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Summary of the Chapter-12-Magnetic Effects of Electric Current:
- A compass needle is essentially a small magnet. One end, which points towards the north, is referred to as the north pole, while the other end, pointing south, is called the south pole.
- The region around a magnet contains a magnetic field, where the magnetic force can be observed.
- Magnetic fields are represented using field lines. These lines depict the path a hypothetical free north pole would follow. The direction of the magnetic field at any point is indicated by the direction a north pole would move if placed at that location. The lines are denser where the magnetic field is stronger.
- A metallic wire with an electric current generates a magnetic field. The field lines around the wire form concentric circles, and their direction can be determined using the right-hand rule.
- The shape of the magnetic field around a conductor depends on the shape of the conductor. The magnetic field around a current-carrying solenoid resembles that of a bar magnet.
- An electromagnet is made by wrapping a soft iron core with a coil of insulated copper wire.
- A conductor carrying current experiences a force when placed in a magnetic field. If the current and the magnetic field are perpendicular to each other, the force acting on the conductor will be perpendicular to both, as described by Fleming’s left-hand rule.
- Household electricity is supplied as alternating current (AC) at 220 V and 50 Hz frequency. The live wire has red insulation, and the neutral wire is black, with a potential difference of 220 V between them. A green earth wire is connected to a metal rod buried in the ground as a safety feature, preventing electric shocks in case of current leakage.
- Fuses are essential safety devices that protect circuits from damage due to short circuits or overloading.
NCERT Based MCQ with Explanation:
1. What happens when an electric current is passed through a copper wire?
(a) It generates heat
(b) It produces a magnetic field
(c) It creates light
(d) It stops the flow of electricity
Explanation: Passing an electric current through a conductor like copper generates a magnetic field around it.
2. Who discovered the relationship between electricity and magnetism?
(a) Nikola Tesla
(b) Albert Einstein
(c) Hans Christian Oersted
(d) Michael Faraday
Explanation: In 1820, Oersted discovered that an electric current could deflect a compass needle, linking electricity to magnetism.
3. Which of the following is true about magnetic field lines?
(a) They start from the south pole and end at the north pole outside a magnet
(b) They are always straight
(c) They form closed loops
(d) They cross each other at various points
Explanation: Magnetic field lines emerge from the north pole and merge at the south pole outside a magnet, and form a continuous loop inside the magnet.
4. What is the direction of the magnetic field inside a solenoid?
(a) From north to south
(b) From south to north
(c) Perpendicular to the wire
(d) It is random
Explanation: Inside a solenoid, the magnetic field lines run from the south pole to the north pole.
5. What is a solenoid?
(a) A device that converts electrical energy into mechanical energy
(b) A coil of many circular turns of insulated copper wire
(c) A magnet used in MRI machines
(d) A device used to store electrical energy
Explanation: A solenoid is a cylindrical coil of wire, often used to create a uniform magnetic field when electric current passes through it.
6. Which rule helps determine the direction of the magnetic field around a current-carrying conductor?
(a) Fleming's right-hand rule
(b) Right-hand thumb rule
(c) Maxwell’s equation
(d) Ampere’s law
Explanation: According to the right-hand thumb rule, if the thumb points in the direction of the current, the curled fingers show the direction of the magnetic field.
7. What is the shape of the magnetic field around a straight current-carrying conductor?
(a) Concentric circles
(b) Straight lines
(c) Zigzag lines
(d) Radial lines
Explanation: The magnetic field lines around a straight conductor form concentric circles, the strength of which diminishes as the distance from the conductor increases.
8. Which device uses electromagnets in its operation?
(a) Compass
(b) Speaker
(c) Battery
(d) Light bulb
Explanation: Electromagnets are used in speakers to convert electrical energy into sound.
9. How can you increase the strength of a solenoid's magnetic field?
(a) Decrease the current
(b) Increase the number of turns in the coil
(c) Use a thicker wire
(d) Reduce the voltage
Explanation: Increasing the number of turns in the solenoid increases the magnetic field as each turn adds to the overall field strength.
10. Which of the following correctly describes the magnetic field near a long straight wire?
(a) Radial lines originating from the wire
(b) Concentric circles centered on the wire
(c) Parallel lines along the wire
(d) Field lines crossing each other
Explanation: The magnetic field around a long straight current-carrying wire forms concentric circles centered on the wire.
11. Which rule is used to determine the direction of force on a current-carrying conductor in a magnetic field?
(a) Fleming’s right-hand rule
(b) Maxwell’s corkscrew rule
(c) Fleming’s left-hand rule
(d) Right-hand thumb rule
Explanation: Fleming’s left-hand rule helps determine the direction of force on a current-carrying conductor placed in a magnetic field by using the thumb, forefinger, and middle finger perpendicular to each other.
