JAMB Physics Past Questions & Answers - Page 147

731.

The half-life of a radio-active element is 9 days, what fraction of atoms has decayed in 36 days?

A.

\( \frac{1}{16} \)

B.

\( \frac{1}{4} \)

C.

\( \frac{1}{2} \)

D.

\( \frac{15}{16} \)

Correct answer is D

No explanation has been provided for this answer.

733.

What is the angle of dip at the magnetic equator

A.

\( 45^{\circ} \)

B.

\( 0^{\circ} \)

C.

\( 90^{\circ} \)

D.

\( 180^{\circ} \)

Correct answer is B

The angle of dip dip is the angle which the resultant earth's magnetic field makes with the horizontal.
At the magnetic equator, which is entirely along the horizontal this angle is zero

734.

A bread toaster uses a current of 4A when plugged in a 24oV line. it takes 1 minute to toast slices of bread. what is the energy consumed by the toaster

A.

5.76 x 104J

B.

1.60 x 104J

C.

3.60 x 103J

D.

1.60 x 102J

Correct answer is A

Electrical Energy = IVt
= 4 x 240 x (1 x 60)
= 57600J
= 5.76 x 104J

735.

Find the frequencies of the first three harmonics of a piano string of length 1.5m, if the velocity of the wave on the string is 120ms-1

A.

40Hz, 80Hz, 120Hz

B.

80Hz, 160Hz, 240Hz

C.

180Hz, 360Hz, 540Hz

D.

369Hz, 180Hz, 90Hz

Correct answer is A

in vibrations in string, the fundamental frequency happens to be the first harmonic, and is given as

f0 = V/2l.

Where V = Velocity of the sound wave, I = Length of the string.

The second harmonic is the first overtone given by

f1 = V/l = 2f0 ;

and the third harmonic happens to be the second overtone, given by

f2 = 3V/2l = 2f0.

Thus if V = 120m/s and l = 1.5m
Thus f1 = 2f0 = 2 x 40 = 80Hz
Thus f2 = 3f0 = 3 x 40 = 120Hz
Thus we have in order, 40Hz, 80Hz, 120Hz