Question Bank — Electricity: Magnetic and Heating Effects

NCERT Class 8 Science · Chapter 4 · CBSE Board Pattern

Class 8 Science Chapter 4 Question Bank CBSE Board Pattern

Section A — MCQs (10 questions, 1 mark each)

  1. When the switch in Activity 4.1 is moved to the ON position, the compass needle deflects. What does this directly show?
    (a) The wire becomes a permanent magnet
    (b) Electric current produces a magnetic field around the wire
    (c) The compass needle is an electromagnet
    (d) Current stops the magnetic effect

  2. In Activity 4.2, iron paper clips cling to the nail only when the circuit is closed. This happens because:
    (a) The nail becomes a permanent magnet
    (b) The current-carrying coil turns the nail into an electromagnet
    (c) The clips are conductors of electricity
    (d) The cell produces heat

  3. Which of the following correctly states the effect of increasing the number of turns in the coil of an electromagnet (keeping current same)?
    (a) Magnetic field becomes weaker
    (b) Magnetic field becomes stronger
    (c) Poles of the electromagnet reverse
    (d) No change occurs

  4. Nichrome wire is preferred over copper wire of the same length and thickness in heating appliances because:
    (a) It offers higher resistance and produces more heat
    (b) It is a better conductor
    (c) It does not allow current to flow
    (d) It remains cold when current passes

  5. A dry cell stops working after some time because:
    (a) The electrolyte becomes solid
    (b) The chemicals inside get used up
    (c) The zinc container melts
    (d) The carbon rod becomes an insulator

Assertion-Reason Questions

  1. Assertion (A): An electromagnet loses its magnetism when the current is switched off.
    Reason (R): The magnetic field is produced only when electric current flows through the coil.
    (a) Both A and R are true and R is the correct explanation of A
    (b) Both A and R are true but R is not the correct explanation of A
    (c) A is true but R is false
    (d) A is false but R is true

  2. Assertion (A): Increasing the current through an electromagnet increases the deflection of a nearby compass needle.
    Reason (R): The strength of the magnetic field of an electromagnet depends on the amount of current flowing through the coil.
    (a) Both A and R are true and R is the correct explanation of A
    (b) Both A and R are true but R is not the correct explanation of A
    (c) A is true but R is false
    (d) A is false but R is true

  3. In a Voltaic cell, electric current is produced due to:
    (a) Heating of electrodes
    (b) Chemical reactions between electrodes and electrolyte
    (c) Magnetic effect inside the container
    (d) Friction between the metal rods

  4. Which statement about rechargeable batteries is correct?
    (a) They can be used only once
    (b) They can be recharged and reused many times
    (c) They do not contain any electrolyte
    (d) They work only on the heating effect of current

  5. When the direction of current in the coil of an electromagnet is reversed:
    (a) The strength of the magnet increases
    (b) The poles of the electromagnet are reversed
    (c) The magnet stops working permanently
    (d) No change occurs in the magnetic effect

Section B — Very Short Answer (6 questions, 2 marks each)

  1. State what happens to the compass needle in Activity 4.1 when the switch is turned ON and then OFF. What does this observation indicate?

  2. Why is an iron core used inside the coil while making an electromagnet for practical use?

  3. List two factors on which the strength of an electromagnet depends.

  4. What is observed when current is passed through a nichrome wire for a short time? Name the effect responsible for this observation.

  5. Why is a dry cell called a “single-use” cell?

  6. In a lifting electromagnet, why do the iron objects fall when the current is switched off?

Section C — Short Answer (5 questions, 3 marks each)

  1. Describe briefly how an electromagnet is made using an iron nail and insulated wire. How can its magnetic effect be stopped?

  2. Explain why the heating effect is more when two cells are used instead of one cell in Activity 4.5 (same nichrome wire and time).

  3. Differentiate between a Voltaic cell and a dry cell on the basis of (i) electrolyte and (ii) portability.

  4. Draw a simple labelled diagram of the setup used in Activity 4.1 to show the magnetic effect of current. Label the cell, wire, switch and compass.

  5. State any three household appliances that work on the heating effect of electric current and mention the common part present in all of them.

Section D — Long Answer (3 questions, 5 marks each)

  1. What is an electromagnet? Describe with a labelled diagram how you would make a simple electromagnet using a coil of wire, an iron nail and a cell. Also explain how its strength can be increased. (Diagram required)

  2. Explain the heating effect of electric current with reference to Activity 4.5. Why does nichrome wire become warm while copper wire of same size does not become as warm? Give two examples of devices where this effect is useful and one situation where it can be harmful.