12. What will happen to the deflection of a compass needle if the current in a nearby conductor is increased?
(a) The deflection decreases
(b) The deflection increases
(c) The deflection remains the same
(d) The needle stops moving
Explanation: The magnetic field produced by a conductor increases with the current, causing a greater deflection of the compass needle.
13. What determines the direction of the magnetic field produced by a current in a straight wire?
(a) The thickness of the wire
(b) The length of the wire
(c) The direction of the current
(d) The material of the wire
Explanation: The direction of the magnetic field around a straight conductor is determined by the direction of the current using the right-hand thumb rule.
14. Why do magnetic field lines never intersect each other?
(a) Because magnetic forces cancel out at intersection points
(b) Because two different directions of force at one point are impossible
(c) Because they are always parallel
(d) Because the magnetic field strength is too weak
Explanation: If magnetic field lines intersect, it would imply that the magnetic field has two directions at the point of intersection, which is not possible.
15. In which direction do the magnetic field lines outside a bar magnet move?
(a) From north to south
(b) From south to north
(c) In concentric circles
(d) They do not move
Explanation: Outside a bar magnet, the magnetic field lines emerge from the north pole and merge at the south pole.
16. What happens to the magnetic field when the current direction is reversed in a wire?
(a) The magnetic field disappears
(b) The magnetic field becomes stronger
(c) The magnetic field's direction reverses
(d) The magnetic field weakens
Explanation: Reversing the current in a wire also reverses the direction of the magnetic field, as indicated by the right-hand thumb rule.
17. Which of the following devices uses a solenoid?
(a) Loudspeaker
(b) Transformer
(c) Electric bell
(d) Battery
Explanation: Solenoids are used in electric bells to generate a magnetic field that moves the hammer to strike the bell.
18. What does the right-hand thumb rule indicate about a straight current-carrying conductor?
(a) The magnitude of the magnetic field
(b) The direction of the magnetic field
(c) The strength of the electric current
(d) The speed of electrons
Explanation: The right-hand thumb rule helps determine the direction of the magnetic field around a straight conductor by aligning the thumb with the current direction and the curled fingers indicating the field lines.
19. What happens to the magnetic field as we move farther away from a current-carrying wire?
(a) It becomes stronger
(b) It remains the same
(c) It becomes weaker
(d) It changes direction
Explanation: The strength of the magnetic field decreases as the distance from the current-carrying wire increases.
20. Which of the following is NOT a property of magnetic field lines?
(a) They form closed loops
(b) They never intersect
(c) They are strongest at the poles
(d) They are straight lines
Explanation: Magnetic field lines are curved and form closed loops; they do not intersect or form straight lines except at points of infinitesimal distance from the source.
21. What is the direction of the magnetic field in a current-carrying circular loop?
(a) Concentric circles inside the loop
(b) Straight lines inside the loop
(c) Radial lines outside the loop
(d) Curved lines moving away from the loop
22. The strength of the magnetic field produced by a current-carrying conductor depends on which factor?
(a) The color of the conductor
(b) The amount of current passing through the conductor
(c) The length of the conductor
(d) The type of material used
23. What is a common use of an electromagnet?
(a) Electric fan
(b) Radio transmission
(c) Magnetic cranes
(d) Lightning rods
24. Which of the following statements about solenoids is correct?
(a) A solenoid has a uniform magnetic field inside it
(b) A solenoid has no magnetic field
(c) A solenoid’s magnetic field decreases as current increases
(d) A solenoid’s magnetic field is strongest at the center
25. What does Fleming’s left-hand rule help determine?
(a) The direction of magnetic force on a conductor
(b) The strength of the magnetic field
(c) The direction of electric current
(d) The magnitude of current in a circuit
26. What happens to the force on a current-carrying conductor when it is placed parallel to a magnetic field?
(a) The force becomes maximum
(b) The force becomes zero
(c) The force decreases but not to zero
(d) The force fluctuates
27. How can you increase the magnetic field strength of a solenoid?
(a) Decrease the number of turns in the coil
(b) Increase the resistance in the circuit
(c) Increase the number of turns in the coil
(d) Use a shorter coil
28. What causes deflection in a compass when placed near a current-carrying wire?
(a) The heat generated by the wire
(b) The magnetic field produced by the current
(c) The electric field produced by the wire
(d) None of the above
29. What is the purpose of using soft iron in an electromagnet?
(a) To reduce the current flow
(b) To increase the magnetic field strength
(c) To increase the electric resistance
(d) To create a permanent magnet
30. Which direction will the force act on a current-carrying conductor placed perpendicular to a magnetic field?
(a) Along the direction of the current
(b) Perpendicular to both the current and magnetic field
(c) Along the direction of the magnetic field
(d) Opposite to the current flow
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