Permanent Magnets and Electromagnets - UNSOLVED PRACTICE SET
Chapter: Magnetism and Matter | Topic: Permanent Magnets and Electromagnets
PERMANENT MAGNETS AND ELECTROMAGNETS - UNSOLVED PRACTICE SET
Topic: Permanent Magnets and Electromagnets
Multiple Choice Questions
Q1. Which property is most desirable for a material used to make permanent magnets?
- Low retentivity and low coercivity
- High retentivity and high coercivity
- Low retentivity and high coercivity
- High retentivity and low coercivity
Q2. An electromagnet is made using:
- Steel core with DC current
- Soft iron core with DC current
- Soft iron core with AC current
- Any material with any current
Q3. The core of a transformer is made of laminated soft iron to:
- Increase electrical conductivity
- Reduce eddy current losses and hysteresis losses
- Make it heavier
- Increase its magnetic field strength
Q4. Which of the following is NOT an application of electromagnets?
- Electric bell
- Loudspeaker
- Compass needle
- MRI machine
Q5. Alnico is an alloy used for making permanent magnets because it has:
- High coercivity and high Curie temperature
- Low coercivity and low retentivity
- High electrical conductivity
- Low density
Q6. When the current in an electromagnet is switched off:
- The magnetic field increases
- The magnetic field disappears immediately (for soft iron core)
- The core becomes permanently magnetized
- The magnetic field reverses direction
Short Answer Questions
Q7. Why is soft iron preferred over steel for making the core of an electromagnet? Give two reasons.
Q8. What is the difference between a permanent magnet and an electromagnet in terms of:
(a) How they are made?
(b) How their magnetism can be controlled?
Q9. Why are permanent magnets made of materials with high coercivity? What would happen if a low coercivity material were used?
Q10. Explain why the core of a transformer is laminated and made of soft iron rather than solid steel.
Q11. A student wants to make a simple electromagnet at home using a nail, copper wire, and a battery. Should the nail be made of iron or steel? Justify your answer.
Q12. Why do we use alloys like Alnico and Carbon Steel for permanent magnets instead of pure iron?
Long Answer Questions
Q13. Describe the construction and working of an electromagnet. Explain why soft iron is preferred as the core material and how the strength of the electromagnet can be increased.
Q14. Compare permanent magnets and electromagnets under the following heads:
(i) Materials used
(ii) Method of production
(iii) Control of magnetic strength
(iv) Applications
Q15. Explain the working of an electric bell with a neat diagram. How does the electromagnet in the bell help in producing the ringing sound continuously?
Numerical & Application-Based Problems
Q16. An electromagnet is made by winding 500 turns of copper wire around a soft iron core of length 25 cm and cross-sectional area 4 cm². A current of 3 A is passed through the coil. The relative permeability of soft iron is 2000.
(a) Calculate the magnetic field intensity (H) inside the core.
(b) Calculate the magnetic field (B) inside the core.
(c) If the current is switched off, explain what happens to the magnetic field and why soft iron is preferred over steel for this application.
Q17. A permanent magnet is made of an alloy with retentivity 1.0 T and coercivity 4 × 10⁴ A/m. It has a volume of 2 × 10⁻⁵ m³.
(a) Calculate the minimum magnetic field intensity required to demagnetize this magnet completely.
(b) Calculate the magnetic dipole moment of the magnet if its pole strength is 5 × 10⁻⁴ Am.
(c) This magnet is used in a school loudspeaker. Explain what would happen to the sound quality if this permanent magnet were replaced by an electromagnet with the same magnetic moment.
Q18. In your school's science exhibition, two teams present their projects:
Team A builds an electromagnet using a soft iron rod (length 20 cm, diameter 1 cm) with 400 turns of wire carrying 2 A current.
Team B uses a permanent bar magnet of magnetic moment 8 Am².
Both are tested by counting how many paper clips each can lift.
(a) Calculate the magnetic moment of Team A's electromagnet (assume the rod behaves like a solenoid).
(b) The permanent magnet lifts 15 paper clips, while the electromagnet lifts 12 paper clips when the current is on, but drops all of them when switched off. Discuss two practical advantages and two disadvantages of using an electromagnet instead of a permanent magnet in a real-world application like a scrap metal lifting crane.