Electricity and Magnetism — Question 18
Back to all questionsQuestion 18
Draw a neat and labelled diagram of an electric bell and describe its working.
Labelled diagram of the electric bell is shown below:
Working of electric bell:
- When the switch (bell push) is pressed, current flows through the coil of electromagnet.
- The electromagnet gets magnetized and attracts the armature. As the armature moves the hammer attached to it moves and strikes the gong so the bell rings.
- During this process the contact between the metallic springy strip and adjusting screw breaks so the circuit becomes incomplete.
- The current stops flowing through the coil of electromagnet and it gets demagnetized.
- As the electromagnet gets demagnetized, the armature comes back to its original position due to spring action of the metallic springy strip and comes in contact with the adjusting screw.
- The circuit becomes complete, if the bell push is pressed again, current will flows through the coils of electromagnet and it gets magnetized.
- It attracts the armature and hammer attached with it strikes the gong again and the bell rings.
- This process of making and breaking the circuit continues and the bell rings as long as the bell push remains pressed.
ICSE Class 7 Physics — Electricity: Complete Study Guide
Electricity is the final chapter in ICSE Class 7 Physics, carrying approximately 12 marks. This introductory chapter covers static electricity, electric current, simple circuits, conductors and insulators, the dry cell, and electrical safety. It builds the foundation for the more detailed study of current electricity, Ohm's law, and electromagnetism in higher classes.
Students begin with static electricity — the build-up of charges on surfaces through friction. Understanding that only electrons (negative charges) transfer during friction, while protons remain fixed in the nucleus, is fundamental. The chapter then transitions to current electricity — the continuous flow of charges through a closed conductor. Drawing and interpreting circuit diagrams using standard symbols is a key practical skill that is tested heavily in exams.
The distinction between conductors (materials that allow current flow, like metals) and insulators (materials that block current, like rubber and plastic) connects to real-world safety design — why wires have copper inside and plastic outside. The structure of a dry cell, which converts chemical energy to electrical energy, is another important topic. The chapter concludes with essential electrical safety precautions that every student should know.
| Concept | Key Points |
|---|---|
| Electric Charge | Positive (+) and negative (−); like repel, unlike attract |
| Static Electricity | Charging by friction; electrons transfer, not protons |
| Electric Current | Flow of charges; SI unit: Ampere (A) |
| Dry Cell | Chemical energy → Electrical energy; carbon rod (+), zinc casing (−) |
Must-Know Concepts
- Like charges repel, unlike charges attract
- Friction causes electron transfer — object losing electrons becomes +, gaining becomes −
- Current flows only in a closed (complete) circuit
- Standard circuit symbols for cell, battery, bulb, switch must be memorised
- Conductors have free electrons (metals); insulators have bound electrons (rubber, plastic)
- Dry cell: carbon rod = +, zinc casing = −
Common Mistakes
- Saying "protons transfer" during friction charging — only ELECTRONS move
- Drawing cell symbol incorrectly — long thin line = +, short thick line = −
- Saying "current is used up by the bulb" — current flows through and returns to cell
- Leaving circuits open in diagrams — always connect wires back to the cell
Exam Tips
- Draw circuit diagrams with a ruler using standard symbols — neatness counts
- For static electricity, explain the electron transfer mechanism for full marks
- Use comparison tables for conductor vs insulator questions
- Label all parts of the dry cell diagram — this is a common diagram question
- Safety questions are easy marks — list at least 5 rules
How does a comb attract paper bits after rubbing on hair?
When a comb is rubbed on dry hair, electrons transfer from hair to the comb, making the comb negatively charged. When brought near neutral paper bits, the charged comb induces a charge separation in the paper — positive charges are pulled closer and negative charges are pushed away. The attraction between the nearby positive charges and the negative comb is stronger than the repulsion with the farther negative charges, so the paper bits are attracted to the comb.
What is the difference between a conductor and an insulator?
Conductors are materials that allow electric current to flow through them because they contain free electrons. Examples: copper, aluminium, iron. Insulators are materials that do not allow current to flow because their electrons are tightly bound to atoms. Examples: rubber, plastic, wood, glass.
Why do electrical wires have plastic coating?
The inner copper wire is a conductor that carries current. The outer plastic coating is an insulator that prevents the current from reaching our hands or other objects, protecting us from electric shocks. This combination provides both efficient current flow and safety.
What is the difference between an open and closed circuit?
A closed circuit has a complete, unbroken path for current to flow — the switch is ON, and devices work. An open circuit has a break in the path (switch OFF, broken wire, or disconnected component) — current cannot flow, and devices do not work.