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JAMB Physics Past Questions & Answers - Page 85

421.

When the r.m.s value of a source of electricity supply is given as 240v, it means that the peak value of the supply is

240v

340v

480v

57600v

View answer & explanation

Correct answer is B

The relationship between the peak value and r.m.s value of electricity supply is given as

V $_{r.m.s}$ = $\frac{V_o}{\sqrt{2}}$

where V_o = peak voltage

∴ V_o = V $_{r.m.s}$ × √2

= 240 × √2

= 339.5v

= 340v

422.

When two objects P and Q are supplied with the same quantity of heat, the temperature change in P is observed to be twice that of Q. The mass of P is half that of Q. The ratio of the specific heat capacity pf P to Q is

1:4

4:1

1:1

2:1

View answer & explanation

Correct answer is C

H = mcθ

H_P = H_Q, θ_P = 2θ,

m_P = $\frac{1}{2}$ m_Q

∴ H_P = m_QC_Pθ_P, also HQ = m_QC_Qθ_Q

Since H_P = H_Q

∴ m_Pc_Pθ_P = m_Qc_Pθ_q

Putting in the conditions

$\frac{1}{2}$ m_Q × c_P × 2θ_Q = m_Q × θ_Q × c_Q

∴ c_P = c_Q

$\frac{C_p}{C_Q}$ = $\frac{1}{1}$

= 1:1

423.

Calculate the height of the image formed by a pinhole camera of length 12cm used to photograph an object 60cm away from the hole and 70cm high

10 cm

16 cm

5 cm

14 cm

View answer & explanation

Correct answer is D

Magnification = $\frac{\text{Length of camera}}{\text{distance of object from pin hole}}$

m = $\frac{12}{60}$

= 0.2

Also magnification m = $\frac{\text{Height of image}}{\text{Height of object}}$

∴ Height of image = m × height of object

= 0.2 × 70

= 14cm

424.

The S.I unit of moment of a force is

kgm

µm $^{-1}$

View answer & explanation

Correct answer is B

Moment of a force is the product of a force and the perpendicular distance from the line of action of the force
Moment = force × distance
= N × m
= Nm

425.

An element and its isotope only differ in the number of

protons

electrons

ions

neutrons

View answer & explanation

Correct answer is D

Isotopy is due to difference in mass number, what causes the difference is the difference in the number of neutrons present