Electrical CircuitsOxford AQA IGCSE Physics: Revision notes
Section 1
How do charges move in a circuit?
Metals conduct electricity easily because they contain free electrons — electrons not bound to a particular atom that can drift through the metal lattice when a potential difference is applied.
Current is the rate of flow of charge:
I = Q / t
where I is current (amps, A), Q is charge (coulombs, C), and t is time (seconds, s).
The voltage (potential difference) of a source is the energy it supplies per unit charge that passes through it, measured in volts (V). The potential difference across a component is the energy transferred per unit charge as charge passes through it:
V = E / Q
Always quote units — a current with no unit or wrong unit loses the mark even if the number is correct.
Section 2
What is static electricity?
Static electricity is an imbalance of charge on an object, caused by friction transferring electrons from one insulating surface to another. Because there is no conducting route for the charge to flow away, it stays put until it finds a path — a discharge.
- If enough charge builds up, it can jump across a gap as a spark
- Lightning is a large-scale discharge between a charged cloud and the ground (or between clouds)
Section 3
How do we draw and use circuit components?
Circuit diagrams use standard symbols for: switch (open/closed), cell, battery, lamp, fuse, voltmeter, ammeter, diode, resistor, variable resistor, LDR, LED and thermistor.
- A thermistor is a resistor whose resistance depends on temperature — used in thermostats to switch heating on/off
- An LDR (light-dependent resistor) has resistance that depends on light intensity — used to switch lights on automatically when it gets dark
A streetlight uses an LDR: as light falls, the LDR's resistance rises, less current flows, and a switching circuit turns the lamp on.
Section 4
What is resistance and how do components behave?
Resistance opposes the flow of current. The three quantities are linked by:
V = I x R
For a given potential difference, greater resistance means smaller current.
An I-V characteristic is a graph of current against potential difference for a component:
| Component | I-V behaviour |
|---|---|
| Resistor (constant temperature) | straight line through the origin — current directly proportional to pd (Ohm's Law) |
| Filament lamp | curve that flattens as pd increases — resistance increases as temperature increases |
| Diode | current flows easily in the forward direction (low resistance) but almost not at all in reverse (very high resistance) |
| Thermistor | resistance decreases as temperature increases |
| LDR | resistance decreases as light intensity increases |
An LED emits light only when current flows in the forward direction, and uses much less current than other forms of lighting for a similar brightness.
When charge flows through a resistor, moving charges collide with the stationary atoms of the material, causing heating.
Students often say a filament lamp 'obeys Ohm's Law' — it does not, because its resistance changes with temperature, so the I-V graph is not a straight line.
Required practical: investigate the I-V characteristics of a filament lamp, a diode, and a resistor at constant temperature — be ready to describe the method and control variables.
Section 5
How do series and parallel circuits differ?
| Series | Parallel | |
|---|---|---|
| Combined resistance | sum of individual resistances | less than the smallest individual resistance |
| Current | same at every point in the circuit | splits between branches, and recombines |
| Potential difference | shared between components | the same across each parallel branch |
In a series circuit, if one component fails, the whole circuit breaks. In a parallel circuit, each branch can operate independently.
Must Know
- I = Q/t and V = E/Q
- Free electrons allow metals to conduct; static electricity is a charge imbalance with no conducting path, released by discharge
- V = I x R; resistors at constant temperature obey Ohm's Law (straight-line I-V graph)
- Thermistor resistance falls as temperature rises; LDR resistance falls as light rises; filament lamp resistance rises as temperature rises
- Diodes/LEDs conduct in the forward direction only, with very high reverse resistance
- Series: same current, resistances add, pd shared. Parallel: same pd across branches, combined resistance is lower than any single branch
That's the notes covered.
Carry on to the next subtopic.