JAMB Physics 1992

23 reviewed questions with answers and explanations.

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Question 1

A ruler has divisions 1 cm apart. A single reading is rounded to the nearest division. What is the maximum magnitude of rounding error? Ignore other measurement errors.

  1. 0.1 cm
  2. 0.5 cm
  3. 1.0 cm
  4. 2.0 cm
Answer and explanation

B: 0.5 cm

When a reading is rounded to the nearest 1 cm division, it can differ from the true value by up to half a division. The rounding-error bound is therefore±0.5 cm.

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Question 2

For a simple pendulum undergoing small oscillations, which quantities affect its period? I. Bob mass. II. Pendulum length. III. Gravitational acceleration.

  1. I, II and III
  2. II and III only
  3. I and III only
  4. I and II only
Answer and explanation

B: II and III only

For small oscillations, T =2π√(L/g). The period depends on length and gravitational acceleration, but not on bob mass. Thus II and III are the relevant quantities.

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Question 10

In an experiment measuring Young’s modulus of a circular wire, the tensile stress depends on the

  1. Load and extension
  2. Load and radius of the wire
  3. Radius of the wire and extension
  4. Extension and original length of the wire
Answer and explanation

B: Load and radius of the wire

Tensile stress is force divided by cross-sectional area. For a circular wire, stress =F/(πr²), so the load and radius determine it. Extension and original length instead determine strain.

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Question 12

If a plastic sphere floats in water (density = 1000 kg m-3) with 0.5 of its volume submerged and floats in oil with 0.4 volume submerged, the density of the oil is

  1. 800 kg m-3
  2. 1 200 kg m-3
  3. 1 250 kg m-3
  4. 2 000 kg m-3
Answer and explanation

C: 1 250 kg m-3

Floating equilibrium gives sphere density =1000 ×0.5 =500 kg/m³. In the second liquid,500 =ρ ×0.4, so its density is 500/0.4 =1250 kg/m³.

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Question 16

At constant pressure, the density of a fixed mass of ideal gas is

  1. Constant as temperature changes
  2. Proportional to its volume
  3. Inversely proportional to its absolute temperature
  4. Independent of its volume
Answer and explanation

C: Inversely proportional to its absolute temperature

At constant pressure, a fixed amount of ideal gas satisfies V/T =constant. Since density is mass divided by volume, density is inversely proportional to absolute temperature.

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Question 17

How much heat is absorbed by 0.05 kg of copper, of specific heat capacity 390 J/(kg·K), when heated from 20 °C to 70 °C?

  1. 3.98 ×10⁻¹ J
  2. 9.75 ×10² J
  3. 3.98 ×10³ J
  4. 9.75 ×10³ J
Answer and explanation

B: 9.75 ×10² J

The temperature rise is 70 −20 =50 K. Heat absorbed is mcΔT =0.05 ×390 ×50 =975 J =9.75 ×10² J.

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Question 21

Which of the following characteristics of a wave is used in the measurement of the depth of the sea?

  1. Diffraction
  2. Interference
  3. Refraction
  4. Reflection
Answer and explanation

D: Reflection

Echo sounding sends a sound pulse downward and measures its return after reflection from the seabed. With sound speed v and round-trip time t, depth is vt/2.

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Question 23

A siren disc has 32 equally spaced holes in one ring. A single air jet produces one pulse per passing hole. If the disc rotates at 25 revolutions per second, what is the sound frequency?

  1. 80 Hz
  2. 600 Hz
  3. 800 Hz
  4. 1 600 Hz
Answer and explanation

C: 800 Hz

Each revolution lets 32 holes pass the air jet, producing 32 pressure pulses. At 25 revolutions per second, the pulse frequency is 32 ×25 =800 Hz.

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Question 24

Which of the following properties make the convex mirror useful as a driving mirror? I The image is real II The image is erect III It has a wide field of view IV The image is magnified

  1. I, II and IV
  2. I, II and III
  3. II and III
  4. I and III.
Answer and explanation

C: II and III

For objects in front of it, a convex mirror gives virtual, upright, diminished images. Its wide field of view lets a driver see more of the surrounding traffic. Therefore II and III apply.

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Question 25

An object is placed between the pole and focus of a concave mirror. The virtual image obtained is

  1. Diminished and upright
  2. Diminished and inverted
  3. Enlarged and inverted
  4. Enlarged and upright.
Answer and explanation

D: Enlarged and upright.

An object between a concave mirror and its focus produces a virtual image behind the mirror. The image is upright and larger than the object.

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Question 28

The speed of light in air is 3.0 ×10⁸ m/s. Find its speed in glass of refractive index 1.50 relative to air.

  1. 1.5 ×10⁸ m/s
  2. 3.0 ×10⁸ m/s
  3. 2.0 ×10⁸ m/s
  4. 6.0 ×10⁸ m/s
Answer and explanation

C: 2.0 ×10⁸ m/s

The refractive index is the speed in air divided by the speed in glass. Thus v =3.0 ×10⁸/1.50 =2.0 ×10⁸ m/s.

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Question 30

Which arrangement gives the conventional order of these electromagnetic bands from shorter to longer typical wavelengths?

  1. Gamma rays, ultraviolet rays, x-rays, infra-red rays.
  2. Gamma rays, x-rays, ultraviolet rays, infra-red rays
  3. Infra-red rays, ultraviolet rays, x-rays, gamma rays.
  4. Gamma rays, ultraviolet rays, infra-red rays, x- rays.
Answer and explanation

B: Gamma rays, x-rays, ultraviolet rays, infra-red rays

In the conventional electromagnetic-spectrum order, gamma rays have the shortest typical wavelengths, followed by X-rays, ultraviolet and infrared.

