Question

Choose true or false.

True False Two straight wires carry electric currents in the positive directions along the x- and y-axes. In the first quadrant, the magnetic field is directed out of the plane of the paper in some places, into the plane of the paper in other places, and in some places it is zero.

True False  A parallel-plate capacitor has a capacitance of 10 mF and is charged with a 20 V power supply. The power supply is then removed and a dielectric of dielectric constant 4 is used to fill the space between the plates. The voltage now across the capacitor is 5 V.
True False  An atom of oxygen that has been doubly ionized is sent through a uniform magnetic field in a direction perpendicular to the field. The period of revolution of the ion is T. The ion picks up an electron, becoming singly ionized, this action having a negligible effect on the speed of the ion. The period of revolution of the ion is now 4T.
True False  A circular loop of wire of radius R carries a current I. The loop lies in the xy-plane with its center at the origin. The direction of the magnetic field at the center of the loop must be in the direction of the negative z-axis.
True False  A 6-ohm resistor, an 8-ohm resistor, and a 24-ohm resistor are connected together. The minimum resistance that can be produced, using all three resistors is 4-ohms.

Answer 1

1. True

If the current in the two wires are equal then magnetic field will be zero. If current in two wires are different then magnetic field will be either in plane or out of the plane.

2. True

Capacitance becomes 4 fold on inserting dielectric. Since Q = C V. Thus voltage decrease by 4 times. Thus V = 5 volt.

3. False

On picking the charge speed drops by a factor of 2. Therfore time period will increase by a factor of 2.

4. False

It depends on the direction of current flowing in the loop. If current flows in counterclockwise then magnetic field will be in z direction and if current in clockwise direction then magnetic field will be in - z direction.

5. False

Minimum resistance is obtained by connecting the resistors in parallel. In this minimum resistance is

(6 x 8 x 24)/(6 x 8 + 8 x 24 + 24 x 6) ohm

= 3 ohm