21.

A uniform electric field and a uniform magnetic field are acting along the same direction in a certain region. If an electron is projected along the direction of the fields with a certain velocity then

• its velocity will decrease.

• its velocity will increase.

• it will turn towards right of direction of motion.

• it will turn towards right of direction of motion.

A charged particle of mass m and charge q travels on a circular path of radius r that is perpendicular to a magnetic field B. The time taken by the particle to complete one revolution is

In a potentiometer experiment, the balancing with a cell is at length 240 cm. On shunting, the cell with a resistance of 2Ω the balancing length becomes 120 cm. The internal resistance of the cell is

• 1 Ω

• 0.5 Ω
  

• 3 Ω

• 3 Ω

The resistance of hot tungsten filament is about 10 times the cold resistance. What will be the resistance of 100 W and 200 V lamp when not in use?

• 40 Ω

• 20 Ω

• 400 Ω

• 400 Ω

A magnetic needle is kept in a non-uniform magnetic field. It experiences

• a torque but not a force

• neither a force nor a torque

• a force and a torque.

• a force and a torque.

A current I ampere flows along an infinitely long straight thin-walled tube, then the magnetic induction at any point inside the tube is

• infinite

• zero

• 
• 

A long wire carries a steady current. It is bent into a circle of one turn and the magnetic field at the centre of the coil is B. It is then bent into a circular loop of n turns. The magnetic field at the centre of the coil will be

• nB

• n²B

• 2nB

• 2nB

The magnetic field due to a current carrying circular loop of radius 3 cm at a point on the axis at a distance of 4 cm from the centre is 54 µT. What will be its value at the centre of the loop?

• 250 µT

• 150 µT

• 125 µT

• 125 µT

In a uniform magnetic field of induction B a wire in the form of semicircle of radius r rotates about the diameter of the circle with angular frequency ω. The axis of rotation is perpendicular to the field. If the total resistance of the circuit is R the mean power generated per period of rotation is

A charged oil drop is suspended in a uniform field of 3 × 10⁴ V/m so that it neither falls nor rises. The charge on the drop will be (take the mass of the charge = 9.9 × 10⁻¹⁵ kg and g = 10 m/s² )

• 3.3 × 10⁻¹⁸ C

• 3.2 × 10⁻¹⁸ C

• 1.6 × 10⁻¹⁸ C

• 1.6 × 10⁻¹⁸ C