  3. Describe the construction of a dry cell with a labelled diagram. How is it different from a rechargeable battery? Why are rechargeable batteries preferred for many modern devices?

Section E — Case/Source-Based (2 questions, 4 marks each)

Case 1

During the science exhibition, Sumana demonstrated a model in which an iron nail wrapped with wire picked up paper clips only when connected to a battery. When the connection was removed, the clips fell.

(i) Name the device formed when current flows through the coil.
(ii) Why did the clips fall when the circuit was opened?
(iii) If Sumana uses two cells instead of one, what change would be observed in the number of clips lifted? Give reason.
(iv) Suggest one real-life application of such a device in factories or scrap yards.

Case 2

A student sets up a circuit with a nichrome wire connected to a cell and switch. After closing the switch for 30 seconds, the wire feels warm. The same wire is later connected to two cells and the heating is noticeably more.

(i) Name the effect responsible for the wire becoming warm.
(ii) Why does the wire heat up more with two cells than with one cell?
(iii) Name the material commonly used for heating elements in electric irons and room heaters.
(iv) State one safety precaution that should be taken while performing such an activity.

Section F — HOTS and Application (4 questions, 3 marks each)

  1. Predict-and-justify: What would happen if the iron nail is removed from the coil in Activity 4.3 while current is still flowing? Would the compass deflection increase, decrease or remain the same? Justify your answer.

  2. Analyse an anomalous observation: In Activity 4.3, a student observes that even after inserting the iron nail, the compass deflection is very small. Give two possible reasons for this unexpected result based on the factors affecting electromagnet strength.

  3. Apply to an unfamiliar real-life situation: A crane in a scrap yard is fitted with a large electromagnet instead of a hook. Explain how the operator can pick up and release heavy steel objects safely without touching them. What would happen if the current could not be switched off?

  4. Compare two situations: Two identical coils are made — one with 50 turns of copper wire and another with 50 turns of nichrome wire. Both are connected one by one to the same cell and a compass is placed near each. In which case will the compass show greater deflection? Justify your answer.

▶ Answer Key Attempt all questions first,
then tap to reveal

Section A

  1. (b)
  2. (b)
  3. (b)
  4. (a)
  5. (b)
  6. (a)
  7. (a)
  8. (b)
  9. (b)
  10. (b)

Section B

  1. Needle deflects when ON, returns when OFF — shows magnetic field is produced only when current flows (1+1).
  2. Iron core makes the electromagnet much stronger (2).
  3. Amount of current; number of turns (1+1).
  4. Wire becomes warm — heating effect of current (1+1).
  5. Chemicals get used up and cannot be recharged (2).
  6. Magnetic field disappears when current stops (2).

Section C

  1. Coil wound on nail + cell connected; magnetic effect stops when current is switched off (1+1+1).
  2. More current with two cells produces more heat (3).
  3. Voltaic: liquid electrolyte; Dry: paste electrolyte; Dry cell more portable (1+1+1).
  4. Correct labelled diagram of wire, nails, cell, switch, compass (3).
  5. Electric iron, heater, kettle; all have heating element/coil (1+1+1).

Section D

  1. Definition (1), labelled diagram of coil on nail with cell (2), increasing current/turns/core (2).
  2. Explanation of heating effect with resistance (2), nichrome vs copper (1), two useful + one harmful example (2).
  3. Labelled diagram of dry cell (2), difference from rechargeable (2), advantage of rechargeable (1).

Section E

Case 1: (i) Electromagnet (1) (ii) Magnetic field disappears (1) (iii) More clips due to stronger field (1) (iv) Lifting scrap steel (1).
Case 2: (i) Heating effect (1) (ii) Larger current produces more heat (1) (iii) Nichrome (1) (iv) Do not touch wire for long / teacher supervision (1).

Section F

  1. Deflection decreases because without iron core the magnetic field is weaker (reasoning chain: iron core concentrates field).
  2. Possible reasons: fewer turns than needed, weak cell / low current, poor contact, nail not fully inside core.
  3. Operator switches current ON to lift, OFF to release; if current cannot be switched off, objects cannot be released safely.
  4. Copper coil shows greater deflection — copper is better conductor, produces stronger current and thus stronger magnetic field than nichrome.
← Revision Summary All Chapters

All questions are answerable from the NCERT chapter text. Reviewed by GFIS faculty.