Magnetic Effects of Electric Current Test

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Magnetic Effects of Electric Current Test - 27

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Question 1
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You are sitting in a room in which a strong magnetic field is directed from your left towards right. A beam of electrons is directed from front towards you. What would be the direction of magnetic force on this beam ?

A
downwards.

B
upwards.

C
towards right

D
towards left

Solution

The direction of the magnetic field is from left to right, the direction of the beam of electrons is from your front hence the direction of the current is from back to front. Now by applying Fleming left hand rule such that middle finger point from back to front and index finger point from left to right then thumb will point downwards hence the magnetic force on the beam of electron is downwards.
Question 2
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Magnetic field lines

A
Cannot interest at all

B
Intersect at infinity

C
Intersect within the magnet

D
Intersect at the neutral points
Solution
The correct answer is option (A).

$$\textbf{Explanation}$$
$$\bullet$$ Tangent on magnetic field lines give direction of magnetic field.
$$\bullet$$If the magnetic field lines intersect at any point, then at the point of intersection, two tangents can be draw which will give two different directions of the field at that point. This is contradictory.
$$\bullet$$So, magnetic field line can not intersect each other.
Hence, magnetic field lines cannot intersect at all.
The correct answer is option (A).
Question 3
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Magnetic lines of force inside a bar magnet

A
Are from north pole to south pole of the magnet

B
Do not exist

C
Depend upon the area of cross section of the bar magnet

D
Are from south pole to north pole of the magnet

Solution

$$\textbf{Correct option: Option (D).}$$

$$\textbf{Explanation:}$$

Magnetic lines of force from a bar magnet always forms closed-loop and by convention, they are taken to be coming out of the north pole and going inside the south pole forming a closed-loop and hence inside a bar magnet the direction of magnetic lines of force is from south pole to north pole of the magnet.

$$\textbf{Thus, option (D) is correct.}$$
Question 4
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A solenoid is_____

A
an electromagnet

B
a temporary magnet

C
a permanent magnet

D
Both 1 and 2

Solution

A solenoid is a type of electromagnet whose purpose is to generate a controlled magnetic field. A solenoid is a long piece of wire which is wound in the shape of a coil. Sometimes an iron core is also inserted inside the coil. When the electric current passes through the coil it creates a relatively uniform magnetic field inside the coil.
So, a solenoid behaves as an electromagnet. We also know that an electromagnet is a temporary magnet as it shows the magnetic properties only when the current is present. Once we withdraw the current, the magnetic properties will vanish.
Question 5
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Which of the following figures represents the magnetic lines of force due to an isolated south pole?

A

B

C

D

Solution

By convention, all the magnetic lines of force emerge from north pole and terminate at the south pole.
Hence for a hypothetical case of an isolated south pole, the magnetic lines of force would enter into it uniformly in all directions.
Hence Option A
Question 6
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Which of the following statements is INCORRECT?

A
Electric current is a scalar quantity.

B
Electric lines of force are closed curves.

C
Magnetic lines of induction are closed curves.

D
Changing magnetic field induces an electric current in a coil.

Solution

Answer is B.

Magnetic field is a quantity that has both direction and magnitude. The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it. Therefore it is taken by convention that the field lines emerge from north pole and merge at the south pole. Inside the magnet, the direction of field lines is from its south pole to its north pole. Thus the magnetic field lines are closed curves.
The relative strength of the magnetic field is shown by the degree of closeness of the field lines. The field is stronger, that is, the force acting on the pole of another magnet placed is greater where the field lines are crowded.
No two field-lines are found to cross each other. If they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.
Question 7
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Which one of the following statements is true?

A
A motor works on the principle of electromagnetic induction

B
An electric motor converts mechanical energy into electrical energy

C
AC generator has slip rings while DC generator has a commutator

D
An electric generator converts electrical energy into mechanical energy

Solution

$$(a)$$ Motor works on the principle of mechanical effect of magnetic field on a current carrying wire.
$$(b)$$Electric motor converts electrical energy into mechanical energy
$$(c)$$AC generator has slip rings while DC generator has commutator or split rings.
$$(d)$$ Electric generator converts mechanical energy into electrical energy.

Answer-(C)
Question 8
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The direction of magnetic field line in case of a bar magnet at point A & B are

A
$$\leftarrow$$ at A and $$\rightarrow$$ at B

B
$$\leftarrow$$ at A and $$\leftarrow$$ at B

C
$$\rightarrow$$ at A and $$\rightarrow$$ at B

D
$$\rightarrow$$ at A and $$\leftarrow$$ at B

Solution

In a bar magnet, the field lines emerge from the north pole and merge at the south pole. Inside the magnet, the direction of field lines is from its south pole to its north pole.
Therefore, The direction of the magnetic field line at point $$A$$ is towards the right $$(\rightarrow)$$.
The direction of the magnetic field line at point $$B$$ is towards the left $$(\leftarrow)$$.
Question 9
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The magnetic field lines in the middle of the current-carrying solenoid are.

A
spirals

B
circles

C
perpendicular to the axis of the tube

D
parallel to the axis of the tube

Solution

Answer is D.

A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid. One end of the solenoid behaves as a magnetic north pole, while the other behaves as the south pole. The field lines inside the solenoid are in the form of parallel straight lines. This indicates that the magnetic field is the same at all points inside the solenoid. That is, the field is uniform inside the solenoid.
Hence, the magnetic field lines in the middle of the current-carrying solenoid are parallel to the axis of the tube.
Question 10
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In a current-carrying solenoid, the magnetic field direction is given by the right hand rule such that the

A
Encircling fingers indicate the direction of electric current

B
Encircling fingers indicate the direction of magnetic field

C
Extended right thumb will point in the direction of the axial magnetic field

D
Both $$(B)$$ and $$(C)$$