Drawing circuits
Section: D.C. Circuits | Syllabus: Cambridge AS Level Physics (9702)
Circuit Symbols
Circuit diagrams use standardised symbols to represent electrical components. You must be able to recognise, draw, and use all symbols listed in the Cambridge 9702 syllabus.
FIG 10.1: Standard Circuit SymbolsDisplay a reference table showing all required circuit symbols arranged in a grid format: Row 1: Cell (single line long, short), Battery (multiple cells), Switch (open and closed positions). Row 2: Resistor (rectangular box), Variable resistor (resistor with arrow through it), Thermistor (resistor with diagonal line through it). Row 3: Light-dependent resistor/LDR (resistor in circle with arrows pointing inward), Lamp/Filament bulb (circle with cross), Light-emitting diode/LED (diode symbol with arrows pointing outward). Row 4: Ammeter (circle with A), Voltmeter (circle with V), Galvanometer (circle with G). Row 5: Diode (triangle pointing to line), Capacitor (two parallel lines), Motor (circle with M). Each symbol should be clearly labelled with its component name.
Exam TipWhen drawing circuit symbols, ensure they are neat and correctly proportioned. A cell has one long line (positive terminal) and one short line (negative terminal). A battery consists of multiple cells in series.
Interpreting Circuit Diagrams
A complete electric circuit requires:
- A source of potential difference (e.g., cell, battery, power supply)
- A complete, unbroken conducting path through which charge can flow
- One or more components that transfer electrical energy to other forms
When interpreting circuit diagrams:
- Identify whether components are connected in series or parallel
- Trace the path of current from the positive terminal through the circuit
- Identify all junctions where current divides or combines
- Note the position of measuring instruments (ammeters and voltmeters)
A closed switch completes the circuit (current flows - circuit is ON). An open switch breaks the circuit (no current flows - circuit is OFF). This is opposite to a water tap!
Series Circuits
In a series circuit, components are connected end-to-end so that there is only one path for current to flow.
FIG 10.2: Series CircuitShow a circuit with a battery connected to three resistors (R₁, R₂, R₃) in series. Include an ammeter in series with the resistors. Label the current I as the same through all components. Show voltmeters connected in parallel across each resistor measuring V₁, V₂, and V₃.
Properties of Series Circuits
| Property | Series Circuit Rule |
|---|---|
| Current | Same through all components: I_total = I_1 = I_2 = I_3 |
| Potential Difference | Divides between components: V_total = V_1 + V_2 + V_3 |
| Resistance | Adds up: R_total = R_1 + R_2 + R_3 |
Current is the same throughout because charge is conserved - electrons cannot accumulate at any point. The p.d. divides because the total energy transferred per coulomb equals the sum of energy transferred in each component.
Parallel Circuits
In a parallel circuit, components are connected across the same two points, providing multiple paths for current to flow.
FIG 10.3: Parallel CircuitShow a circuit with a battery connected to three resistors (R₁, R₂, R₃) arranged in parallel. Show the main current I splitting at a junction into I₁, I₂, and I₃ through each branch, then recombining. Indicate that all three resistors have the same p.d. V across them.
Properties of Parallel Circuits
| Property | Parallel Circuit Rule |
|---|---|
| Current | Divides between branches: I_total = I_1 + I_2 + I_3 |
| Potential Difference | Same across all branches: V_total = V_1 = V_2 = V_3 |
| Resistance | Reciprocals add: 1R_total = (1/R_1) + (1/R_2) + (1/R_3) |
The p.d. is the same across parallel branches because both ends of each branch connect to the same two points. Current divides because charge carriers can take different paths - like water in a river splitting around an island.
Combination Circuits
Most practical circuits contain both series and parallel sections. To analyse these:
- Identify which components are in series and which are in parallel
- Calculate the combined resistance of parallel sections first
- Then add series resistances to find total resistance
- Use V = IR to find currents and potential differences
Show a circuit with a 12V battery connected to: resistor R₁ (3Ω) in series with a parallel combination of R₂ (6Ω) and R₃ (6Ω). Label all components with their resistance values.
Worked Example: Combination CircuitProblem: For the circuit above, calculate (a) the total resistance, (b) the total current, and (c) the p.d. across R₁.
Solution:
Step 1: Find combined resistance of parallel section (R₂ and R₃): (1/R_P) = (1/6) + (1/6) = (2/6) = (1/3) R_P = 3\,Ω Step 2: Add series resistance: R_total = R_1 + R_P = 3 + 3 = 6\,Ω Step 3: Calculate total current: I = (V/R) = (12/6) = 2 A Step 4: Calculate p.d. across R₁: V_1 = IR_1 = 2 × 3 = 6 VPlacement of Ammeters and Voltmeters
Ammeters
- Connected in series with the component whose current is being measured
- Must have very low resistance (ideally zero) so they do not significantly reduce the current in the circuit
- All the current being measured must pass through the ammeter
Voltmeters
- Connected in parallel across the component whose p.d. is being measured
- Must have very high resistance (ideally infinite) so they draw negligible current from the circuit
- Measures the energy transferred per unit charge between two points
Show a simple circuit with a battery, lamp, ammeter (in series with the lamp), and voltmeter (in parallel across the lamp). Use arrows to indicate current flow. Label the ammeter as measuring current through the lamp and the voltmeter as measuring p.d. across the lamp.
Why These Requirements?An ammeter with high resistance would reduce the current it is trying to measure. A voltmeter with low resistance would draw significant current, affecting the circuit and giving an inaccurate reading of the true p.d.
Common Exam Mistakes
- Drawing an ammeter in parallel (it must be in series)
- Drawing a voltmeter in series (it must be in parallel)
- Forgetting to take the reciprocal at the end when calculating parallel resistance
- Confusing open switch (OFF) with closed switch (ON)
- Assuming current is "used up" as it passes through components - current is the same in series
- Adding resistances directly when they are in parallel
Concept Check Questions
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