Electricity — Class 8 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 8 Science, Physics — Chapter 5. Static charges, circuits and the effects of current.
1. About this chapter
This chapter covers static electricity (charging by friction and induction), the electroscope, conductors and insulators, the electric circuit, series and parallel circuits, the effects of current, and the fuse.
2. Static electricity
- Rubbing two bodies produces static charge by the transfer of electrons. The body that gains electrons becomes negative; the one that loses electrons becomes positive.
- Example: when an ebonite rod is rubbed with fur, electrons move to the ebonite — the fur becomes positive and the ebonite negative.
- Like charges repel; unlike charges attract.
3. Electroscope and charging by induction
- An electroscope is a device used to detect electric charge and to charge a body by induction.
- Charging by induction: a charged body brought near an uncharged body induces an opposite charge on the near side (no contact needed).
4. Circuits, effects and the fuse
- Conductors (metals, salt water) carry current; insulators (rubber, plastic) do not.
- A simple circuit must have a battery (cell), connecting wires and a switch (and a device such as a bulb).
- In a parallel circuit, the voltage is the same across all components; in a series circuit the same current flows through each.
- Effects of current: heating (heater), magnetic (electromagnet), chemical (electroplating).
- A fuse is a protective device that breaks the circuit when too much current flows.
5. Worked examples
Example 1. When ebonite is rubbed with fur, what charge does the fur get? Positive (it loses electrons to the ebonite).
Example 2. What does an electroscope do? It detects electric charge and can charge a body by induction.
Example 3. Name the three things a simple circuit must have. A battery, wires and a switch.
6. Book-back questions (Samacheer Kalvi)
I. Choose the correct answer
- When an ebonite rod is rubbed with fur, the charge acquired by the fur is — (a) positive / (b) negative. Ans: (a) positive.
- Electrification of two bodies by rubbing is due to the transfer of — (a) protons / (b) electrons. Ans: (b) electrons.
- A simple circuit must have — (a) battery, wire, switch / (b) only a wire. Ans: (a) battery, wire, switch.
- A fuse is a — (a) decorative item / (b) protective device for breaking a circuit. Ans: (b) protective device.
II. True or False 5. A charged body induces an opposite charge on an uncharged body brought near it. — True. 6. An electroscope is used to charge a body by induction. — True. 7. Water can conduct electricity. — True.
III. Answer briefly 8. What is charging by induction? 9. Differentiate conductors and insulators with an example each.
7. Common mistakes
- Mistake: Saying rubbing creates new charge. Fix: Rubbing only transfers electrons between bodies.
- Mistake: Thinking pure water conducts well. Fix: Pure water is a poor conductor; salt water conducts.
- Mistake: Mixing up series and parallel. Fix: Parallel → same voltage; series → same current.
8. Quick revision
- Physics Ch 5 · static electricity, electroscope, circuits, effects.
- Charging by friction = transfer of electrons; gain → negative, lose → positive.
- Like charges repel, unlike attract; electroscope detects charge / charges by induction.
- Simple circuit: battery + wire + switch; parallel → same voltage.
- Effects: heating, magnetic, chemical; fuse breaks the circuit on overload.
