G3 Physics and O-Level Current Electricity Hub
G3 Physics and O-Level Current Electricity hub: electric current, e.m.f., potential difference, resistance and I–V characteristic graphs.
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
Current electricity links charge flow to energy transfer and resistance. The main route covers current, voltage quantities, resistance and the three required I–V characteristics; the thermistor comparison is kept separate as optional support.
Start here
Recall charge and electron transfer, then study:
- Electric current, direction and Q = It
- E.m.f. and potential difference
- Sources in series
- Resistance, wire dimensions and temperature
- Metallic conductor I–V graph
- Filament lamp I–V graph
- Semiconductor diode I–V graph
Use the I–V Characteristics Lab after lessons 5–7 to connect circuit readings to graph shapes.
Lessons
Current and voltage quantities
Electric Current
Current, conventional direction, electron flow and Q = It.
E.M.F. & Potential Difference
E.m.f. and p.d. as distinct energy-per-charge quantities.
Cells in Series
Total e.m.f. for aiding and opposing series sources.
Resistance and required I–V characteristics
Resistance Basics
R = V/I, wire dimensions and temperature effects in metals.
Metallic Conductors
Complete straight-line characteristic at constant temperature.
Filament Lamps
The symmetric curve caused by increasing filament temperature.
Semiconductor Diodes
Forward conduction, reverse blocking and the full asymmetric graph.
Supporting transfer: thermistor I–V graph
The thermistor I–V graph is not required in Topic 14. Use it only to compare a component whose resistance decreases as it heats. Thermistors become core later as input transducers in potential-divider circuits.
What you will learn
Each group below points to the lesson where the idea is explained and practised.
| You need to be able to… | Main lesson |
|---|---|
| define current and the ampere, distinguish conventional current from electron flow, and apply Q = It | Electric current |
| distinguish e.m.f. from potential difference as energy per unit charge | E.m.f. and potential difference |
| calculate the total e.m.f. of series sources, including opposing sources | Sources in series |
| apply R = V/I and explain how wire dimensions and temperature affect resistance | Resistance |
| sketch and interpret the complete I–V characteristics of a metallic conductor, filament lamp and diode | Metallic conductor, filament lamp and diode |
Revision
Definitions and equations
| Quantity | Symbol | Formula | Unit |
|---|---|---|---|
| Current | I | I = Q/t | ampere (A) |
| Charge | Q | Q = It | coulomb (C) |
| E.m.f. or p.d. | V | V = W/Q | volt (V) |
| Resistance | R | R = V/I | ohm (Ω) |
Use 1 A = 1 C s⁻¹, 1 V = 1 J C⁻¹ and 1 Ω = 1 V A⁻¹ as unit checks. An ammeter goes in series; a voltmeter goes in parallel across the component.
Exam traps
- Conventional current and electron drift point in opposite directions in a metal.
- E.m.f. is energy supplied per coulomb by a source; p.d. is energy transferred per coulomb through a component.
- R = V/I defines resistance at a point. Ohm’s law additionally requires I ∝ V at constant physical conditions.
- On an I-against-V straight line, gradient is 1/R; on a V-against-I line, gradient is R.
- Required graph sketches should show both positive and negative regions, especially the diode’s reverse-bias branch.
Practice
G3 Physics / O-Level practice check
The Current of Electricity check covers current and charge, e.m.f. and p.d., then resistance and I–V graphs, and points you to the lesson to revisit first.
- Start with the Current Electricity Quiz for quick error detection.
- Then complete Structured Current Electricity for definitions, calculations and graph explanations.
- Use the I–V Characteristics Lab to gather paired readings and compare V/I at different operating points.
- If definitions remain mixed, retake the Current of Electricity check, then revisit the first lesson it identifies.
Continue learning
Continue to D.C. Circuits, where current, p.d. and resistance are applied to series, parallel and potential-divider networks. For deeper, out-of-syllabus treatment such as drift velocity and resistivity, use A-Level Current of Electricity.
Course and syllabus information
- Course
- SEC G3 Physics
- Edition
- SEC G3 Physics 2027