41.

Four charges equal to −Q are placed at the four corners of a square and a charge q is at its centre. If the system is in equilibrium the value of q is

• 
• 
• 
• 

Alternating current can not be measured by D.C. ammeter because

• A.C. cannot pass through D.C.

• A.C. changes direction

• average value of current for complete cycle is zero

• average value of current for complete cycle is zero

The total current supplied to the circuit by the battery is

• 1A

• 2 A

• 4 A

• 4 A

The resistance of the series combination of two resistances is S. When they are joined in parallel through total resistance is P. If S = nP, then the minimum possible value of n is

• 4

• 3

• 2

• 2

An electric current is passed through a circuit containing two wires of the same material, connected in parallel. If the length and radii of the wires are in the ratio of 4/3 and 2/3, then the ratio of the currents passing through the wire will be

• 3

• 1/3

• 8/9

• 8/9

In a metre bridge experiment null point is obtained at 20 cm from one end of the wire when resistance X is balanced against another resistance Y. If X < Y, then where will be the new position of the null point from the same end, if one decides to balance a resistance of 4X against Y?

• 50 cm

• 80 cm

• 40 cm

• 40 cm

The electrochemical equivalent of a metal is 3.3 × 10⁻⁷ kg per coulomb. The mass of the metal liberated at the cathode when a 3 A current is passed for 2 seconds will be

• 19.8 × 10⁻⁷ kg

• 9.9 × 10⁻⁷ kg

• 6.6 × 10⁻⁷ kg

• 6.6 × 10⁻⁷ kg

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