PART A: BASIC CONCEPTS
Q1. What is the magnetic effect of electric current?
Answer: When an electric current flows through a conductor, it produces a magnetic field around it. This phenomenon is called the magnetic effect of electric current. The magnetic field exists as long as the current flows and disappears when the current stops.
Q2. Who discovered the magnetic effect of electric current and when?
Answer: Hans Christian Oersted discovered the magnetic effect of electric current in 1820. He observed that a compass needle placed near a current-carrying wire deflected from its north-south direction.
Q3. State Oersted's experiment and its conclusion.
Answer: Experiment: Oersted placed a compass needle parallel to a straight wire. When current was passed through the wire, the compass needle deflected. Conclusion: Electric current produces a magnetic field around the conductor.
Q4. How can you demonstrate that a current-carrying conductor has a magnetic field around it?
Answer:
- Place a compass needle near a current-carrying straight wire
- Switch on the current - the compass needle deflects
- Reverse the direction of current - the compass needle deflects in the opposite direction
- Switch off the current - the compass needle returns to its original position
PART B: MAGNETIC FIELD PATTERNS
Q5. What is the shape of the magnetic field around a straight current-carrying conductor?
Answer: The magnetic field around a straight current-carrying conductor consists of concentric circles with the conductor at the center. The field lines are closer near the conductor and farther apart at greater distances.
Q6. State the right-hand thumb rule.
Answer: If you hold a current-carrying straight conductor in your right hand such that the thumb points in the direction of current, then the fingers wrapped around the conductor will point in the direction of the magnetic field lines.
Q7. What factors affect the strength of the magnetic field around a current-carrying conductor?
Answer:
- Current strength: Greater the current, stronger the magnetic field
- Distance from conductor: Closer to the conductor, stronger the field
- Nature of the medium: Magnetic field is stronger in magnetic materials
Q8. Describe the magnetic field pattern around a circular current-carrying conductor.
Answer:
- At the center: Field lines are straight and perpendicular to the plane of the coil
- Near the wire: Field lines are curved
- The field is strongest at the center
- Direction can be found using the right-hand rule
PART C: SOLENOID AND ELECTROMAGNET
Q9. What is a solenoid?
Answer: A solenoid is a coil of wire wound in the form of a cylinder. When current passes through it, it behaves like a bar magnet with distinct north and south poles.
Q10. Describe the magnetic field of a current-carrying solenoid.
Answer:
- Inside the solenoid: Field lines are parallel and uniform
- Outside the solenoid: Field lines resemble those of a bar magnet
- The field is strongest inside the solenoid
- One end acts as north pole, the other as south pole
Q11. How do you determine the poles of a current-carrying solenoid?
Answer: Use the right-hand rule: If you curl the fingers of your right hand in the direction of current flow in the solenoid, your thumb points toward the north pole.
Q12. What is an electromagnet?
Answer: An electromagnet is a temporary magnet made by placing a soft iron core inside a current-carrying solenoid. It becomes magnetic only when current flows through the coil.
Q13. List the advantages of electromagnets over permanent magnets.
Answer:
- Magnetic strength can be controlled by changing current
- Magnetism can be switched on/off
- Poles can be reversed by reversing current direction
- Very strong magnetic fields can be produced
- Shape and size can be varied as needed
PART D: FORCE ON CURRENT-CARRYING CONDUCTOR
Q14. What happens when a current-carrying conductor is placed in a magnetic field?
Answer: A current-carrying conductor placed in a magnetic field experiences a mechanical force. This is due to the interaction between the magnetic field of the conductor and the external magnetic field.
Q15. State Fleming's left-hand rule.
Answer: Stretch the first finger, middle finger, and thumb of your left hand mutually perpendicular to each other. If the first finger points in the direction of the magnetic field, middle finger in the direction of current, then the thumb points in the direction of force on the conductor.
Q16. On what factors does the force on a current-carrying conductor in a magnetic field depend?
Answer:
- Current strength: Greater current, greater force
- Magnetic field strength: Stronger field, greater force
- Length of conductor: Longer conductor, greater force
- Angle between conductor and field: Maximum when perpendicular
Q17. When is the force on a current-carrying conductor maximum and minimum?
Answer:
- Maximum: When the conductor is perpendicular to the magnetic field
- Minimum (Zero): When the conductor is parallel to the magnetic field
PART E: ELECTRIC MOTOR
Q18. What is an electric motor?
Answer: An electric motor is a device that converts electrical energy into mechanical energy. It works on the principle that a current-carrying conductor placed in a magnetic field experiences a force.
Q19. Explain the working principle of a DC motor.
Answer:
- A rectangular coil is placed in a magnetic field
- When current flows through the coil, it experiences force
- The coil rotates due to this force
- Commutator reverses the current direction every half rotation
- This ensures continuous rotation in the same direction
Q20. What is the function of a commutator in a DC motor?
Answer: The commutator reverses the direction of current in the coil every half rotation. This ensures that the force on the coil always acts in the same direction, maintaining continuous rotation.
Q21. What is the function of brushes in a DC motor?
