow
- Apply charge equals current multiplied by time
- Define source e.m.f. as work done per unit charge around a circuit
- Calculate total e.m.f. for sources in series
- Define component potential difference as work done per unit charge
- State resistance as potential difference divided by current
- Apply resistance equals potential difference divided by current
- Apply wire-resistance proportionalities for length and cross-sectional area
- Describe the effect of temperature on metallic resistance
- Sketch and interpret required current–voltage characteristics
Syllabus statements covered
- state that current is the rate of flow of charge and that it is measured in amperes
- distinguish between conventional current and electron flow
- recall and apply the relationship charge = current × time to new situations or to solve related problems
- state that the electromotive force (e.m.f.) of a source is the work done per unit charge by the source in driving charges around a complete circuit and that it is measured in volts
- calculate the total e.m.f. where several sources are arranged in series
- state that the potential difference (p.d.) across a component in a circuit is the work done per unit charge in driving charges through the component and that it is measured in volts
- state that resistance = p.d. / current
- apply the relationship R = V / I to new situations or to solve related problems
- recall and apply the relationship of the proportionality between resistance and the length and cross-sectional area of a wire to new situations or to solve related problems
- describe the effect of temperature increase on the resistance of a metallic conductor
- sketch and interpret the I-V characteristic graphs for a metallic conductor at constant temperature (ohmic conductor), for a filament lamp and for a semiconductor diode.
Lessons
Work through them in order.
Current and voltage quantities
- Current and charge flowDefine current as the rate of flow of charge, use Q = It and compare conventional current with electron flow.
- E.m.f. and potential differenceDistinguish e.m.f. from potential difference as energy transferred per unit charge.
- Cells in seriesFind the total e.m.f. of cells in series when they aid or oppose each other.
Resistance and required I–V characteristics
- Resistance and Ohm’s lawUse R = V/I to find resistance, current or p.d., and tell the definition of resistance apart from Ohm’s law.
- Wire resistance and temperatureCompare wires using R ∝ L and R ∝ 1/A, including a change of diameter, and explain why a hot metal resists more.
- I–V graph of a metallic conductorSketch the straight-line I–V graph of a metal at constant temperature and find its resistance.
- I–V graph of a filament lampExplain why a filament lamp's I–V graph curves as it heats up, and find its resistance at a point.
- I–V graph of a semiconductor diodeSketch a diode's I–V graph and explain forward conduction and reverse blocking.
Beyond the syllabus
Practise and check
Or choose
Topic reference
Quick reference
| Quantity | Symbol | Equation | Unit |
|---|---|---|---|
| current | I | I = Q/t | ampere (A), 1 A = 1 C s⁻¹ |
| charge | Q | Q = It | coulomb (C) |
| e.m.f. | E | E = W/Q | volt (V), 1 V = 1 J C⁻¹ |
| p.d. | V | V = W/Q | volt (V) |
| resistance | R | R = V/I | ohm (Ω), 1 Ω = 1 V A⁻¹ |
| cells in series | Eₜₒₜₐₗ | add aiding e.m.f.s, subtract opposing ones | volt (V) |
| wire resistance | R | R ∝ L, R ∝ 1/A | ohm (Ω) |
Core knowledge to remember
- Current is the rate of flow of charge. Conventional current flows from the positive terminal round the circuit to the negative terminal; in a metal, electrons drift the other way.
- E.m.f. is the work done per unit charge by a source in driving charges around a complete circuit.
- P.d. is the work done per unit charge in driving charges through a component.
- Resistance is p.d. divided by current. A metal at constant temperature obeys Ohm’s law: I ∝ V.
- A hotter metal has a larger resistance, because its ions vibrate more and the electrons collide with them more often.
- An ammeter goes in series and has a very small resistance; a voltmeter goes in parallel and has a very large resistance.
Common mistakes
- Using minutes instead of seconds in Q = It.
- Mixing up e.m.f. (energy given by a source) and p.d. (energy transferred in a component).
- Calling every use of R = V/I Ohm’s law; it defines resistance at one point.
- Treating the gradient of an I-against-V graph as the resistance. For a straight line through the origin it is 1/R; for a curve, use V/I at the point.
- Halving the resistance for a doubled diameter: the area, and so the factor, is four.
- Sketching only the positive half of an I–V graph, or drawing a diode’s graph the same on both sides.