Electrical Circuits (Copy)
1. Core Concepts
Key Quantities
| Quantity | Symbol | Unit | Measured Using |
|---|---|---|---|
| Current | I | Ampere (A) | Ammeter (in series) |
| Voltage (Potential Difference) | V | Volt (V) | Voltmeter (in parallel) |
| Resistance | R | Ohm (Ω) | Calculated via Ohm’s Law |
| Charge | Q | Coulomb (C) | Q = I × t |
| Energy | E | Joule (J) | E = V × Q or E = VIt |
Ohm’s Law
V = I × R
(Voltage = Current × Resistance)
Use to calculate any of the three when two are known.
2. Objectives of Electrical Circuit Experiments
- Investigate the relationship between current, voltage, and resistance
- Determine the resistance of a component
- Compare how current and voltage behave in series and parallel circuits
- Study V–I characteristics of resistors, filament lamps, and diodes
3. Common Apparatus
| Apparatus | Use |
|---|---|
| Power supply or dry cells | Source of voltage |
| Ammeter | Measures current (in series) |
| Voltmeter | Measures voltage (in parallel) |
| Variable resistor (rheostat) | To vary current/voltage |
| Fixed resistor / filament bulb / diode | Components under test |
| Connecting wires + switch | Complete circuit |
| Multimeter (sometimes) | Measures V, I, R in one tool |
4. Standard Experimental Setups
A. To Investigate Ohm’s Law (V ∝ I)
Objective: Show that voltage across a resistor is directly proportional to the current through it.
Method:
- Set up circuit with resistor, ammeter in series, and voltmeter in parallel.
- Use a variable resistor or vary voltage to change current.
- Record multiple readings of V and I.
- Plot a graph of V (y-axis) vs I (x-axis).
Result:
- Straight line through origin = Ohm’s Law obeyed
- Gradient = Resistance (R)
B. Filament Lamp V–I Characteristics
Objective: Investigate non-linear resistance of a filament lamp.
Observation:
- As current increases, filament heats up, resistance increases
→ Graph curves (not straight) — Ohm’s Law is not obeyed
C. Series vs Parallel Circuits (Current & Voltage Distribution)
| Quantity | Series Circuit | Parallel Circuit |
|---|---|---|
| Current | Same in all components | Splits at branches |
| Voltage | Splits across components | Same across each branch |
Typical Task:
- Build both circuits
- Measure V and I in different parts
- Record and compare how values differ
D. Measuring Resistance of a Wire
Objective: Find resistance of wire using measurements of V and I
Method:
- Connect wire to circuit
- Measure current (A) and voltage (V)
- Calculate:
R = V / I - Repeat with different lengths of wire to see effect of length on R
5. Sample Data Table (Ohm’s Law)
| Voltage (V) | Current (A) | Resistance (Ω) |
|---|---|---|
| 0.5 | 0.10 | 5.0 |
| 1.0 | 0.20 | 5.0 |
| 1.5 | 0.30 | 5.0 |
→ Constant R = straight line graph
6. Graphical Analysis
| Graph | Interpretation |
|---|---|
| V vs I (resistor) | Straight line → constant R |
| V vs I (lamp) | Curved graph → resistance increases |
| I vs V (diode) | Current flows only above threshold in one direction |
✔️ Label axes:
- “Voltage / V”
- “Current / A”
✔️ Use best-fit line or smooth curve
✔️ Use gradient to determine resistance
7. Common Errors and How to Avoid Them
| Error | Fix |
|---|---|
| Ammeter placed in parallel | Must be in series |
| Voltmeter placed in series | Must be in parallel |
| Readings fluctuate | Wait for needle/digital value to stabilize |
| Loose connections | Ensure tight and clean connections |
| Changing two variables at once | Vary only one variable at a time |
8. Variables to Control and Identify
| Type | Example |
|---|---|
| Independent | Voltage (by varying power supply or rheostat) |
| Dependent | Current through component |
| Control | Temperature, same resistor, same circuit layout |
9. Improvement Suggestions
- Use digital meters for more precise readings
- Switch off between readings to avoid heating wires
- Take repeat readings and average current
- Use fixed resistors if consistent resistance is needed
- Use low voltage to protect components
10. ATP-Style Question Types
| Question | Example |
|---|---|
| Design an experiment | “Describe how to determine the resistance of a wire using a voltmeter and ammeter.” |
| Draw a circuit | “Draw a circuit to measure current through and voltage across a filament bulb.” |
| Identify error | “Voltmeter is connected incorrectly.” |
| Plot graph | “Plot V vs I and determine resistance.” |
| Compare readings | “Explain why voltage across resistors is different in series.” |
| Suggest improvement | “Use a digital voltmeter for more precise readings.” |
11. Exam Tips
- Always quote:
→V = IR
→R = V / I
→Q = ItorE = VIt - Use correct units:
→ A, V, Ω, J, C, s - When asked for total resistance:
→ Series:R_total = R₁ + R₂ + ...
→ Parallel:1/R_total = 1/R₁ + 1/R₂ + ... - When drawing circuits:
- Power supply symbol → ⎓
- Ammeter = circle with A
- Voltmeter = circle with V
- Resistor = rectangle
- Diode = triangle with line
- Variable resistor = resistor with arrow
- Always write:
“Repeat and average readings to improve reliability.”
“Ensure all meters are correctly connected and zeroed before starting.”