A 200 W sodium street lamp emits yellow light of wavelength 0.6 um. Assuming it to be 25% efficient in converting electrical energy to light, the number of photons of yellow light it emits per second is
1.5 x 1020
6 x 1018
62 x 1020
62 x 1020
A.
1.5 x 1020
Efficient power P=
A body of mass m taken from the earth's surface to the height equal to twice the radius (R) of the earth. The change in potential energy of the body will be
mg2R
2/3 mgR
3 mgR
3 mgR
B.
2/3 mgR
Change in potential energy,
Two cities are 150 km apart. electric power is sent from one city to another city through copper wires. The fall of potential per km is 8 V and the average resistance per km is 8 V and the average resistance per km is . The power loss in the wire is,
19.2 W
19.2 kW
19.2 J
12.2 kW
B.
19.2 kW
Potential difference (drop) between two cities = 150 x 8 = 1200 V
Average resistance of total wire = 0.5 x 150 =
Power loss, P =
= 19.2 kW
The potential energy of a system increase if work is done
By the system against a conservative force
By the system against a nonconservative
upon the system by a conservative force
upon the system by a conservative force
A.
By the system against a conservative force
The potential energy of a system increases if work is done by the system against a conservative force.
-ΔU = Wconservative face
A ball is thrown vertically downwards from a height of 20m with an initial velocity vo, It collides with the ground, loses 50% of its energy in collision and rebounds to the same height. The initial velocity vo is,
(Take g = 10 ms-2)
14 ms-1
20 ms-1
28 ms-1
28 ms-1
B.
20 ms-1
Suppose a ball rebounds with speed v,