JAMB Physics 1987

23 reviewed questions with answers and explanations.

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Question 1

Which unit is equivalent to kg·m·s⁻¹?

  1. N·s⁻¹
  2. N·m·s
  3. N·s
  4. J·s⁻¹
Answer and explanation

C: N·s

One newton is kg·m·s⁻². Multiplying by one second gives N·s = kg·m·s⁻¹, the unit of momentum.

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Question 6

A 4800 kg elevator is supported by a cable whose maximum safe tension is 60000 N. What is the maximum upward acceleration? Take g = 10 m/s².

  1. 2.5 m/s²
  2. 5.0 m/s²
  3. 7.5 m/s²
  4. 10.0 m/s²
Answer and explanation

A: 2.5 m/s²

For upward acceleration, T − mg = ma. At the cable limit, a = 60000/4800 − 10 = 2.5 m/s². Any greater acceleration would exceed the stated tension limit.

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Question 12

If a beaker is filled with water, it is observed that the surface of the water is not horizontal at the glass-water interface. This behaviour is due to

  1. Friction
  2. Viscosity
  3. Surface tension
  4. Evaporation.
Answer and explanation

C: Surface tension

The curved meniscus results from surface tension together with adhesion between water and glass. Water wets the glass, so its surface rises near the wall.

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Question 13

The ideal mechanical advantage of a frictionless inclined plane depends on

  1. Its length alone
  2. Its height alone
  3. The product of its length and height
  4. The ratio of its length to its height
Answer and explanation

D: The ratio of its length to its height

For an ideal frictionless inclined plane, effort × length = load × height. Mechanical advantage is load/effort = length/height.

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Question 16

The 0 °C and 100 °C fixed points of a centigrade thermometer are 20 cm apart. What temperature is indicated when mercury is 4.5 cm above the lower mark?

  1. 22.5 °C
  2. 29.0 °C
  3. 90.0 °C
  4. 100.0 °C
Answer and explanation

A: 22.5 °C

The 20 cm interval represents 100 °C. A rise of 4.5 cm therefore gives T = (4.5/20) × 100 = 22.5 °C.

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Question 18

A metallic cube has side 5.0 cm at 20 °C and linear expansivity 4.0 × 10⁻⁵ K⁻¹. Using the first-order thermal expansion approximation, find its volume at 120 °C.

  1. 126.50 cm³
  2. 126.25 cm³
  3. 126.00 cm³
  4. 125.00 cm³
Answer and explanation

A: 126.50 cm³

Initial volume is 5³ = 125 cm³. To first order, volume expansivity is 3α, so V = 125[1 + 3(4 × 10⁻⁵)(120 − 20)] = 126.50 cm³.

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Question 19

Air trapped above a mercury barometer makes it read 73.5 cm when the true height is 75.0 cm. The trapped air volume is 8.0 cm³. At the same temperature, it later reads 74.0 cm and the trapped volume is 6.0 cm³. Find the true barometric height.

  1. 72.0 cm
  2. 74.5 cm
  3. 75.1 cm
  4. 76.0 cm
Answer and explanation

D: 76.0 cm

Initially the trapped air pressure is 75.0 − 73.5 = 1.5 cmHg. At constant temperature, p₂ = 1.5 × 8/6 = 2.0 cmHg. True atmospheric pressure is then 74.0 + 2.0 = 76.0 cmHg.

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Question 21

Hot water is mixed with three times its mass of water at 10 °C, giving a final temperature of 20 °C. Neglect heat loss. What was the hot water’s initial temperature?

  1. 100 °C
  2. 80 °C
  3. 50 °C
  4. 40 °C
Answer and explanation

C: 50 °C

Neglecting heat loss, hot-water heat loss equals cold-water heat gain: mc(T − 20) = 3mc(20 − 10). Thus T − 20 = 30 and T = 50 °C.

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Question 23

Calculate the heat required to convert 2 kg of ice at −2 °C to water at 0 °C. The specific heat capacity of ice is 2090 J/(kg·°C), and its specific latent heat of fusion is 333 kJ/kg.

