JAMB Physics Past Questions & Answers - Page 352

1,756.

A metal is illuminated with a radiation of energy 6.88eV If the kinetic energy of the emitted electrons is 1.5OeV, calculate the work function of the metal.

A.

0.22eV

B.

4.59eV

C.

5.38eV

D.

8.38eV

E.

10.32eV

Correct answer is C

Work function = Total energy - kinetic Energy
= 6.88 - 1.50 = 5.38 ev

1,758.

A length of wire has a frequency of 225Hz when stretched by a force of 225N. If the force increased to 324N, what is the new frequency of vibration?

A.

270 Hz

B.

512 Hz

C.

488 Hz

D.

356 Hz

Correct answer is A

f1 = 225Hz, T1 = 225N
f2 = ? T2 = 324N
But f ∝ √T => f = K√T
=> K = f/√T
∴ f1/√T1 = f2/√T2
225/√225 = f2/√324

f2 = 225√324
√225

f2 = 270Hz

 

 

 

1,759.

An image in a convex lens is upright and magnified 3 times. If the focal length of the lens is 15 cm, what is the object distance?

A.

10 cm

B.

25 cm

C.

16 cm

D.

14 cm

Correct answer is A

m = v/u = -3
v = -3u
f = 15cm
1/v + 1/U = 1/f
1/-3U + 1/u = 1/15
2/3u = 1/15
3U = 15 x 2
U = 30/3
= 10 cm

1,760.

The correct expression for the potential at a point distance r from a charge q in an electric field is?

A.

_q2
4πεor2

B.

_q2
4πεor

C.

_q
4πεor2

D.

\(\frac{q}{4πΣ_or}\)

Correct answer is D

Electric potential is work done in taking a unit mass or charge from infinity to that point

V = w = Fr = Q1Q2 x r
Q Q 4πεor2 Q1

 

= _q
4πεor