JAMB Physics 1989

19 reviewed questions with answers and explanations.

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Question 1

Which of the following is a set of vectors?

  1. Force, mass and moment of force
  2. Acceleration, velocity and moment of force
  3. Mass, weight and density
  4. Mass, volume and density
Answer and explanation

B: Acceleration, velocity and moment of force

Acceleration and velocity have magnitude and direction. Moment of force, or torque, is also a vector with direction given by the right-hand rule. Mass, volume and density are scalars.

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Question 2

The magnitude of the resultant of two mutually perpendicular forces, F1 and F2 is 13N. If the magnitude of F1 is 5N, what is the magnitude of F2?

  1. 2.6 N
  2. 8.0 N
  3. 12.0 N
  4. 18.0 N
Answer and explanation

C: 12.0 N

Perpendicular forces obey R² = F₁² + F₂². Therefore F₂ = √(13² − 5²) = √144 = 12 N.

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Question 3

A velocity–time graph contains points (5 s, 10 m/s) and (20 s, 20 m/s). Find the mean acceleration between them to two decimal places.

  1. 0.67 m/s²
  2. 0.83 m/s²
  3. 1.50 m/s²
  4. 2.00 m/s²
Answer and explanation

A: 0.67 m/s²

Mean acceleration is change in velocity divided by elapsed time: (20 − 10)/(20 − 5) = 10/15 = 0.67 m/s² to two decimal places.

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Question 11

Which of the following devices are used to measure pressure? I Aneroid barometer II Hydrometer III Hygrometer IV Manometer.

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

D: I and IV.

An aneroid barometer measures atmospheric pressure and a manometer measures pressure or pressure difference. A hydrometer measures liquid density, while a hygrometer measures humidity.

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Question 16

An electric heater melts 1.5 kg of ice initially at 0 °C. It draws 20 A from a 12 V battery. Assuming all its heat melts the ice, find the time taken. Use specific latent heat of fusion 336 × 10³ J/kg.

  1. 76.0 min
  2. 35.0 min
  3. 21.0 min
  4. 2.9 min.
Answer and explanation

B: 35.0 min

Heat needed is mL = 1.5 × 336000 = 504000 J. Heater power is VI = 12 × 20 = 240 W. Time = 504000/240 = 2100 s = 35.0 min.

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Question 17

In the kinetic theory of an ideal gas, absolute temperature is a

  1. Form of energy proportional to the total kinetic energy of its molecules
  2. Form of energy proportional to the average kinetic energy of its molecules
  3. Physical property proportional to the total kinetic energy of its molecules
  4. Physical property proportional to the average translational kinetic energy of its molecules
Answer and explanation

D: Physical property proportional to the average translational kinetic energy of its molecules

Absolute temperature is a physical property, not a quantity of energy. For an ideal gas, the mean translational kinetic energy per molecule is 3kT/2, so it is proportional to kelvin temperature.

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Question 22

In a sound wave in air, the adjacent rarefactions and compressions are separated by a distance of 17cm. If the velocity of the sound wave is 340ms-1, determine the frequency.

  1. 10 Hz
  2. 20 Hz
  3. 1 000 Hz
  4. 5 780 Hz
Answer and explanation

C: 1 000 Hz

A compression and the adjacent rarefaction are half a wavelength apart. Thus λ = 2 × 17 cm = 0.34 m. Frequency f = v/λ = 340/0.34 = 1000 Hz.

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Question 24

Light of free-space wavelength 5000 × 10⁻⁸ cm enters glass of refractive index 1.5. Find its wavelength in glass, rounding the coefficient of 10⁻⁸ cm to the nearest integer.

  1. 3333 × 10⁻⁸ cm
  2. 5000 × 10⁻⁸ cm
  3. 6666 × 10⁻⁸ cm
  4. 7500 × 10⁻⁸ cm
Answer and explanation

A: 3333 × 10⁻⁸ cm

Frequency stays unchanged on entering glass, while speed becomes c/n. Therefore wavelength becomes λ/n = (5000/1.5) × 10⁻⁸ cm ≈ 3333 × 10⁻⁸ cm.

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Question 26

The magnification of the image of an object placed in front of a convex mirror is 1/3. If the radius of curvature of the mirror is 24cm, what is the distance between the object and its image?

  1. 8 cm
  2. 16 cm
  3. 24 cm
  4. 32 cm
Answer and explanation

D: 32 cm

For a convex mirror, f = −12 cm and v = −u/3. The mirror equation gives −1/12 = 1/u − 3/u = −2/u, so u = 24 cm and v = −8 cm. Their separation is 24 + 8 = 32 cm.

