Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Quick Summary
Electricity is part of Term 2 in Class 8 Science. This chapter covers electric current, potential difference, resistance, Ohm's law, series and parallel circuits, and the heating effect of electric current — all essential for understanding everyday electrical appliances.
Key Definitions
- Electric current (I): The rate of flow of electric charge; measured in Amperes (A).
- Potential difference (V): The work done per unit charge to move charge between two points; measured in Volts (V).
- Resistance (R): Opposition to the flow of electric current; measured in Ohms (Ω).
- Conductor: A material that allows electric current to flow easily (e.g., copper, aluminium).
- Insulator: A material that does not allow electric current to flow (e.g., rubber, wood, glass).
- Ohm's Law: V = IR (voltage = current × resistance).
- Heating effect: When current flows through a resistance, electrical energy is converted to heat energy.
Key Formulae
| Formula | Meaning |
|---|---|
| V = IR | Ohm's Law |
| P = VI = I²R = V²/R | Electric power |
| H = I²Rt | Heat produced (Joule's law) |
| Series: R = R₁ + R₂ + R₃ | Total resistance in series |
| Parallel: 1/R = 1/R₁ + 1/R₂ | Total resistance in parallel |
Types of Circuits
Series Circuit
- Same current flows through all components.
- Total resistance = sum of individual resistances.
- If one component fails, the whole circuit breaks.
- Voltage is divided among components.
Parallel Circuit
- Same voltage across all components.
- Total resistance is less than the smallest individual resistance.
- If one component fails, others continue to work.
- Current is divided among branches.
Heating Effect of Current
- When current flows through a resistance, heat is generated.
- Given by Joule's law: H = I²Rt
- Applications: electric heater, electric iron, toaster, incandescent bulb, fuse wire.
- Fuse wire: melts when excess current flows, protecting the circuit.
Symbols Used in Circuit Diagrams
- Cell: short and long parallel lines
- Battery: multiple cells
- Switch: break in the line
- Resistance: zig-zag line
- Ammeter (A): measures current, connected in series
- Voltmeter (V): measures voltage, connected in parallel
Important Questions
One Mark
- State Ohm's law.
- What is the SI unit of resistance?
- Name one application of the heating effect of current.
- In which type of circuit is the same current maintained through all components?
- What is the function of a fuse in an electric circuit?
Short Answer (3 marks)
- Distinguish between series and parallel circuits with two differences each.
- Calculate the resistance if the voltage is 12 V and current is 3 A.
- Explain the heating effect of electric current with one application.
Long Answer (5 marks)
- State Ohm's law. Draw a circuit diagram to verify Ohm's law. Plot a V-I graph and explain what the slope represents.
- Explain series and parallel combinations of resistances with formulae and circuit diagrams. Give one practical application of each.
Common Mistakes
- Confusing ammeter and voltmeter connections (ammeter in series, voltmeter in parallel).
- Forgetting to include units in numerical answers.
- Using incorrect formula for parallel resistance.
Revision Checklist
- [ ] Textbook reading completed
- [ ] Ohm's law formula and numerical practice completed
- [ ] Circuit diagrams drawn for series and parallel circuits
- [ ] Heating effect applications listed
- [ ] Important questions answered in notebook
Frequently Asked Questions
At constant temperature, the current flowing through a conductor is directly proportional to the potential difference (voltage) across its ends. V = IR.