Electricity: Magnetic and Heating Effects — Mastery

Q1. Sumana's working model at the science exhibition used an iron nail wrapped with wire and connected to a battery. Instead of a hook, the nail itself picked up paper clips when the switch was closed. The model is an example of

• A. A permanent bar magnet
• B. An electromagnet (a current-carrying coil with an iron core)
• C. A simple voltaic cell
• D. A nichrome heating coil

Q2. Mohini and Aakarsh recall that magnetic materials are attracted by magnets and that iron is a magnetic material. Yet in Sumana's model there was no permanent magnet — only an electric circuit. They were surprised because

• A. They thought iron always behaves as a magnet on its own
• B. They had been told that electricity has no effect on iron
• C. Sumana's lemon was acting as a magnet
• D. An electric current was producing a magnetic effect, which they had not learnt before

Q3. A compass placed below a straight wire deflects to the right when current flows in one direction. If the battery terminals are reversed, the needle is MOST LIKELY to

• A. Deflect to the right by the same angle
• B. Stay along its original direction
• C. Stop reacting to the current
• D. Deflect to the left by about the same angle

Q4. The region around a magnet or a current-carrying wire where the magnetic effect can be felt, such as by the deflection of a compass needle, is called the

• A. Electrolyte
• B. Magnetic field
• C. Electric field
• D. Filament

Q5. Hans Christian Oersted is said to have made his famous discovery when, during a demonstration, he noticed that

• A. A magnet attracted a glass rod
• B. Water boiled on its own near a wire
• C. The needle of a magnetic compass deflected whenever a circuit was closed or opened
• D. An iron nail glowed red hot in a coil

Q6. A student makes four coils with 25, 50, 75 and 100 turns and connects them one by one to the same cell. The compass placed in the same position records the deflection in each case. The student's BEST conclusion is

• A. Number of turns does not affect the strength of the electromagnet
• B. As the number of turns increases, the strength of the electromagnet increases
• C. The 25-turn coil gives the strongest magnet
• D. The deflection depends only on the material of the wire

Q7. An electromagnet, using a single cell, attracts MORE iron paper clips than the same electromagnet with a battery of 2 cells. This statement is

• A. True — fewer cells always give more current
• B. True — single cells store the most energy
• C. False — a battery with more cells gives a larger current and a stronger magnet, attracting more clips
• D. True — clips are attracted only when a single cell is used

Q8. A lifting electromagnet at a scrap yard works on which of the following effects of electric current?

• A. Heating effect of electric current
• B. Chemical effect of electric current
• C. Magnetic effect of electric current
• D. Light effect of electric current

Q9. Although the heating effect is useful in many appliances, in long electric transmission wires it causes

• A. More magnetic flow
• B. Energy loss in the form of heat
• C. Permanent magnets to form
• D. Cooling of the surroundings

Q10. While conducting Activity 4.5 (the nichrome-wire heating activity), which of the following is the BEST instruction for a Class 8 teacher to give the students BEFORE switching ON the cell?

• A. Hold the nichrome wire tightly the whole time so it does not move
• B. Do not touch or hold the nichrome wire for an extended period to avoid injuries
• C. Pour water over the wire after a few seconds for safety
• D. Bring an iron magnet near the wire to test its strength

Q11. Volta replaced the frog's leg with saltwater-soaked paper kept between two different metals and still obtained an electric current. This experiment proved that

• A. The electricity actually came from the frog
• B. Only living tissue can produce electricity
• C. Only one type of metal is needed to produce electricity
• D. It is the combination of metals and a liquid (electrolyte) that generates current

Q12. When the external circuit of a Voltaic cell is connected, electric current flows through the circuit from

• A. The negative terminal to the positive terminal of the cell
• B. The positive terminal to the negative terminal of the cell
• C. Only inside the cell, not in the external circuit
• D. From the centre of the electrolyte outwards in all directions

Q13. In the lemon-cell activity (Activity 4.6), the two metal electrodes are

• A. Two iron nails
• B. Two copper wires
• C. A copper wire and a wooden stick
• D. A copper wire and an iron nail

Q14. Aakarsh has no lemons. He wants to set up a Voltaic cell of the same kind. According to the chapter, what is a reasonable substitute for the lemon juice?

• A. Pure distilled water
• B. Cooking oil
• C. Petrol
• D. A salt solution

Q15. Dry cells are called 'dry' because

• A. Their outer surface always feels dry to touch
• B. They cannot be exposed to water at all
• C. Their inside is completely empty
• D. Their electrolyte is not a liquid but a thick moist paste

Q16. Which of these statements about a dry cell is correct?

