CBSE Class 8 Science Electricity Magnetic and Heating Effects (Class 8 NCERT New) Quiz 3

1. An engineer is designing an electromagnet to lift heavy steel objects in a scrapyard. She needs to maximize the lifting force. Which combination of changes would best achieve this goal?

A Decrease the current and decrease the number of coil turns B Increase the current, increase the number of coil turns, and use a soft iron core C Use a copper core and decrease the number of coil turns D Increase the voltage only and use a wooden core

2. Two identical metal rods, one made of copper and one made of iron, are heated at one end. After the same amount of time, the copper rod feels hotter at the other end. What conclusion can be drawn, and how does this relate to thermal conductivity?

A Iron has higher thermal conductivity than copper B Copper has higher thermal conductivity than iron, allowing heat to transfer faster along its length C Both rods have the same thermal conductivity but different densities D The result is due to the difference in the length of the rods

3. A student notices that the magnetic field lines around a straight current-carrying conductor are circular. If the current in the conductor is reversed, what happens to the direction of the magnetic field lines, and how can this be determined?

A The field lines remain in the same direction regardless of current direction B The field lines reverse direction, which can be determined using the right-hand thumb rule C The field lines become straight when the current is reversed D The field lines disappear when the current is reversed

4. In an induction cooktop, the pot heats up but the cooktop surface itself remains relatively cool. Using your knowledge of induction heating and thermal conductivity, explain which of the following best describes why this happens?

A The cooktop surface is made of a material with very high thermal conductivity that quickly dissipates heat B Eddy currents are induced directly in the pot due to the changing magnetic field, generating heat in the pot itself, while the cooktop surface is made of a poor electrical conductor that does not generate significant eddy currents C The cooktop surface reflects all the heat back into the pot D The magnetic field only heats materials that are in direct contact with the coil

5. A hospital uses an MRI machine that relies on a very strong electromagnet. The machine must maintain a constant and powerful magnetic field. Which of the following design choices best supports this requirement, and why?

A Use a permanent magnet because it does not require electricity B Use a superconducting electromagnet because it can carry very large currents with negligible resistance, producing an extremely strong and stable magnetic field C Use a simple iron bar magnet because it is the strongest type of magnet D Use a low-current electromagnet to avoid overheating

6. A student is comparing two electromagnets. Electromagnet P has 50 turns and carries a current of . Electromagnet Q has 100 turns and carries a current of . Both have the same iron core. Which electromagnet is stronger, and what reasoning supports this?

A Electromagnet P is stronger because it carries more current B Both electromagnets are equally strong because the product of turns and current () is the same for both C Electromagnet Q is stronger because it has more turns D Neither electromagnet produces a magnetic field without a permanent magnet

7. A metal rod of length , cross-sectional area , and thermal conductivity transfers heat at a rate given by . If the length of the rod is doubled and the cross-sectional area is halved while keeping the temperature difference constant, how does the rate of heat transfer change?

A The rate of heat transfer doubles B The rate of heat transfer becomes of the original C The rate of heat transfer remains the same D The rate of heat transfer becomes of the original

8. A technician is troubleshooting an induction heating system that is not heating a metal workpiece efficiently. The system uses a high-frequency alternating current. Which of the following changes would most likely improve the heating efficiency, and why?

A Replace the alternating current with direct current to create a stable magnetic field B Increase the frequency of the alternating current to induce stronger eddy currents in the workpiece, thereby increasing heat generation C Reduce the frequency of the alternating current to allow more time for heat to build up D Replace the metal workpiece with a plastic one to reduce energy loss

9. Electroplating is based on ___________of an electric current.

A Physical effect B Chemical effect C Heating effect D Magnetic effect

10. Which of these is plated on the iron to prevent it from chemically reacting with food?

A Gold B Chromium C Tin D Zinc