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SCIENCE CLASS- 8

CHAPTER-4 (Electricity: Magnetic and Heating Effects)

CBSEChapter 4 Keep the curiosity alive

Question 1. Fill in the blanks.

(i) The solution used in a Voltaic cell is called __________.

Answer: Electrolyte

Explanation: The liquid solution used in a voltaic cell that allows the flow of ions and helps produce electricity is called an electrolyte.

(ii) A current carrying coil behaves like a __________.

Answer: Magnet

Explanation: When electric current flows through a coil, it produces a magnetic field around it and behaves like a magnet.

Question 2. Choose the correct option.

(i) Dry cells are less portable compared to Voltaic cells. (True/False)

Answer: False

Explanation: Dry cells are more portable because the electrolyte is in paste form and cannot spill easily.

(ii) A coil becomes an electromagnet only when electric current flows through it. (True/False)

Answer: True

Explanation: An electromagnet works only when electric current passes through the coil.

(iii) 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

Explanation: A battery of two cells produces a stronger current, making the electromagnet stronger and capable of attracting more paper clips.

Question 3. 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

Explanation:

  • Nichrome wire becomes warm because of the heating effect of electric current.
  • A magnetic field is produced around the wire due to the magnetic effect of electric current, causing the compass needle to deflect.

Question 4. Match the items in Column A with those in Column B.

Column A Column B
Voltaic cell Generates electricity by chemical reactions
Electric iron Works on heating effect of electric current
Nichrome wire Best suited for electric heater
Electromagnet Works on magnetic effect of electric current

Answer:

  • (i) Voltaic cell → (d)
  • (ii) Electric iron → (c)
  • (iii) Nichrome wire → (a)
  • (iv) Electromagnet → (b)

Question 5. Nichrome wire is commonly used in electrical heating devices because it

(i) is a good conductor of electricity.

(ii) generates more heat for a given current.

(iii) is cheaper than copper.

(iv) is an insulator of electricity.

Answer: (ii) generates more heat for a given current.

Explanation: Nichrome has high resistance and can withstand high temperatures without melting, making it suitable for heating devices.

Question 6. Electric heating devices (like an electric heater or a stove) are often considered more convenient than traditional heating methods (like burning firewood or charcoal). Give reasons.

Answer:

  • They are easy to use and control.
  • They produce heat quickly.
  • They do not produce smoke or ash.
  • They reduce indoor air pollution.
  • They are cleaner and safer.
  • They help protect forests by reducing the use of firewood.

Question 7. Look at Fig. 4.4a.

(i) Draw an arrow on the diagram to show the path of electric current.

Answer:

Conventional current flows from the positive terminal of the battery through the wire and coil and returns to the negative terminal.

(ii) Explain why the compass needle moves when current flows.

Answer:

When electric current passes through the coil, a magnetic field is produced around it. The compass needle, being a small magnet, aligns itself with this magnetic field and therefore gets deflected.

(iii) Predict what would happen to the deflection if you reverse the battery terminals.

Answer:

The direction of current will reverse. As a result, the direction of the magnetic field will also reverse and the compass needle will deflect in the opposite direction.

Question 8. Sumana forgets to move the switch of her lifting electromagnet model to OFF position. After some time, the iron nail no longer picks up iron paper clips, but the wire wrapped around the iron nail is still warm. Why did the lifting electromagnet stop lifting the clips?

Answer:

The wire became warm because current continued to flow through it.

Possible reasons for the electromagnet stopping its work are:

  • The battery became weak after prolonged use.
  • The current flowing through the coil decreased.
  • The magnetic field produced by the coil became too weak.
  • As a result, the electromagnet could no longer attract the paper clips.

Question 9. In Fig. 4.12, in which case will the LED glow when the switch is closed?

Answer:

The LED will glow in Fig. 4.12(a).

Explanation:

Lemon juice contains dissolved acids and ions, making it a good conductor of electricity.

Pure water contains very few ions and is a poor conductor of electricity. Therefore, the LED will not glow in Fig. 4.12(b).

Question 10. Neha keeps the coil exactly the same as in Activity 4.4 but slides the iron nail out, leaving only the coiled wire. Will the coil still deflect the compass? If yes, will the deflection be more or less than before?

Answer:

Yes, the coil will still deflect the compass.

However, the deflection will be less than before.

Explanation:

The coil itself produces a magnetic field when current flows through it. The iron nail strengthens the magnetic field. When the nail is removed, the magnetic field becomes weaker, resulting in smaller compass deflection.

Question 11. We have four coils of similar shape and size made up from iron, copper, aluminium and nichrome as shown in Fig. 4.13.

When current is passed through the coils, compass needles placed near the coils will show deflection.

Choose the correct option.

(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) In all four circuits

Explanation:

Whenever electric current flows through a conductor, a magnetic field is produced around it irrespective of whether the conductor is made of iron, copper, aluminium or nichrome.

Therefore, the compass needle will show deflection in all four circuits.

Key Learning Points

  • Electric current produces both heating and magnetic effects.
  • Electromagnets work only when current flows through them.
  • Nichrome wire is commonly used in heating appliances due to its high resistance.
  • Current-carrying coils behave like magnets.
  • Magnetic fields around conductors can deflect compass needles.
  • Lemon juice conducts electricity better than pure water.
  • Reversing battery terminals reverses the direction of the magnetic field.