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Question 27

In a common three-mirror kaleidoscope with an equilateral triangular cross-section, adjacent plane mirrors are placed

  1. At 60°
  2. Parallel to one another
  3. Perpendicular to one another
  4. At 45°
Answer and explanation

A: At 60°

A common three-mirror kaleidoscope uses mirrors forming an equilateral triangular tube. Adjacent mirrors meet at 60°, creating repeated symmetric images by reflection.

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Question 29

Under white light, an ideal yellow card is viewed through an ideal blue filter. What colour does the card appear?

  1. Black
  2. Green
  3. Red
  4. White
Answer and explanation

A: Black

In the ideal colour model, a yellow card reflects red and green light. A blue filter transmits blue and blocks red and green, leaving essentially no light from the card, so it appears black.

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Question 30

Dispersion of light by a glass prism is due to the

  1. Different hidden colours of the glass
  2. Different speeds of the various colours in glass
  3. Defects in the glass
  4. High density of glass.
Answer and explanation

B: Different speeds of the various colours in glass

Glass has a refractive index that depends on wavelength. Different colours therefore travel at different speeds in it and refract by different amounts, separating the white light.

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Question 31

Which of the following pairs is NOT part of the electromagnetic spectrum? I Radio waves II Beta rays III Gamma rays IV Alpha rays

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

D: II and IV.

Alpha radiation consists of helium nuclei and beta radiation of electrons or positrons. They are particles with mass. Radio waves and gamma rays are electromagnetic radiation.

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Question 32

Two insulated conducting spheres of different sizes carry opposite charges. They are connected by a metal wire. Charge flows until both spheres

  1. Carry the same magnitude and sign of charge.
  2. Are at the same potential.
  3. Are at the same temperature.
  4. Are of the same size.
Answer and explanation

B: Are at the same potential.

Charges redistribute through the connecting conductor until there is no potential difference between the spheres. At electrostatic equilibrium the connected conductor is equipotential.

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Question 35

A uniform wire 5 m long has cross-sectional area 0.2 × 10⁻⁶ m² and resistance 0.425 Ω. Find its resistivity.

  1. 1.10 × 10⁻⁶ Ω·m
  2. 4.25 × 10⁻⁶ Ω·m
  3. 2.40 × 10⁻⁷ Ω·m
  4. 1.70 × 10⁻⁸ Ω·m
Answer and explanation

D: 1.70 × 10⁻⁸ Ω·m

For a uniform wire R = ρL/A. Thus ρ = RA/L = 0.425 × (0.2 × 10⁻⁶)/5 = 1.70 × 10⁻⁸ Ω·m.

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Question 36

Resistors of 250 Ω, 500 Ω and 1 kΩ are connected in series across a 6 V battery. Find the voltage across the 250 Ω resistor to two decimal places.

  1. 0.20V
  2. 0.86V
  3. 1.71V
  4. 3.43V.
Answer and explanation

B: 0.86V

Series resistance is 250 + 500 + 1000 = 1750 Ω. The 250 Ω resistor gets the fraction 250/1750 of the voltage: 6 × 250/1750 = 0.86 V to two decimal places.

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Question 43

An ammeter of resistance 0.1 Ω has full-scale current 50 mA. A 0.0111 Ω shunt is connected across it. Find the new full-scale total current to the nearest milliampere.

  1. 400 mA
  2. 450 mA
  3. 500 mA
  4. 550 mA.
Answer and explanation

C: 500 mA

At full scale, the meter voltage is 0.050 × 0.1 = 0.005 V. The shunt carries 0.005/0.0111 ≈ 0.45045 A. Adding the meter’s 0.050 A gives 0.50045 A, or 500 mA to the nearest milliampere.

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Question 44

The electrochemical equivalent of silver is 0.0012 g/C. What steady current deposits 36.0 g in 5.0 minutes?

  1. 6000 A
  2. 100 A
  3. 10 A
  4. 1 A
Answer and explanation

B: 100 A

Deposited mass is m = ZIt. With time 5 × 60 = 300 s, I = 36/(0.0012 × 300) = 100 A.

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Question 47

For a fixed metal surface in the ordinary single-photon photoelectric effect, the maximum kinetic energy of an emitted electron depends on the

  1. Intensity of the radiation
  2. Source of the radiation
  3. Wavelength of the radiation
  4. Detection device for the electron.
Answer and explanation

C: Wavelength of the radiation

For a fixed metal in the ordinary photoelectric effect, Kmax = hc/λ − φ. The wavelength sets photon energy and therefore the maximum emitted-electron energy above the work function.

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