Answer: Brushes maintain electrical contact between the external circuit and the rotating commutator. They allow current to flow into the rotating coil.
PART F: ELECTROMAGNETIC INDUCTION
Q22. What is electromagnetic induction?
Answer: Electromagnetic induction is the phenomenon of producing electric current in a conductor when there is a change in magnetic flux linked with it.
Q23. State Faraday's laws of electromagnetic induction.
Answer: First Law: Whenever there is a change in magnetic flux linked with a conductor, an EMF is induced in it. Second Law: The magnitude of induced EMF is directly proportional to the rate of change of magnetic flux.
Q24. State Lenz's law.
Answer: The direction of induced current is such that it opposes the change that produces it. This law is a consequence of the conservation of energy.
Q25. What is Fleming's right-hand rule?
Answer: Stretch the first finger, middle finger, and thumb of your right hand mutually perpendicular. If the first finger points in the direction of magnetic field and the thumb in the direction of motion of conductor, then the middle finger points in the direction of induced current.
PART G: ELECTRIC GENERATOR
Q26. What is an electric generator?
Answer: An electric generator is a device that converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction.
Q27. Explain the working of an AC generator.
Answer:
- A rectangular coil rotates in a magnetic field
- As the coil rotates, magnetic flux through it changes
- This induces an EMF in the coil
- The EMF and current change direction twice in each rotation
- Slip rings maintain continuous contact with external circuit
Q28. What is the difference between AC and DC generators?
Answer: AC Generator:
- Uses slip rings
- Produces alternating current
- Current changes direction
- More efficient
DC Generator:
- Uses commutator
- Produces direct current
- Current flows in one direction
- Less efficient due to sparking
Q29. Why is AC preferred over DC for power transmission?
Answer:
- AC can be easily stepped up or down using transformers
- High voltage AC reduces power loss during transmission
- AC generators are simpler and more efficient
- AC motors are more efficient and reliable
PART H: APPLICATIONS AND NUMERICAL PROBLEMS
Q30. List some applications of electromagnetic induction.
Answer:
- Electric generators (AC and DC)
- Transformers
- Induction motors
- Magnetic braking systems
- Induction cookers
- Metal detectors
Q31. A conductor of length 50 cm moves with velocity 10 m/s perpendicular to a magnetic field of 0.5 T. Calculate the induced EMF.
Answer: Given: l = 50 cm = 0.5 m, v = 10 m/s, B = 0.5 T EMF = Blv = 0.5 × 0.5 × 10 = 2.5 V
Q32. Why does a compass needle deflect when brought near a current-carrying conductor?
Answer: A current-carrying conductor produces a magnetic field around it. This magnetic field interacts with the magnetic field of the compass needle, causing it to deflect from its north-south orientation.
Q33. How can you increase the strength of an electromagnet?
Answer:
- Increase the current through the coil
- Increase the number of turns in the coil
- Use a soft iron core
- Reduce the air gap in the magnetic circuit
Q34. Why is soft iron used as the core of an electromagnet?
Answer:
- Soft iron has high magnetic permeability
- It gets easily magnetized and demagnetized
- It has low hysteresis loss
- It increases the strength of the magnetic field
PART I: CONCEPTUAL QUESTIONS
Q35. Can you shield yourself from a magnetic field? Explain.
Answer: Unlike electric fields, magnetic fields cannot be completely shielded. However, materials with high magnetic permeability (like soft iron) can redirect magnetic field lines, providing partial shielding.
Q36. Why don't we feel the magnetic field around current-carrying wires in our homes?
Answer:
- The current in household wires is relatively small
- AC current produces rapidly changing magnetic fields
- The magnetic field strength decreases rapidly with distance
- Our bodies are not sensitive to weak magnetic fields
Q37. What happens to the magnetic field when the current in a solenoid is doubled?
Answer: When the current in a solenoid is doubled, the magnetic field strength also doubles. This is because the magnetic field is directly proportional to the current.
Q38. Explain why the magnetic field inside a solenoid is uniform.
Answer: Inside a solenoid, the magnetic field lines from all the turns of the coil add up in the same direction, creating parallel and equally spaced field lines, which represent a uniform field.
Q39. How is the direction of force on a current-carrying conductor related to the direction of current and magnetic field?
Answer: The force, current, and magnetic field are mutually perpendicular to each other. The direction of force can be determined using Fleming's left-hand rule.
Q40. Why is electromagnetic induction important in modern technology?
Answer: Electromagnetic induction is the basis for:
- Power generation in generators
- Power transformation in transformers
- Working of electric motors
- Wireless charging technology
- Magnetic braking systems
- Induction heating
IMPORTANT FORMULAS TO REMEMBER:
- Force on current-carrying conductor: F = BIL sin θ
- Induced EMF (motional): EMF = BLv
- Induced EMF (Faraday's law): EMF = -dΦ/dt
- Magnetic flux: Φ = BA cos θ
STUDY TIPS:
- Practice drawing magnetic field patterns regularly
- Use right-hand and left-hand rules frequently
- Understand the difference between motor and generator action
- Solve numerical problems step by step
- Connect concepts to real-life applications
- Review the working of electric motors and generators thoroughly