Magnetic Effects of Electric Current Test

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Magnetic Effects of Electric Current Test - 31

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Question 1
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The magnet is strongest near the

A
Poles of the magnet

B
Ends of the magnet

C
Centre of the magnet

D
One quarter point from the poles of the magnet

Solution

A magnet is strongest near the poles at that point all the field lines are present and its density is maximum thus strongest.
Question 2
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The diagram below shows a wire carrying conventional current in the direction shown by the red arrows.
The magnetic field is straight downward, as indicated by the blue X's.
What is the direction of the force exerted by the magnetic field on the wire?

A
Upward

B
Downward

C
Northward

D
Southward

E
Westward

Solution

The direction of force on a current carrying conductor placed normal to a magnetic field is given by Fleming's left hand rule , if we apply this rule for the given diagram , the direction of force will be downward.
Question 3
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A straight wire carries current toward the east.
What is the direction of the magnetic field directly below the wire?

A
Northward

B
Southward

C
Eastward

D
Westward

E
Up

Solution

According to the right hand thumb rule, if one holds the wire carrying current in right hand so that the thumb points along the wire in the direction of the current, then the fingers will encircle the wire in the direction of the lines of magnetic force.
In this case, when thumb points towards east, the fingers curl to point towards north directly below the wire.
Question 4
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A straight wire carries conventional current toward the sky.
What is the direction of the magnetic field due to the current at a point west of the wire?

A
North

B
South

C
East

D
West

E
Up

Solution

Using Right-hand thumb rule, the direction of magnetic field due to a straight wire is shown in the figure above.
If we align our thumb in direction of current and curl our fingers anti-clockwise then at P out fingertips will point SOUTH.
Thus magnetic field at point P points in the south direction i.e option B is correct.
Question 5
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As a negatively charged bowling ball rolls eastward toward some bowling pins, in what direction does it experience a force, due to the earth's magnetic field (Assume the earth's field is pointing straight north)?

A
North

B
South

C
East

D
West

E
Down

Solution

According to the figure, $$\vec{v} = v\hat{y}$$ $$\vec{B} = -B\hat{x}$$
Charge of the ball $$Q = -q$$
$$\therefore$$ Magnetic force $$\vec{F} = -q(\vec{v}\times \vec{B}) = -q [v\hat{y} \times (-B)\hat{x}] = qvB (\hat{y} \times \hat{x}) =qvB $$ $$(-\hat{z})$$
Thus the ball experience a magnetic force in downward direction.
Question 6
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All of the following magnets pictured have magnetic field lines like the ones pictured in the diagram above. For which one are the poles correctly labeled?

A

B

C

D

E

Solution

The magnetic field lines originate at the North pole $$(N)$$ and terminate at the South pole $$(S)$$ making closed loops.
Hence option B is correct.
Question 7
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An electron enters a magnetic field directed to the right, with a velocity toward the bottom of the screen. What is the direction of the force on the electron?

A
To the left

B
Into the screen

C
To the right

D
Out of the screen

Solution

It is given that the direction of the magnetic field is rightwards and the direction of the electron is from top to bottom of the screen hence the direction of the current is from bottom to top of the screen. Now by applying Fleming's left-hand rule, the direction of the force is directed into the screen.
Question 8
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In alternating current:

A
The direction of current is always positive

B
The direction of current is always negative

C
The direction of current changes constantly

D
None of these

Solution

A current that changes direction after equal intervals of time is called an alternating current.
Question 9
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Energy in a current - carrying coil is stored in the form of :

A
Electric field

B
Magnetic field

C
Dielectric strength

D
Heat

Solution

Energy is actually stored in the magnetic field generated by the current flowing through the inductor
Question 10
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The magnetic field lines due to a bar magnet correctly shown are:

A

B

C

D

Solution

As the magnetic field lines are closed curves.
Thus they must pass through the magnets as well and its direction is from south to north for same sense (direction)