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 Symbols Display 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 Tip When 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) Switch Convention 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 Circuit Show 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 Physical Explanation 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 Circuit Show 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: fraction = fraction + fraction + fraction Physical Explanation 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 FIG 10.4: Combination Circuit 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 Circuit Problem: 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): fraction = fraction + fraction = fraction = fraction R_P = 3\, Step 2: Add series resistance: R_total = R_1 + R_P = 3 + 3 = 6\, Step 3: Calculate total current: I = fraction = fraction = 2 A Step 4: Calculate p.d.
across R₁: V_1 = IR_1 = 2 × 3 = 6 V Placement of Ammeters and Voltmeters Ammeters Connected in series with the component whose current is being measur…
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