Current of Electricity: Complete O Level Physics Cheatsheet
O Level Physics, Chapter 16 · Read time: ~9 minutes
These notes come from TGC Academy's O Level Physics tuition in Singapore.
Key Formulas
Current and charge
Current is the rate of flow of charge, measured in amperes (A). Q = It.
Conventional current flows from the positive terminal to the negative terminal. Electron flow is in the opposite direction, from negative to positive, because electrons are negatively charged.
An ammeter is connected in series and has a very low resistance.
Trap: Circuit diagrams always show conventional current. Only use the electron flow direction when a question specifically asks about electrons.
E.m.f. and potential difference
Electromotive force (e.m.f.): the work done per unit charge by the source in driving charge around a complete circuit. Unit: volt.
Potential difference (p.d.): the work done per unit charge in driving charge through a component. Unit: volt.
Cells in series, all facing the same way, have a total e.m.f. equal to the sum of their e.m.f.s. A cell connected the opposite way subtracts.
A voltmeter is connected in parallel across a component and has a very high resistance.
Trick: Think of e.m.f. as energy given to the charge by the source, and p.d. as energy the charge gives up in a component. Both are 'work done per unit charge', the difference is where the energy goes.
Resistance and I-V graphs
Resistance = p.d. ÷ current, R = V/I, measured in ohms (Ω).
For a wire, resistance is proportional to length and inversely proportional to cross-sectional area. Doubling the length doubles R. Doubling the area halves R.
For a metallic conductor, resistance increases with temperature, because the vibrating atoms obstruct the flow of electrons more.
Ohmic conductor (metal wire at constant temperature): a straight line through the origin, so R is constant.
Filament lamp: the curve bends towards the voltage axis, because the filament heats up and its resistance increases as current rises.
Semiconductor diode: almost no current in the reverse direction, then current rises sharply in the forward direction once a small voltage is passed.
Trap: Resistance at a point on an I-V graph is V ÷ I at that point, not the gradient. For a curve like the filament lamp's, the gradient gives a different, wrong answer.
Worked Example
A 3.0 m length of nichrome wire has a resistance of 6.0 Ω. It is connected to a 12 V supply for 2 minutes.
(a) Calculate the current in the wire. [1]
I = V / R = 12 ÷ 6.0 = 2.0 A
(b) Calculate the charge that flows through the wire in 2 minutes. [2]
t = 2 × 60 = 120 s
Q = It = 2.0 × 120 = 240 C
(c) The wire is replaced by one of the same material, 1.5 m long, with twice the cross-sectional area. Calculate its resistance. [2]
Halving the length halves R: 6.0 → 3.0 Ω.
Doubling the area halves R again: 3.0 → 1.5 Ω.
Why this question is a good test of the topic: it combines a time conversion with Q = It, then tests proportional reasoning on resistance, where students often change only one factor or multiply instead of divide.
Frequently Asked Questions
What is the difference between conventional current and electron flow?
Conventional current flows from positive to negative. Electrons actually flow from negative to positive.
What is the difference between e.m.f. and potential difference?
E.m.f. is the work done per unit charge by the source around the whole circuit. P.d. is the work done per unit charge through one component.
How does the length of a wire affect its resistance?
Resistance is proportional to length. A wire twice as long has twice the resistance.
Why is the I-V graph of a filament lamp curved?
As the current increases, the filament gets hotter and its resistance increases, so current rises less and less for each extra volt.
What does a diode's I-V graph show?
It allows current in one direction only. There is almost no current in reverse, and current rises steeply in the forward direction.
I-V graphs still not clicking?
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