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Question 32

The angle between the direction of the earth’s magnetic field and the horizontal is called the

  1. Angle of deviation
  2. Magnetic declination
  3. Magnetic meridian
  4. Angle of dip
Answer and explanation

D: Angle of dip

Magnetic dip, also called inclination, is the angle the magnetic field makes with the horizontal. Declination instead measures the horizontal difference between geographic and magnetic north.

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Question 33

Find the magnitude of the force on an electron of charge−1.6 ×10⁻¹⁹ C in an electric field of magnitude 10⁸ V/m.

  1. 1.6 ×10⁻¹¹ N
  2. 1.6 ×10⁻¹⁴ N
  3. 1.6 ×10⁻¹⁶ N
  4. 1.0 ×10⁻¹⁶ N
Answer and explanation

A: 1.6 ×10⁻¹¹ N

Force magnitude is |q|E =(1.6 ×10⁻¹⁹)(10⁸) =1.6 ×10⁻¹¹ N. Because an electron is negatively charged, the force points opposite the electric field.

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Question 35

Which change does not alter the equilibrium emf of a primary cell, with chemistry and other conditions otherwise unchanged?

  1. Changing temperature
  2. Changing the cell’s physical size
  3. Changing the electrode materials
  4. Changing the electrolyte
Answer and explanation

B: Changing the cell’s physical size

Cell emf depends on electrode reactions, electrolyte conditions and temperature. Scaling up an otherwise identical cell increases available charge and can change internal resistance, but does not change its equilibrium emf.

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Question 39

A 5 V battery is connected across an initially uncharged 2.0 μF capacitor. After a long time, what is the magnitude of charge on either plate?

  1. 1.5 μC
  2. 10.0 μC
  3. 20.0 μC
  4. 50.0 μC
Answer and explanation

B: 10.0 μC

Charge magnitude on either plate is Q =CV =(2.0 ×10⁻⁶) ×5 =10 ×10⁻⁶ C =10 μC. The two plates carry equal and opposite charges.

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Question 40

For resistive devices with the same proportional protection margin, which has the smallest operating current and therefore needs the lowest fuse current rating?

  1. 60 W, 240 V
  2. 60 W, 40 V
  3. 40 W, 12 V
  4. 40 W, 5 V
Answer and explanation

A: 60 W, 240 V

Operating current is P/V. The four currents are 0.25 A,1.5 A,3.33 A and 8 A. The 60 W,240 V device draws the smallest current and needs the lowest current rating when the same protection margin is used.

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Question 42

For steady current I through a constant resistance R for time t, the heat produced is

  1. Inversely proportional to the current and the resistance
  2. Directly proportional to the current, resistance and the time.
  3. Inversely proportional to the square of the resistance, current and the time.
  4. Directly proportional to the square of the current, the resistance and the time.
Answer and explanation

D: Directly proportional to the square of the current, the resistance and the time.

Joule heating is H =I²Rt for a steady current through a constant resistance. Heat is proportional to current squared, resistance and elapsed time.

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Question 44

A galvanometer has resistance 10 Ω and full-scale current 0.05 A. What added resistance converts it into a 100 V voltmeter, and how is it connected?

  1. 20000 Ω in parallel
  2. 19990 Ω in series
  3. 1990 Ω in series
  4. 100 Ω in series
Answer and explanation

C: 1990 Ω in series

The total required resistance is V/I =100/0.05 =2000 Ω. Subtracting the galvanometer’s 10 Ω gives R =1990 Ω. It is connected in series to limit the current through the meter.

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Question 45

The units of impedance, RMS voltage and resonance frequency are respectively

  1. Volt, ampere and hertz
  2. Ohm, volt and hertz
  3. Watt, ohm and radian
  4. Ohm, hertz and joule.
Answer and explanation

B: Ohm, volt and hertz

Impedance is voltage divided by current, so its unit is the ohm. RMS voltage is measured in volts, and resonance frequency in hertz, or cycles per second.

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Question 46

For a sinusoidal alternating current with peak I₀, which relation gives its RMS current Iᵣₘₛ?

  1. I₀ = Iᵣₘₛ/√2
  2. Iᵣₘₛ = I₀/√2
  3. Iᵣₘₛ = √2/I₀
  4. I₀ = √2/Iᵣₘₛ
Answer and explanation

B: Iᵣₘₛ = I₀/√2

For sinusoidal current, the mean of sin² over a cycle is 1/2. Thus the mean square current is I₀²/2, and its square root is Iᵣₘₛ =I₀/√2.

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Question 48

In the ordinary photoelectric effect, the threshold frequency depends on the

  1. Intensity of incident light
  2. Frequency of incident light
  3. Material of the photocathode
  4. Collecting potential difference between cathode and anode
Answer and explanation

C: Material of the photocathode

The threshold frequency is f₀ =φ/h, whereφ is the photocathode’s work function. It is therefore determined by the emitting material and its surface condition.

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Question 49

After three half-lives, what fraction of the original radioactive isotope has decayed?

  1. 1/8
  2. 1/3
  3. 2/3
  4. 7/8
Answer and explanation

D: 7/8

After three half-lives, the fraction remaining is(1/2)³ =1/8. The fraction decayed is 1 −1/8 =7/8.

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