NECO Physics Past Questions & Answers - Page 36

176.

In 90 seconds, the mass of a radioactive element reduces to \(\frac{1}{16}\) of its original value. Determine the half- life of the element.

A.

15.0s

B.

18.0s

C.

22.5s

D.

30.0s

E.

45.0s

Correct answer is C

\(\frac{1}{16} = (\frac{1}{2})^n\)

\((\frac{1}{2})^4 = (\frac{1}{2})^n\)

\(n = 4\)

90 secs = 4 half lives

1 half life = \(\frac{90}{4}\) secs

= 22.5 secs

178.

A transformer is required to give 120V from a 240V mains supply. If the primary has 5500 turns, how many turns has the secondary?

A.

232

B.

450

C.

550

D.

2750

E.

11000

Correct answer is D

No explanation has been provided for this answer.

179.

A short chain is usually attached to the rear side of petro tanker trailing behind it to ensure that the

A.

charges generated by friction in the tanker are conducted to the earth

B.

chain vibrates in resonance with the tanker’s engine

C.

heat generated by friction in the engine is conducted to the earth

D.

petrol tanker is balanced on the road

E.

tanker does not exceed a certain speed limit

Correct answer is A

No explanation has been provided for this answer.

180.

Calculate the critical angle for light traveling from water to air. (Refractive index of water = 1.33)

A.

36.9°

B.

41.3°

C.

48.8°

D.

49.2°

E.

51.4°

Correct answer is C

critical angle formula = \(sin^{-1}\) \(\frac{n_2}{n_1}\)

→ \(sin^{-1}\) \(\frac{1}{1.33}\) = \(sin^{-1}\) 0.7518

= 48.75º