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Magnetism is the force that attracts a metal object towards a magnet. All magnets possess a magnetic field around it which has its influence running from the north pole of the magnet to the south pole. Magnets in today’s world have a lot of applications. Electromagnets are another category of man-made magnets where by passing an electric field through a conductor, the metal acts as a magnet until the constant electric field is applied.
Gauss’s law is one of the most important equations in magnetism. The surface through which the magnetic fields are passing is known as the Gaussian surface. Gauss’s law states that the electric field passing through any closed surface is the ratio of the charge “q” it encloses and the volume enclosed by it. Unlike electric charges, magnetic poles cannot exist alone, they are always found in pairs. There it is almost impossible to have a unipolar magnet. Therefore, Gauss’s law for magnetism states that “The net magnetic flux through any closed surface is zero
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MCQs on Magnetism and Matter
Ques. The magnetic field lines of force inside a bar magnet:
- From S pole to N pole of the magnet
- Do not exist
- From N pole to S pole of the magnet
- Area of the cross-section of the magnet
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Ans. a) From S pole to N pole of the magnet
Explanation - We know that the magnetic lines of force are always present in a closed loop. So, by convention, the field lines are taken to the outward direction from the N pole to the S pole of the magnet. However, inside the magnetic field, the field lines are present from the S pole to the N pole.
Ques. The magnetic lines of force are:
- Closed curves
- Intersect far away from the poles
- Always intersect
- Do not pass through a vacuum
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Ans. a) Closed curves
Explanation - The magnetic field lines are a closed curve. Inside the magnetic field, lines are directed from the S pole to the N pole, and outside the magnetic field lines from the north pole to the south pole.
Magnetic field lines
Ques. What is the value of the angle of dip at the magnetic equator?
- 45 degree
- 0 degree
- 90 degree
- 60 degree
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Ans. b) 0 degree
Explanation - In the magnetic field, the immersion needle stays horizontally at zero-degree angles while, on magnetic poles, the magnetic needle stays upright, at a 90-degree angle. In all other areas, the angle of dip is between 0 and 90 degrees.
Ques. What is a permanent magnet?
- The magnetic field is removed
- It contains a metal core made from iron
- Do not lose their magnetic characteristics
- Controlled by electric current
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Ans. c) Do not lose their magnetic characteristics
Explanation - Permanent magnets are the most commonly used. They are known as permanent magnets because they do not lose their magnetic properties when they are magnetized. The four types of permanent magnets are -
- Ceramic
- Alnico
- Samarium cobalt (SmCo)
- Neodymium iron boron (NIB)
Ques. A magnet of magnetic moment M revolves by 360° in a magnetic field of magnitude H. What will be the work done?
- 0
- -MH
- MH
- 2MH
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Ans. a) Zero
Explanation - Work done will be denoted as W
W = -MH (cosθ? - cosθ?)
= -MH (cos360° - cos 0°)
= 0
Ques. Ferrites can be:
- Ferromagnetic
- Ferrimagnetic
- Ant. Ferromagnetic
- None of the above
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Ans. a) Ferromagnetic
Explanation - The ferrites are ferromagnetic materials that have compounds of iron, boron, barium, or strontium and molybdenum. Ferrite material has a high magnetic field, which allows it to hold magnets stronger than iron, and they are also known as ceramic magnets. Ferrite ceramics and ferromagnetic materials have many uncoated electrons and are aligned to form a measurable macroscopic field.
Ques. Which option is not correct about the uses of ferrites and ferromagnetic?
- Making powder
- Used as a radar-absorbing material
- Used in electric conductors
- Used in leather making
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Ans. d) Used in leather making
Explanation - Ferrite ceramics and ferromagnetic materials are usually produced as powders, which can be soaked in solid water. Ferrite cores are used in electrical inductors, transformers, and electromagnets where the high electrical resistance of ferrite leads to very low current eddy losses. Previous computer memory stores data in the remaining magnetic fields of the ferrite cores, which are grouped as the primary memory chain. Their powders are used in magnetic tape recording tapes. Ferrite particles are also used as part of radar absorbers used in low-cost aircraft and expensive suction tiles lined the chambers used for magnetic compliance measurements.
Ques. If a hole is created at the center of the magnetic bar, then its magnetic moment will be -
- Vanishes
- Decreases
- Increases
- Does not change
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Ans. d) Does not change
Explanation - The magnetic bar has pole power and length. These two numbers determine the so-called magnetic moment of the magnet bar. Magnetic time is defined as the force of the pole multiplied by the division of the poles north and south of the bar. The split between poles can be considered as the magnetic length of the magnet (although the pole split is less than the length). When the hole is cut at the magnet bar, the pole strength does not change, as the number of free poles eventually does not change. The length does not change. Therefore, the magnetic properties of the magnetic field should not change.
Ques. In a coil galvanometer, we use a radial magnetic field so that the galvanometer scale is -
- Linear
- Algebraic
- Exponential
- logarithmic
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Ans. a) Linear
Explanation - Radial magnetic field is that field, where the coil plane in all positions corresponds to the magnetic field. A radial magnetic field is applied to the galvanometer of a moving coil to produce constant torque from the coil. It is used to measure the relationship between a current and an indirect angle so that the current cannot be easily measured.
Ques. Tangent law is applicable when:
- Two magnetic fields exist
- Uniform magnetic fields are used
- Two uniform and perpendicular magnetic fields exist
- Horizontal components of Earth’s magnetic field are present
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Ans. c) Two uniform and perpendicular magnetic fields exist
Explanation - When a magnet is placed in two magnetic fields on the same plane, the magnet creates an θ angle with a magnetic field provided as -
B = BH tanθ
Ques. The magnetic dipole moment of a solenoid having N turns is given as -
- NIA2
- NIA
- NI2A
- NI2A2
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Ans. b) NIA
Explanation - Magnetic moment is defined as the magnetic field that interacts with an applied field to produce a working moment. In the current loop, m = IA, A vector value, a position is considered a vertical vector. So, the magnetic moment of a solenoid is m = NIA.
Where N = number of turns
I = solenoid current
A = cross-sectional area of the solenoid coil
Ques. A magnetic needle is lying parallel to the magnetic requires W units of work to turn it through 60°. Find the value of torque needed to maintain the needle in this position.
- W
- W√3
- 2W
- (W/2) √3
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Ans. b) W√3
Explanation - W = MB (1 - cos 60°) = MB/2
Now, torque = MB sin 60°
= √3 MB/2
= W√3
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