Question 9
Describe with the help of a diagram an instrument which can be used to detect a charged body.
The instrument used to detect a charged body is known as electroscope.
It consists of a metal rod on which two leaves of aluminium foil are fixed to one end and a metal disc at the other end. The leaves are kept inside a conical flask, and it is corked to isolate them from the atmospheric air.
When a charged body comes in contact with the metal disc, the aluminium leaves move away from each other because some charges get transferred to aluminium leaves through the metal rod. This process is called charging by conduction. The charges on the leaves and the charged body are of the same nature, and thus, the leaves of aluminium repel each other. If the body is not charged then they would attract each other.
Chapter Overview: Some Natural Phenomena
This chapter covers two major natural phenomena: lightning (caused by static electricity in clouds) and earthquakes (caused by tectonic plate movement). Key concepts include charging by rubbing, types of charges, electroscope, earthing, lightning conductor, seismograph, and Richter scale. Safety measures for both phenomena are practically important.
Board Exam Weightage: 5-7 marks | Difficulty: Moderate
Key Formulas & Concepts
| Concept | Details |
|---|---|
| Law of Charges | Like charges repel; unlike charges attract |
| Conservation of Charge | Total charge before and after rubbing remains the same (electrons transfer, not created) |
| Richter Scale | Logarithmic: each unit increase = 10 times more ground shaking |
| Lightning Conductor | Metal rod on building + copper wire + earth plate = safe path for lightning to ground |
Must-Know Concepts
- Charging by rubbing transfers electrons from one object to another (does not create charge)
- Glass rubbed with silk becomes positive; plastic rubbed with fur/wool becomes negative
- Electroscope detects charge: foil leaves diverge when charged
- Earthing provides a conducting path for excess charge to flow safely to the ground
- Lightning is a massive electrical discharge caused by charge build-up in clouds
- Earthquakes are caused by sudden movement of tectonic plates; measured on Richter scale
Lightning vs Earthquake
| Feature | Lightning | Earthquake |
|---|---|---|
| Cause | Charge build-up in clouds | Tectonic plate movement |
| Energy | Electrical | Mechanical (seismic waves) |
| Warning | Thunder, dark clouds | Usually no warning |
| Measurement | Not measured on standard scale | Richter scale / seismograph |
| Safety | Stay indoors, avoid tall objects/trees | Duck under table, move to open area |
Common Mistakes to Avoid
- Thinking rubbing creates new charges (it only transfers electrons)
- Confusing earthing and lightning conductor (conductor uses earthing principle)
- Thinking Richter scale is linear (it is logarithmic)
- Confusing epicentre (surface) and focus (underground)
Scoring Tips
- Learn safety rules for both lightning and earthquakes (frequently asked)
- Draw and label the electroscope and lightning conductor
- Remember: epi-centre is on top (epi = above), focus is underground
- Practice charge interaction problems: which attract, which repel
Frequently Asked Questions
Q: How many marks does this chapter carry in the exam?
A: Approximately 5-7 marks in the annual exam.
Q: What question types are commonly asked?
A: MCQs (1 mark), Very Short Answer (2 marks), Short Answer (3 marks), and Long Answer / Diagram (5 marks).
Q: Is this chapter important for competitive exams?
A: Yes, concepts from this chapter appear in NTSE, Olympiad, and other science competitions.