Wave Optics — Physics STD 12 Science — Question

An ionization chamber with parallel conducting plates as anode and cathode has 5  electrons and the same number of singly-charged positive ions per . The electrons are moving at 0.4 m/s. The current density from anode to cathode is .  The velocity of positive ions moving towards cathode is

(a) 0.4 m/s

(b) 16 m/s

(c) Zero

(d) 0.1 m/s

Assertion : Fax is a modulating and demodulating device.

Reason     : It is necessary for exact reproduction of a document.

(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.

(c) If assertion is true but reason is false.

(d) If assertion is false but reason is true.

A paramagnetic material is kept in a magnetic field. The field is increased till the magnetization becomes constant. If the temperature is now decreased, the magnetization:

Two concentric coplanar circular loops of radii  and  carry currents of respectively  and  in opposite directions (one clockwise and the other anticlockwise.) The magnetic induction at the centre of the loops is half that due to  alone at the centre. If .  the value of   is

(a) 2

(b) 1/2

(c) 1/4

(d) 1

The difference in the lengths of a mean solar day and a sidereal day is about

(a) 1 min

(b) 4 min

(c) 15 min

(d) 56 min

A river is flowing from west to east at a speed of 5 metres per minute. A man on the south bank of the river, capable of swimming at 10 metres per minute in still water, wants to swim across the river in the shortest time. He should swim in a direction:

1. Due north.
2. 30° east of north.
3. 30° north of west.
4. 60° east of north.

Kirchoff’s I law and II law of current, proves the

(a) Conservation of charge and energy

(b) Conservation of current and energy

(c) Conservation of mass and charge

(d) None of these

A wire of length L and 3 identical cells of negligible internal resistances are connected in series.  Due to current, the temperature of the wire is raised by ∆ T in a time t.  A number N of similar cells is now connected in series with a wire of the same material and cross – section but of length 2 L.  The temperature of the wire is raised by the same amount ∆ T in the same time t.  the value of N is

(a) 4

(b) 6

(c) 8

(d) 9

In a hydrogen discharge tube it is observed that through a given cross-section 3.13   electrons are moving from right to left and 3.12  protons are moving from left to right. What is the electric current in the discharge tube and what is its direction

(a) 1 mA towards right	(b) 1mA towards left
(c) 2mA towards left	(d) 2mA towards right