Charges & Magnetism Test

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Charges & Magnetism Test - 12

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Question 1
1 / -0

When a magnetic compass needle is carried nearby to a straight wire carrying current, then
(i) the straight wire cause a noticeable deflection in the compass needle.
(ii) the alignment of the needle is tangential to an imaginary circle with straight wire as its centre and has a plane perpendicular to the wire

A
I is correct

B
II is correct

C
both (i) and (ii) are correct

D
neither (i) and (ii) is correct

Solution

Both points are correct and these are the result of experiments done by Danish physicist Hans Christian Oersted in 1820.
Question 2
1 / -0

Which one of the following is not correct about Lorentz force?

A
In presence of electric field and magnetic field the force on a moving electric charge is

B
The force, due to magnetic field on a negative charge is opposite to that on a positive charge.

C
The force due to magnetic field become zero if velocity and amgnetic field are parallel or anti-parallel.

D
For a static charge the magnetic force is maximum.

Solution

If charge is not moving then the magnetic force is zero.
Since
As for stationary charge ∴ .
Question 3
1 / -0

A strong magnetic field is applied on a stationary electron. Then the electron

A
moves in the direction of the field.

B
remains stationary.

C
moves perpendicular to the direction of the field.

D
moves opposite to the direction of the field.

Solution

As
As the electron is stationary. ∴ velocity
∴ So, electron will remain stationary.
Question 4
1 / -0

In an inertial frame of reference, the magnetic force on a moving charged partic le is Its value in another inertial frame of reference will be

A
remain same

B
changed due to change in the amount of charge

C
changed due to change in velocity of charged particle

D
changed due to change in field direction

Solution

which shows magnetic field is velocity dependent due to which it differs from one intertial frame to another.
Question 5
1 / -0

The magnetic force on a current carrying conductor of length l in an external magnetic field is given by

A

B

C

D
Question 6
1 / -0

A straight wire having mass of 1.2 kg and length of 1m carries a current of 5A. If the wire is suspended in mid-air by a uniform horizontal magnetic field, then the magnitude of field is

A
0.65 T

B
1.53 T

C
2.4 T

D
3.2 T

Solution

For mid-air suspension the upward force F on wire due to magnetic field B must be balanced by the force due to gravity, then
IIB = mg; B = mg/II
Here, m = 1.2 kg, g = 10 m s^-2, l = 5 A, l = 1m
Question 7
1 / -0

A circular loop of radius R carrying a current I is placed in a uniform magnetic field B perpendicular to the loop. The force on the loop is

A
2πRIB

B
2πRI²B³

C
2πR²IB

D
zero
Question 8
1 / -0

A 2.5 m long straight wire having mass of 500 is suspended in mid air by a uniform horizontal magnetic field B. If a current of 4 A is passing through the wire then the magnitude of the field is (Take g = 10 m s^-2)

A
0.5 T

B
0.6 T

C
0.25 T

D
0.8 T

Solution

Here, m = 500g = 0.5kg, l = 4A, l = 2.5m
As F = IIB sinθ
mg = IIB sin90^°, (∵ θ= 90^°and F = mg)
Question 9
1 / -0

A current of 10 A is flowing in a wire of length 1.5 m. A force of 15 N acts on it when it is placed in a uniform magnetic field of 2 T. The angle between the magnetic field and the direction of the current is

A
30^°

B
45^°

C
60^°

D
90^°

Solution

F = IlBsinθ or sinθ = F/IlB
or θ = 30^°
Question 10
1 / -0

The horizontal component of earth's magnetic field at a certain place is 3.0 x 10^-5 T and having a direction from the geographic south to geographic north. The force per unit length on a very long straight conductor carrying a steady current of 1.2 A in east to west direction is

A
3.0 x 10^-5 N m^-1

B
3.2 x 10^-5 N m^-1

C
3.6 x 10^-5 N m^-1

D
3.8 x 10^-5 N m^-1

Solution

Force per unit length f = F/l = IB sinθ
when the current is flowing from east to west then θ = 90°, hence
f = IBsin 90° = 1.2 x 3 x 10^-5 x 1 = 3.6 x 10^-5N m^-1.
Question 11
1 / -0

A circular coil of 20 turns and 10 cm radius is placed in a uniform magnetic field of 1.10T normal to the plane of the coil. If the current in the coil is 5A, cross-sectional area is 10^-5 m² and coil is made up of copper wire having free electron density about 10²⁹ m^-3, then the average force on each electron in the coil due to magnetic field is

A
2.5 x 10^-25N

B
5 x 10^-25N

C
4 x 10^-25N

D
3 x 10^-25N

Solution

Force on each electron,
ev_dB = IB/nA
Here, I = 5 A, B = 0.1 T, n = 10²⁹ m^-3,
A = 10^-5m²
So,
Question 12
1 / -0

The magnetic force per unit length on a wire carrying a current of 10 A and making an angle of 45° with the direction of a uniform magnetic field of 0.20 T is

A
2√2 N m^-1

B
2/√2 N m^-1

C
√2/2 N m^-1

D
4√2 N m^-1

Solution

l = 10 A, θ = 45°, B = 0.2 T
∴ F = IlB sin θ
∴ F/l = IB sin 45^° = 10 x 0.2 x 1/√2 = 2/√2 N m^-1
Question 13
1 / -0

An 8 cm long wire carrying a current of 10 A is placed inside a solenoid perpendicular to its axis. If the magnetic field inside the solenoid is 0.3 T, then megnetic force on the wire is

A
0.14 N

B
0.24 N

C
0.34 N

D
0.44 N

Solution

F = lIB sinθ
Here, θ = 90⁰, l = 10 A,
l = 8cm = 8 x 10^-2m, B = 0.3T
F = 10 x 8 x 10^-2 x 0.3 x sin 90⁰ = 0.24 N
Question 14
1 / -0

A rectangular coil ABCD is hung from one side of a balance as shown in figure. A 500 g mass is added to the other arm to balance the weight of the coil. A current of 9.8 A is passed through the coil and a constant magnetic field of 0.4 T acting inward (in xz plane) is switched on such that only arm CD of length 1.5 cm lies in the field. The additional mass m must be added to regain the balance is

A
4g

B
5g

C
6g

D
7g

Solution

In absence of magnetic field the weight added in one pan balances the rectangular coil in the other pan of balance,
∴ Mgl = W_coill or W_coil = Mg = 0.5 x 9.8 N
When current I is passed through the coil and the magnetic field is switched on.
Let m mass be added in the first pan to regain the balance
Then Mgl + mgl = W_coill + IBL sin90° l
mgl = IBLI
or
= 6 x 10^-3 kg = 6 g
Question 15
1 / -0

Which one of the following is correct statement about magnetic forces?

A
Magnetic forces always obey Newton’s third law

B
Magnetic forces do not obey Newton’s third law

C
For very high current, magnetic forces obey Newton’s third law

D
Inside low magnetic field, magnetic forces obey Newton’s third law

Solution

As the action and reaction are instantaneous in III^rd law. But magnetic forces transmit with finite speed c.