Electric Current and Its Effects - Class 7 Science (CBSE)
Based on the 2025-26 NCERT syllabus for Class 7 Science. This chapter explores the practical effects of electric current -- heating, magnetic, and safety mechanisms.
1. Why this chapter matters
Electricity powers our homes, industries, and devices. Understanding the effects of electric current helps us use electricity safely and efficiently. In CBSE exams, this chapter contributes 6-8 marks with diagram-based questions on circuits and electromagnets.
2. Circuit symbols
Electric circuits are drawn using standard symbols instead of realistic pictures. This makes circuit diagrams clean and universally understandable.
Common circuit symbols
| Component | Symbol description |
|---|---|
| Cell | Two parallel lines: longer line (+), shorter thick line (-) |
| Battery | Multiple cells in series |
| Bulb (lamp) | A circle with a cross inside |
| Switch (open) | Two dots with a gap |
| Switch (closed) | Two dots connected by a line |
| Wire | Straight line connecting components |
| Resistor | Zigzag line or rectangle |
| Fuse | A rectangle with a line through it |
3. Heating effect of electric current
When electric current flows through a conductor, the conductor gets heated up. This is called the heating effect of electric current.
Cause
The flow of electrons through the conductor encounters resistance. The energy lost by electrons is converted into heat energy.
Applications
- Electric iron: Heating element (coiled wire) gets hot and irons clothes.
- Electric heater: Nichrome wire coils glow red hot.
- Electric bulb: Tungsten filament heats up and emits light.
- Electric fuse: Wire melts when current exceeds safe limit, breaking the circuit.
Factors affecting heating
- Higher resistance produces more heat.
- Higher current produces more heat.
- Longer duration produces more heat.
4. Magnetic effect of electric current
When electric current flows through a wire, it behaves like a magnet. This is called the magnetic effect of electric current.
Discovery
Hans Christian Oersted discovered in 1820 that a compass needle deflects when placed near a current-carrying wire.
Electromagnet
An electromagnet is a temporary magnet created by passing electric current through a coil wound around a soft iron core.
Properties of electromagnet
- It behaves like a magnet only when current flows.
- Strength can be increased by: increasing the number of turns, increasing the current, or using a soft iron core.
- The polarity can be reversed by reversing the current direction.
Electromagnet vs permanent magnet
| Feature | Electromagnet | Permanent Magnet |
|---|---|---|
| Magnetism | When current flows | Always present |
| Strength | Adjustable | Fixed |
| Polarity | Reversible | Fixed |
| Applications | Cranes, motors | Compasses, speakers |
Applications of electromagnets
- Electric cranes in scrap yards to lift heavy iron objects.
- Electric bells, buzzers, and buzzers.
- Motors, generators, speakers, and MRI machines.
5. Electric fuse
An electric fuse is a safety device that protects electrical circuits from overloading.
How it works
A fuse contains a thin wire with a low melting point. When the current exceeds a safe value, the wire gets heated and melts (blows), breaking the circuit and stopping current flow.
Importance
Fuses prevent damage to appliances and reduce the risk of electrical fires.
6. Miniature Circuit Breaker (MCB)
An MCB is a modern alternative to a fuse. It automatically switches off the circuit when current exceeds a safe limit.
MCB vs fuse
| Feature | Fuse | MCB |
|---|---|---|
| Operation | Wire melts | Switch trips |
| Reusability | Must be replaced | Reset by switching back on |
| Response time | Slower | Very fast |
| Safety | Good | Better |
7. Worked examples
Example 1: Why does an electric bulb get hot but the wires connecting it do not?
The filament of the bulb is made of tungsten, which has high resistance. The connecting wires have very low resistance. Heating is directly proportional to resistance.
Example 2: How can you increase the strength of an electromagnet?
Increase the number of turns in the coil, increase the electric current, or use a soft iron core inside the coil.
Example 3: Draw a circuit diagram showing a battery, a bulb, and a closed switch connected in series.
Draw the battery (longer and shorter parallel lines), connect wire to closed switch (dots connected), then to the bulb (circle with cross), and back to the battery.
8. Common mistakes and how to fix them
| Mistake | Fix |
|---|---|
| Drawing battery with both lines same length | Positive terminal is longer; negative is shorter and thicker |
| Confusing heating and magnetic effects | Heating: thermal energy. Magnetic: magnetic field |
| Forgetting the iron core in electromagnet | An electromagnet needs a soft iron core, not just a coil |
| Thinking fuse protects from high voltage | Fuse protects from EXCESS CURRENT, not voltage |
| Assuming MCB and fuse work identically | MCB resets; fuse must be replaced after blowing |
9. CBSE exam focus
| Question type | Marks | Frequency |
|---|---|---|
| Circuit symbols and diagrams | 2-3 marks | 1 question |
| Heating effect applications | 2 marks | 1 question |
| Electromagnet construction and uses | 3 marks | 1 question |
| Fuse and MCB comparison | 2-3 marks | 1 question |
| Oersted experiment explanation | 2 marks | Occasional |
10. Self-test
- Draw the circuit symbols for: cell, bulb, open switch, closed switch.
- What is the heating effect of electric current? Give two applications.
- Describe how an electromagnet is made.
- Differentiate between a fuse and an MCB.
- How did Oersted demonstrate the magnetic effect of electric current?
- Why is an electromagnet used in a scrap yard crane instead of a permanent magnet?
11. Answer key
- Cell: long and short parallel lines. Bulb: circle with cross. Open switch: dots with gap. Closed switch: dots connected.
- Heat produced when current flows through a conductor. Applications: electric iron, electric heater.
- An electromagnet is made by winding an insulated copper wire around a soft iron core and passing current through it.
- Fuse: wire melts, must be replaced. MCB: switch trips, can be reset.
- Oersted placed a compass near a current-carrying wire and observed the needle deflect, proving current produces a magnetic field.
- An electromagnet can be switched on/off. It can be used to pick up and release scrap as needed. Its strength can also be adjusted.
12. Quick revision
- Heating effect: current produces heat in conductors.
- Magnetic effect: current-carrying wire behaves like a magnet.
- Electromagnet: coil + soft iron core + current.
- Fuse: safety device that melts at excess current.
- MCB: resettable circuit breaker.
- Circuit diagrams use standard symbols for clarity.
- Increasing turns or current strengthens electromagnet.
