By Subhash Kumar | Science Educator | UREducator
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Electric Current and Its Effects
Complete Teaching Resource for CBSE Class 7 Science Chapter 10
Class: VII
Subject: Science
Chapter: 10
Duration: 4 Hours
What Students Will Achieve
Knowledge
- Understand electric circuit components and their symbols
- Explain heating and magnetic effects of electric current
- Describe the construction and working of an electromagnet
- Identify safety devices like fuses and MCBs
Understanding
- Explain why wires heat up when current flows
- Understand how electromagnets work
- Analyze circuit diagrams and identify components
- Explain the importance of fuses in electrical safety
Skills
- Draw circuit diagrams using standard symbols
- Construct simple electric circuits
- Build and test an electromagnet
- Identify problems in simple circuits
Prior Knowledge Assessment
Starter Questions:
- What happens when you switch on a torch?
- Why do some electrical appliances like heaters become hot?
- Have you ever seen a crane lifting heavy metal objects? How might it work?
Understanding Electric Circuits
Circuit Components and Symbols
Electric Cell
Longer line = positive terminal
Electric Bulb
Switch (ON)
Switch (OFF)
Battery
Combination of cells
Connecting Wire
Building a Battery
Cell vs Battery
- A cell is a single unit that converts chemical energy to electrical energy
- A battery is a combination of two or more cells connected together
- Positive terminal of one cell connects to negative terminal of next cell
Making a Cell Holder
- Materials needed: Wooden block, iron strips, rubber bands
- Iron strips act as conductors
- Rubber bands hold cells tightly in place
- Ensures proper connection between cells
Activity: Drawing Circuit Diagrams
Objective: Practice representing circuits using standard symbols
- Draw the circuit shown in Fig. 10.7 of NCERT textbook
- Convert it to a circuit diagram using symbols
- Compare with Fig. 10.8 in textbook
- Draw alternative circuit layouts that would work the same way
- Identify problems in faulty circuits and correct them
Heating Effect of Electric Current
Understanding the Heating Effect
Why Wires Heat Up
- When current flows, electrons collide with atoms in the wire
- These collisions produce heat
- More current = more collisions = more heat
- Depends on wire material, length and thickness
Practical Applications
- Electric heaters, irons, toasters
- Incandescent bulbs (though inefficient)
- Fuses (safety devices that melt when overheated)
- Electric kettles and cookers
Lighting Technologies Comparison
| Type | How It Works | Efficiency | Lifespan |
|---|---|---|---|
| Incandescent | Heated filament glows | Low (10 lumens/watt) | 1,000 hours |
| Fluorescent | Gas discharge excites phosphor | Medium (50-100 lumens/watt) | 8,000-15,000 hours |
| CFL | Compact fluorescent version | Medium (50-70 lumens/watt) | 8,000-10,000 hours |
| LED | Light emitting diodes | High (80-100+ lumens/watt) | 25,000-50,000 hours |
Experiment: Observing Heating Effect
Objective: Demonstrate how current causes heating
- Set up circuit with cell, switch and bulb
- Touch bulb when switch is OFF (note temperature)
- Turn switch ON for 1 minute
- Touch bulb carefully (note temperature difference)
- Turn switch OFF and observe cooling
- Repeat with nichrome wire instead of bulb
Electrical Safety Devices
Fuses
- Contains wire that melts when current is too high
- Breaks circuit to prevent damage/fire
- Must be replaced after blowing
- Different ratings for different applications
MCBs (Miniature Circuit Breakers)
- Modern alternative to fuses
- Automatically switches off when current exceeds limit
- Can be reset after tripping
- More convenient than replacing fuses
Magnetic Effect of Electric Current
Discovery by Hans Christian Oersted
- First observed in 1820 by Danish scientist Oersted
- Noticed compass needle deflected when near current-carrying wire
- Demonstrated connection between electricity and magnetism
- Foundation for electromagnetism and many modern technologies
Building an Electromagnet
Materials Needed
- Iron nail (6-10 cm long)
- Insulated copper wire (about 75 cm)
- Battery/cell
- Switch
- Small metal objects (pins, paperclips)
Construction Steps
- Wind wire tightly around nail (leave some wire free at ends)
- Connect free ends to battery through switch
- Place metal objects near nail
- Turn switch ON - observe objects being attracted
- Turn switch OFF - observe magnetism disappears
Factors Affecting Electromagnet Strength
| Factor | Effect | How to Increase Strength |
|---|---|---|
| Number of coils | More coils = stronger magnet | Increase number of wire turns around core |
| Current | More current = stronger magnet | Use more cells/batteries in series |
| Core material | Iron core greatly increases strength | Use soft iron nail instead of other materials |
| Core size | Larger core can concentrate more flux | Use thicker/longer iron core |
Experiment: Testing Electromagnet Strength
Objective: Investigate factors affecting electromagnet strength
- Build basic electromagnet (nail + 20 wire turns)
- Test how many paperclips it can lift
- Change one variable at a time (number of turns, battery power, core material)
- Record results for each configuration
- Identify which factors have greatest effect
Applications of Electromagnets
Industrial Uses
- Cranes for lifting heavy metal objects
- Separating magnetic materials from junk
- Electric motors and generators
- Magnetic locks and relays
Everyday Devices
- Electric bells and buzzers
- Speakers and headphones
- MRI machines in hospitals
- Credit card magnetic strips
Project: Building an Electric Bell
Objective: Construct a working model of an electric bell
- Study the circuit diagram of an electric bell
- Gather materials: electromagnet, metal strip, contact screw, gong
- Assemble components to make hammer mechanism
- Test and adjust until bell rings properly
- Explain how breaking/remaking circuit creates ringing
Evaluation Strategies
Formative Assessments
- Circuit diagram drawing exercises
- Lab reports on heating/magnetic effects
- Quiz on circuit components and symbols
- Class participation in discussions
Summative Assessments
- Chapter test (MCQs + short answer)
- Practical exam on circuit building
- Electromagnet construction and testing
- Research project on applications of electromagnets
Extended Learning: Visit to Electrical Repair Shop
Objective: Observe real-world applications of concepts
- Interview electrician about common electrical problems
- Examine different types of fuses and MCBs
- Learn about proper wiring techniques
- Understand safety precautions in electrical work
- Prepare report with photographs/diagrams
Frequently Asked Questions
Why does a bulb glow when current passes through it?
An incandescent bulb glows because:
- It contains a thin tungsten filament
- When current flows, the filament heats up due to resistance
- At high temperatures (about 2500°C), the filament glows white hot
- This produces visible light (though much energy is wasted as heat)
How is an electromagnet different from a permanent magnet?
Key differences:
| Feature | Electromagnet | Permanent Magnet |
|---|---|---|
| Magnetism | Only when current flows | Always present |
| Strength | Can be adjusted | Fixed |
| Poles | Can be reversed | Fixed |
| Materials | Iron core + copper wire | Hard magnetic materials |
