Chapter Overview
1820
Oersted's Discovery
1791
Galvani's Observation
1800
Volta's Battery
50+
Coil Turns in Activity
What You'll Learn
Magnetic Effect of Current
Overview: Electric current produces a magnetic field, discovered by Oersted, with applications in electromagnets and devices.
Electromagnets
Overview: Current-carrying coils act as magnets, strengthened by iron cores, used in lifting and appliances.
Heating Effect of Current
Overview: Current through conductors generates heat due to resistance, used in heaters and lamps.
Batteries and Cells
Overview: Chemical reactions in cells like Voltaic, dry, and rechargeable generate electricity.
Historical Context
This chapter explores the magnetic and heating effects of electricity through activities and explanations. It begins with Oersted's 1820 discovery linking electricity and magnetism, leading to electromagnets. The heating effect is demonstrated with nichrome wires, applied in household appliances. Battery generation is covered from Volta's pile to modern rechargeable types.
Key Highlights
Key phenomena include magnetic fields from current, electromagnets with iron cores, heating due to resistance, and chemical energy conversion in cells. Practical applications range from cranes to heaters, with safety and recycling emphasized.
Comprehensive Chapter Summary
1. Introduction and Probe Questions
The chapter starts with questions on detecting current without lamps, making temporary magnets, generating heat electrically, and recharging cells. A story introduces electromagnets at a science exhibition.
2. Magnetic Effect of Electric Current
Discovery and Activity
Activity 4.1 shows current deflects a compass needle, indicating a magnetic field. Oersted's 1820 discovery linked electricity and magnetism.
Magnetic Field
The region around a current-carrying wire where magnetic effects are felt. Disappears when current stops.
Applications
Used in electromagnets, bells, motors, fans, loudspeakers.
3. Electromagnets
Construction and Strength
Activities 4.2-4.4: Coils with current act as magnets, stronger with iron cores and more turns/cells. Poles reverse with current direction.
Lifting Electromagnets
Strong electromagnets in cranes for lifting metal, controlled by switching current.
Earth's Magnetism
Earth behaves as a magnet due to core currents, aiding navigation and protection.
4. Heating Effect of Electric Current
Observation and Explanation
Activity 4.5: Nichrome wire heats due to resistance converting electrical to heat energy.
5. Applications and Safety
Household and Industrial Uses
Heaters, stoves, irons; industrial furnaces for steel recycling.
Safety Measures
Avoid overheating to prevent fires; use rated wires and safety devices.
6. Battery Generation of Electricity
Voltaic cells use chemical reactions; dry cells with paste electrolyte; rechargeable batteries reusable.
Questions and Answers from Chapter
Short Questions
Q1. The solution used in a Voltaic cell is called ________.
Answer: Electrolyte.
Q2. A current carrying coil behaves like a _______ .
Answer: Magnet.
Q3. Dry cells are less portable compared to Voltaic cells. (True/False)
Answer: False.
Q4. A coil becomes an electromagnet only when electric current flows through it. (True/False)
Answer: True.
Q5. An electromagnet, using a single cell, attracts more iron paper clips than the same electromagnet with a battery of 2 cells. (True/False)
Answer: False.
Q6. What is the phenomenon when electric current flows through a conductor producing a magnetic field?
Answer: Magnetic effect of electric current.
Q7. What is the region around a current-carrying wire where magnetic effect is felt?
Answer: Magnetic field.
Q8. Who discovered the magnetic effect of electric current?
Answer: Hans Christian Oersted.
Q9. What material is used to make the core of an electromagnet stronger?
Answer: Iron.
Q10. What causes the heating effect in a wire?
Answer: Resistance to current flow.
Q11. What is a Voltaic cell also known as?
Answer: Galvanic cell.
Q12. Why are dry cells called 'dry'?
Answer: Electrolyte is a paste.
Q13. Can all cells be recharged?
Answer: No.
Q14. What wire is used in heating appliances?
Answer: Nichrome.
Q15. What generates Earth's magnetic field?
Answer: Core currents.
Medium Questions
Q1. An electric current flows through a nichrome wire for a short time. (i) The wire becomes warm. (ii) A magnetic compass placed below the wire is deflected. Choose the correct option.
Answer: (c) Both (i) and (ii) are correct. The heating effect warms the wire due to resistance, and the magnetic effect deflects the compass. (3 marks)
Q2. Match the items in Column A with those in Column B.
Answer: (i) Voltaic cell - (d) chemical reactions; (ii) Electric iron - (c) Works on heating effect; (iii) Nichrome wire - (a) Best suited for electric heater; (iv) Electromagnet - (b) Works on magnetic effect. (3 marks)
Q3. Nichrome wire is commonly used in electrical heating devices because it...
Answer: (ii) generates more heat for a given current. It has higher resistance than copper, converting more energy to heat. (3 marks)
Q4. Electric heating devices are often considered more convenient than traditional heating methods. Give reason(s) considering societal impact.
