21.

An ideal coil of 10H is connected in series with a resistance of 5Ω and a battery of 5V. 2 seconds after the connection is made, the current flowing in amperes in the circuit is

• (1-e)

• e

• e^-1

• e^-1

A long straight wire of radius ‘a’ carries a steady current i. The current is uniformly distributed across its cross section. The ratio of the magnetic field at a/2 and 2 a is 

• 1/4

• 4

• 1

• 1

A current I flows along the length of an infinitely long, straight, thin walled pipe. Then 

• the magnetic field is zero only on the axis of the pipe

• the magnetic field is different at different points inside the pipe

• the magnetic field at any point inside the pipe is zero

• the magnetic field at any point inside the pipe is zero

Charges are placed on the vertices of a square as shown. Let E be the electric field and V the potential at the centre. If the charges on A and B are interchanged with those on D and C respectively, then

•  remains unchanged, V changes 

• Both  and V change

•  and V remain unchanged
•  and V remain unchanged

A charged particle with charge q enters a region of constant, uniform and mutually orthogonal fields , with a velocity perpendicular to both  , and comes out without any change in magnitude or direction of  .Then

The potential at a point x (measured in µm) due to some changes situated on the x-axis is given by V (x) = 20 /x²- 4) volts. The electric field E at x = 4 µm is given by

• 5/3 Volt/µm and in the –ve x direction

• 5/3 Volt/µm and in the +ve x direction

• 10/9 Volt/µm and in the -ve x direction

• 10/9 Volt/µm and in the -ve x direction

Two identical conducting wires AOB and COD are placed at right angles to each other. The wire AOB carries an electric current I₁ and COD carries a current I₂. The magnetic field on a point lying at a distance ‘d’ from O, in a direction perpendicular to the plane of the wires AOB and COD, will be given by

The resistance of a wire is 5 ohm at 50°C and 6 ohm at 100°C. The resistance of the wire at 0 °C will be

• 2 ohm

• 1 ohm

• 4 ohm

• 4 ohm

A parallel plate condenser with a dielectric of dielectric constant K between the plates has a capacity C and is charged to a potential V volts. The dielectric slab is slowly removed from between the plates and then reinserted. The net work done by the system in this process is

• ½ (K – 1) CV²

• CV² (K – 1) /K

• (K –1) CV²

• (K –1) CV²

If M₀ is the mass of an oxygen isotope ₈O¹⁷ , Mp and M_N are the masses of a proton and a neutron respectively, the nuclear binding energy of the isotope is

• (M₀ – ⁸M_P) C²

• (M₀ – ⁸M_P – ⁹M_N) C²

• M₀C²

• M₀C²