Alternating Current — Physics STD 12 Science — Question

An electron changes its position from orbit n = 4 to the orbit n = 2  of an atom. The wavelength of the emitted radiation’s is (R = Rydberg’s constant)

(a)

(b)

(c)

(d)

The plane faces of two identical plano-convex lenses each having focal length of 40 cm are pressed against each other to form a usual convex lens. The distance from this lens, at which an object must be placed to obtain a real, inverted image with magnification one is

(a) 80 cm

(b) 40 cm

(c) 20 cm

(d) 162 cm

In order to see diffraction the thickness of the film is

(a) 100 Å

(b) 10,000 Å

(c) 1 mm

(d) 1 cm

Two resistances R and 2R are connected in parallel in an electric circuit. The thermal energy developed in R and 2R are in the ratio:

1. 1 : 2
2. 2 : 1
3. 1 : 4
4. 4 : 1

A nichrome wire 50 cm long and one square millimetre cross-section carries a current of 4A when connected to a 2V battery.  The resistivity of nichrome wire in ohm metre is

(a) 1

(b) 4

(c) 3

(d) 2

Two metallic charged spheres whose radii are 20 cm  and 10 cm respectively, have each 150 micro - coulomb  positive charge. The common potential after they are connected by a conducting wire is

(a) 9 ×

(b) 4.5 ×

(c) 1.8 ×

(d) 13.5 ×

As shown in the figure, charges +q and -q  are placed at the vertices B and C  of an isosceles triangle. The potential at the vertex A is

(a) .

(b) Zero

(c)

(d)

Two condensers of capacities 2C and C are joined in parallel and charged upto potential V. The battery is removed and the condenser of capacity C is filled completely with a medium of dielectric constant K. The p.d. across the capacitors will now be

(a)

(b)

(c)

(d)

Unit of magnetic permeability is

(a) Amp/metre

(b) Amp/

(c) Henry

(d) Henry/metre