JAMB Physics Past Questions & Answers - Page 439

2,191.

The velocity y of a particle in a time t is given by the equation
y = 10 + 2t²
Find the instantaneous acceleration after 5 seconds

60 ms^-2

20 ms^-2

15 ms^-2

10 ms^-2

View answer & explanation

Correct answer is B

The velocity V = 10 + 2t²
But acceleration, a = dv/dt
thus if V = 10 + 2t²
dv/dt = 4t
And t = 5s then a = 4x5 = 20m/s²
∴a = 20ms

2,192.

In the series a.c circuit shown above, the p.d across the inductor is 8V_r.m.s and that across the resistor is 6V_r.m.s. The effective voltage is

10V

14V

48V

View answer & explanation

Correct answer is E

The effective V =
√ V_R² + V_L²

=
√ 6² + 8²

=
√ 36 + 64

=
√ 100

10V

2,193.

In the diagram above, if the internal resistance of the cell is zero, the ration of the powers P₁ and P₂ dissipated by R₁ and R₂

^R₂/_R₁

^R₁/_R₂

^R₁+R₂/_R₁

^R₁+R₂/_R₂

View answer & explanation

Correct answer is A

Power = IV = I²R = ^V²/_R
In terms of e.m.f E, of a cell we have
IE = I²R = E²/R
∴P₁ = ^E²/_R; P₂ = ^E²/_R₂/^E²/_R₂
= ^E²/_R₁ x ^R₂/_E² = ^R₂/_R₁
∴^P₁/_P₂ = ^R₂/_R₁

2,194.

In the circuit above, The potential across each capacitor is 100V. The total energy stored in the two capacitors is

3.0 x 10⁴J

3.0 x 10²J

2.5 x 10^-2J

6.0 x 10^-3J

View answer & explanation

Correct answer is C

Energy stored in a capacitor is given by
E = 1/2CV2
But effective capacitance for the parallel arrangement is given by
C = C1 + C2
= 2 + 3
= 5μF = 5 x 10-6 x (100)2
∴E = 1/2CV2
= 1/2 x 5 x 10-6 x 104
2.9 x 10-2J

2,195.

In the circuit above, the ammeter reads a current of 5.0 A when R = 8Ω and reads 7.0A when R = 5Ω. The value of the unknown resistance X is

10.0Ω

7.5Ω

5.0Ω

2.5Ω

View answer & explanation

Correct answer is D

From Ohms Law, E = I(R +X)
=> E = I₁(R₁ + X)
again, E = I₂(R₂ + X)
=> I₁(R₁ + X) = I₂(R₂ + X)
∴ 5(8 + X) = 7(5 + X)
40 + 5X = 35 + 7X
7X - 5X = 40 - 35
2X = 5
X = ⁵/₂
= 2.5Ω