Answer: They are cleaner, easier to control, reduce pollution from fuels, and are efficient for urban use. (3 marks)
Q5. Look at the Fig. 4.4a. If the compass placed near the coil deflects: (i) Draw an arrow... (ii) Explain... (iii) Predict...
Answer: (i) Arrow from positive to negative. (ii) Current creates magnetic field. (iii) Deflection reverses. (3 marks)
Q6. Suppose Sumana forgets to move the switch... Why did the lifting electromagnet stop lifting the clips?
Answer: Battery weakened, current reduced; wire heated but magnetism lost. (3 marks)
Q7. In Fig. 4.11, in which case the LED will glow when the switch is closed?
Answer: (a) Lemon juice conducts; pure water does not. (3 marks)
Q8. Neha keeps the coil... but slides the iron nail out... Will the coil still deflect the compass?
Answer: Yes, but deflection less without iron core. (3 marks)
Q9. We have four coils... When current is passed... compass needles... will show deflection.
Answer: (iv) In all four circuits. All conductors produce magnetic fields. (3 marks)
Q10. Why does a current-carrying wire get hot?
Answer: Resistance opposes flow, converting energy to heat. (3 marks)
Q11. How can the strength of an electromagnet be increased?
Answer: More turns, higher current, iron core. (3 marks)
Q12. What happens when current stops in an electromagnet?
Answer: Loses magnetism, objects fall. (3 marks)
Q13. Why is nichrome used in heaters?
Answer: High resistance generates more heat. (3 marks)
Q14. What is inside a dry cell?
Answer: Zinc container, carbon rod, paste electrolyte. (3 marks)
Q15. Why can't all cells be recharged?
Answer: Chemical reactions irreversible in non-rechargeable. (3 marks)
Long Questions
Q1. Explain with activity how electric current produces a magnetic effect.
Answer: In Activity 4.1, connect wire over compass to cell and switch. When ON, needle deflects due to magnetic field from current. Off, returns to original. Oersted discovered this in 1820. Field disappears without current. Applications in motors, bells.
Q2. Describe how to make and test an electromagnet.
Answer: Activity 4.3: Wind 50 turns insulated wire on cylinder, connect to cell. Compass deflects, attracts clips with iron nail inserted for strength. Poles like bar magnet, reverse with current direction.
Q3. Discuss factors affecting electromagnet strength.
Answer: More cells increase current, more turns strengthen field. Iron core amplifies. Single cell weak; battery stronger. Direction change reverses poles.
Q4. Explain heating effect with activity and applications.
Answer: Activity 4.5: Nichrome wire between nails, connected to cell. Heats when current flows due to resistance. More cells, more heat. Used in heaters, irons; industrial furnaces.
Q5. Describe Voltaic cell and its history.
Answer: Electrodes in electrolyte produce current via reaction. Galvani's frog leg, Volta's pile in 1800 using metals and saltwater. Activity 4.6: Lemon cell with copper/iron.
Q6. Compare dry cells and rechargeable batteries.
Answer: Dry: Zinc/carbon with paste, single-use. Rechargeable: Reversible reactions, used in phones, vehicles; Li-ion common, recyclable but wear out.
Q7. Why does Earth behave like a magnet?
Answer: Liquid iron core currents generate field. Aids navigation, protects from particles. Magnets align north-south.
Q8. Explain lifting electromagnets and their use.
Answer: Strong coils on cranes, switch ON to lift metal, OFF to release. Used in factories, scrap yards for efficiency.
Q9. Discuss safety in heating effects.
Answer: Overheating causes fires, melting. Use rated wires, safety devices. Avoid prolonged touch.
Q10. How do batteries generate electricity?
Answer: Chemical reactions between electrodes and electrolyte. Positive to negative flow. Dead when chemicals used up.
Q11. Suppose Sumana forgets to move the switch... Why did the lifting electromagnet stop lifting the clips? Give possible reasons.
Answer: Battery depleted reducing current, magnetism weakened while wire stayed warm from heating effect. Continuous use drains cell quickly.
Q12. Neha keeps the coil exactly the same as in Activity 4.4 but slides the iron nail out... Will the coil still deflect the compass? If yes, will the deflection be more or less than before?
Answer: Yes, coil alone acts as electromagnet but deflection less without iron core amplifying field.
Q13. We have four coils... When current is passed... compass needles will show deflection. (i) Only in circuit (a) (ii) Only in circuits (a) and (b) (iii) Only in circuits (a), (b), and (c) (iv) In all four circuits
Answer: (iv) All produce magnetic fields regardless of material, though strength varies.
Q14. Electric heating devices... are often considered more convenient... Give reason(s) considering societal impact.
Answer: Reduce fossil fuel use, cleaner, controllable; societal: less pollution, efficient energy, safer indoors.
Q15. In Fig. 4.11, in which case the LED will glow when the switch is closed?
Answer: (a) Lemon juice electrolyte conducts; (b) Pure water does not, no current.