• A. It is a single-use cell — once it is used up, it has to be disposed of
• B. It can be recharged any number of times
• C. It uses copper as its negative terminal
• D. It uses water as its electrolyte

Q17. A mobile phone's rechargeable battery needs charging more often after a year or two of use. The chapter explains this as

• A. The phone's screen has dimmed
• B. Rechargeable batteries slowly wear out after being charged and used many times
• C. Lithium has changed into iron
• D. Charging cables become weaker with time

Q18. According to the 'A step further' note, scientists are working on the next big leap — solid-state batteries — in which

• A. There is no electrolyte at all
• B. There is no positive terminal
• C. The liquid or paste-like electrolyte is replaced with solid materials
• D. The battery is made entirely of wood and plastic

Q19. Used batteries that no longer work are BEST disposed of by

• A. Burning them in an open fire
• B. Throwing them with vegetable peels in regular garbage
• C. Burying them in a wet field
• D. Taking them to special e-waste recycling facilities, if available

Q20. A Class 8 teacher wants the class to 'observe' the invisible magnetic field around a current-carrying wire. Which of the following is the BEST classroom strategy from the chapter?

• A. Place a magnetic compass beneath the wire and watch the needle deflect when the switch is moved between ON and OFF
• B. Ask students to memorise the definition of magnetic field aloud
• C. Switch on the wall socket to make the deflection larger
• D. Heat the wire with a candle and look for sparks

Q21. Ravi tells his teacher: 'Once an iron nail is wrapped with current-carrying wire, the nail becomes a permanent magnet forever.' Which of the following is the BEST response from the teacher?

• A. 'Yes, that is exactly what the chapter says.'
• B. 'Let us open the switch now. If the nail is a permanent magnet, the paper clips should still stick — try it.'
• C. 'Just remember the textbook says so.'
• D. 'Iron is non-magnetic, so it cannot become a magnet.'

Q22. Sumana's model surprised Mohini and Aakarsh because it contained no permanent magnet — only an electric circuit. The model worked because

• A. The iron nail had been heated before the exhibition
• B. The iron nail produced electricity by itself
• C. The cardboard base attracted the paper clips
• D. The current in the wire wrapped around the nail produced a magnetic effect

Q23. The chapter recalls (from Class 6) that the magnetic effect of a magnet on a compass needle can act through

• A. Vacuum only
• B. Pure water only
• C. Any non-magnetic material kept between the magnet and the compass
• D. Only iron sheets

Q24. The chapter says that in household circuits, safety devices are placed to minimise incidents caused by overheating. These safety devices are needed because, otherwise, overheating may

• A. Melt plastic parts of plugs and sockets, or even lead to fires
• B. Make all bulbs become rechargeable
• C. Turn copper wires into nichrome
• D. Cool the wires below room temperature

Q25. While making the electromagnet in Activity 4.3, students noticed two things at once: (i) iron clips were attracted to the nail, and (ii) the wire ends became warm. The BEST explanation is

• A. The same current produces both the magnetic effect (attracting clips) and the heating effect (warming the wire)
• B. Only the heating effect is real; clips were attracted by the cardboard
• C. Only the magnetic effect is real; warmth was caused by sunlight
• D. Different cells were used for the two effects

Q26. Electric heating devices (like an electric heater or a stove) are often considered more convenient than traditional heating methods (like burning firewood or charcoal). Which of the following is the BEST societal-impact reason?

• A. Electric heaters do not produce smoke or ash indoors, so indoor air stays cleaner and households face fewer respiratory problems
• B. Burning firewood is technically illegal in all parts of India
• C. Electric heaters always cost less money than firewood for poor families
• D. Firewood cannot reach high enough temperatures to cook food

Q27. A teacher wants the class to discover for themselves that 'more cells make a stronger electromagnet'. The MOST suitable activity is to

• A. Read aloud the paragraph from the textbook
• B. Connect the same coil first to one cell and then to a battery of two cells, and compare the number of paper clips attracted
• C. Replace the iron nail with a copper rod
• D. Switch the same circuit ON and OFF many times

Q28. A teacher asks Class 8 students to PREDICT, then OBSERVE: 'If we double the number of turns on the coil, but keep the same cell, will the number of clips picked up by the nail increase, stay the same, or decrease?' The expected scientific prediction is

• A. Stay the same — number of turns has no effect
• B. Decrease — more turns weaken the magnet
• C. Increase — more turns make the electromagnet stronger
• D. Become zero — the wire will heat up and stop working

Q29. When the teacher opens the switch of Sumana's electromagnet model, the clips fall off the iron nail. A student claims this is because 'the nail has cooled down'. The CORRECT diagnosis is

• A. The student is right — heat is what was holding the clips
• B. The student is wrong — clips fall because current stopped, so the magnetic effect disappeared
• C. The student is right — nails attract clips only when warm
• D. The student is wrong — clips fall because gravity increases when current stops

Q30. Which of the following is true about rechargeable batteries, as stated in the chapter?

• A. They last forever and never need replacement
• B. They are used only in dry cells
• C. Even rechargeable batteries do not last forever — they slowly wear out after being charged and used many times
• D. They cannot be used in mobile phones