# Magnetism and Electromagnetism MCQ Question & Answer

Ques.1. The strength of an electromagnet can be increased by

1. I
2. Increasing the number of turns
3. Increasing current supply
4. All of the above

Answer.4. All of the above

The strength of an electromagnet is directly proportional to

⇒ Increasing current supply:- The strength of the current flowing through the electromagnetic coil. By increasing the current through a solenoid. the strength electromagnet is increased.

⇒ Increasing the number of turns:- Greater the number of turns of winding more stronger is the electromagnet.

⇒ Cross-sectional Area:- By increasing the cross-sectional Area the resistance decrease, therefore the amount of current in the coil Increase

Also, the use of a core material of high permeability will increase an electromagnet’s strength.

Ques.2. What is the magneto-motive force (MMF) of a wire with 8 turns carrying three amperes of current?

1. 2,400 At
2. 24 At
3. 240 At
4. 2.4 At

Given

Number of turns = 8

Current = 3 A

MMF = Current × Number of turns

Fm = N × I

Fm = 8 × 3 = 300 At

Ques.3. What is the magneto-motive force in a 75-turn coil of wire when there are 4 A of current through it?

1. 300 At
2. 18.75 At
3. 187 At
4. 30 At

Given

Number of turns = 75

Current = 4 A

MMF = Current × Number of turns

Fm = N × I

Fm = 75 × 4 = 24 At

Ques.4. Which of the following is a vector quantity?

1. Relative permeability
2.
3. Magnetic potential

Answer.3. Magnetic field intensity

Scalar Quantity:-  The scalar is a quantity whose value may be represented by a single real number, which may be positive or negative. The direction is not at all required in describing a scalar. Thus, A scalar is a quantity which is wholly characterized by its magnitude. The various examples of scalar quantity are temperature, mass, volume, density, speed, Relative permeability, Flux density, Magnetic potential, electric charge etc.

Vector Quantity:- A quantity that has both, a magnitude and a specific direction in space is called a vector.

The magnetic field intensity is a vector quantity H with units of amperes/meter. Its direction is around the conductor. Its magnitude is given by:

H = (I/2πr) Ampere/meter

Ques.5. The magnetism of a magnet can be destroyed by

1. Hammering
2. Heating
3. By Alternating current
4. By all the above methods

Answer.4. By all the above methods

Materials that are attracted to a magnet are magnetic materials. Magnets and magnetic materials contain “tiny magnets’ called magnetic domains, and materials exhibit magnetic properties when these magnetic domains become aligned. Magnets come in a variety of sizes, shapes, and strengths. Heating, jarring, shaking, or dropping a magnet can destroy its magnetism.

By alternating current:- The strong magnetic field produced in the solenoid by the alternating current changes its direction many times a second. This means that the magnet inside the solenoid is magnetized first one way and then the opposite way, repeatedly. As the magnet is withdrawn from the solenoid, the magnetism becomes weaker and weaker until finally, the magnet is completely demagnetized.

Demagnetization by Heating and Hammering:- Other methods of demagnetization include heating and hammering. If a magnet is hammered vigorously while lying in an east-west direction, its magnetism will become weaker or even disappear. The magnetism of a magnet may also be destroyed if the magnet is heated to red-hot and then allowed to cool while lying in an east-west direction.

Ques.6. One maxwell is equal to

1. 10-8 webers
2. 10-4 webers
3. 108 webers
4. 104 webers

The Maxwell is the CGS unit of magnetic flux.

Weber is equivalent to 100000000 or 108 Maxwell.

So 1 Maxwell is equal to 10-8 webers

Ques.7. What will be the magnetic potential difference across the air gap of 2 cm length in the magnetic field of 200 AT/m?

1. 10 AT
2. 6 AT
3. 4 AT
4. 2 AT

The magneto-motive force or difference of magnetic potential along a magnetic circuit is

MMF = H × L

Where

H =  magnetic field strength measured in Amperes per meter = 200 AT/m

L = Length in meter = 2cm = 2/100 m

MMF = 200 × 2/100

MMF = 4 AT

Ques.8. A magnetic field exists around

1. Moving charges
2. Copper
3. Iron
4. Aluminum

The energy of electric fields affects both moving and motionless charges, whereas that of magnetic fields affects only the moving charges. The static electric field is present around motionless charges, and the static magnetic field exists in the vicinity of conductors of direct current (DC) or permanent magnets.

Ques.9. Temporary magnets are used in

1. Electric-bell
2. Generators
3. Motors
4. All of the above

Answer.4. All of the above

Temporary magnets:-  Iron or soft steel will retain magnetism so long as the magnetizing force is applied. Once the magnetizing force is removed, they will lose nearly all their magnetism. Magnets made from such materials are called temporary magnets. They are used where temporary magnetism is required.

Electromagnets are temporary and are used where one wants to be able to vary the strength of the magnetic field (by varying the current) and switch it on and off. As well as being used in cranes to lift iron objects, scrap iron, etc.  Electromagnets are an essential part of many electrical devices such as an electric bell, magnetic locks, relays, and practical motors and generators.

Que.10. When an iron piece is placed in a magnetic field

1. The magnetic lines of force will bend away from their usual paths in order to go away from the piece
2. The magnetic lines of force will bend away from their usual paths in order to pass through the piece
3. The magnetic field will not be affected
4. The iron piece will break

Answer.2. The magnetic lines of force will bend away from their usual paths in order to pass through the piece

Magnetic Shields

Because all materials have some ability to conduct magnetic lines of force, it is not practical to make a magnetic insulator. Strong magnets placed near electrical instruments can cause permanent damage. Therefore, it is necessary to prevent the magnetic flux (lines of force) from passing through the instrument. Magnetic flux can be bent, distorted, and guided by low-reluctance materials inserted in the magnetic field.

The external field of a magnet is distorted when any magnetic substance is placed in that field because it is easier for the lines of force to travel through the magnetic substance than through the air. The opposition of a material to magnetic flux is called reluctance and compares to resistance in an electric circuit. When a piece of soft iron is placed near a magnet, the field is distorted and the lines tend to pass through the iron instead of the air leaving the field weaker elsewhere. The effect of an iron ring or of a cylinder placed in a magnetic field is shown in Figure.  Iron is a better conductor of magnetism than air. This fact is utilized in constructing magnetic shields. Many expensive instruments that may be subjected to magnetic influences are surrounded by iron or iron alloys. The iron provides a magnetic path around the sensitive parts. There is little tendency for the flux lines to leave the iron and pass through an air medium and to return again to the same iron path.

Magnetic screening is of frequent application in telecommunication equipment to prevent undesirable magnetic interference between adjacent components.

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