  1. 666 J
  2. 8360 J
  3. 666000 J
  4. 674360 J
Answer and explanation

D: 674360 J

First warm the ice by 2 °C: Q₁ = 2 × 2090 × 2 = 8360 J. Then melt it: Q₂ = 2 × 333000 = 666000 J. Total heat is 674360 J.

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Question 24

In which of the following are the substances arranged in descending order of their thermal conductivities?

  1. Copper, steel, glass
  2. Steel, copper, glass
  3. Steel, glass, copper
  4. Copper, glass, steel.
Answer and explanation

A: Copper, steel, glass

Copper conducts heat much better than steel, and steel conducts much better than glass. Their descending order of thermal conductivity is copper, steel, glass.

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Question 25

The vacuum in the Thermos flask helps to reduce heat transfer by

  1. Convection and radiation
  2. Convection and conduction
  3. Conduction and radiation
  4. Radiation only.
Answer and explanation

B: Convection and conduction

The vacuum greatly reduces conduction and convection across the space between the flask walls because there is almost no matter there. Electromagnetic radiation can still cross that space.

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Question 26

Why is a house with a white-painted roof generally cooler in hot sunshine than an otherwise similar house with a black-painted roof?

  1. Conduction
  2. Convection
  3. Refraction
  4. Reflection.
Answer and explanation

D: Reflection.

A white coating generally reflects more incoming sunlight than a black coating. With less solar energy absorbed, the roof gains less heat under otherwise similar conditions.

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Question 29

Which instrument is commonly used to produce a nearly pure tone?

  1. Guitar
  2. Vibrating string
  3. Tuning fork
  4. Siren
Answer and explanation

C: Tuning fork

A tuning fork is designed to vibrate mainly at one frequency and gives a nearly pure tone after brief higher-frequency transients fade. Guitar and ordinary string sounds contain strong harmonics.

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Question 33

An object is placed between two plane mirrors inclined at 90°. How many images are formed?

  1. Five
  2. Four
  3. Three
  4. Two.
Answer and explanation

C: Three

Two plane mirrors at right angles form one image by reflection in each mirror and a third by successive reflections in both. Thus there are three images.

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Question 34

An object 3.0cm high is placed 60.0cm from a converging lens whose focal length is 20.0cm. Calculate the size of the image formed.

  1. 0.5 cm
  2. 1.5 cm
  3. 2.0 cm
  4. 6.0 cm
Answer and explanation

B: 1.5 cm

The lens formula gives 1/v = 1/20 − 1/60 = 1/30, so v = 30 cm. Magnification magnitude is v/u = 30/60 = 0.5. Image height is 0.5 × 3.0 = 1.5 cm.

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Question 37

Four lenses are being considered for use as a microscope objective. Which of the following focal lengths is most suitable?

  1. -5mm
  2. +5mm
  3. –5cm
  4. +5cm
Answer and explanation

B: +5mm

A microscope objective uses a converging lens with a short focal length. Positive 5 mm provides stronger convergence than positive 5 cm, making it the suitable choice here.

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Question 38

At a place with magnetic declination 10°E, a compass reads N40°E. What is the true geographic bearing?

  1. N50°E
  2. N40°E
  3. N30°E
  4. N25°E
Answer and explanation

A: N50°E

Magnetic north is 10° east of true north. A compass direction 40° east of magnetic north is therefore 10° + 40° = 50° east of true north: N50°E.

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Question 40

Capacitors of 6 μF and 8 μF are in series. What additional capacitor, connected in series with them, gives a total capacitance of 3 μF?

  1. 3 μF
  2. 16 μF
  3. 24 μF
  4. 30 μF
Answer and explanation

C: 24 μF

For series capacitors, reciprocals add: 1/3 = 1/6 + 1/8 + 1/C. Thus 1/C = 1/24 and the additional capacitance is 24 μF.

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Question 42

An ideal transformer operating with a load has more turns on its secondary than its primary. It

  1. Has a smaller secondary current than primary current
  2. Has greater output power than input power
  3. Is a step-down transformer
  4. Increases the total energy output above its input
Answer and explanation

A: Has a smaller secondary current than primary current

An ideal transformer with more secondary turns steps voltage up. Conservation of power gives VsIs = VpIp, so the secondary current is smaller than the primary current.

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Question 44

The direction of the magnetic field at a point in the vicinity of a bar magnet is

  1. Always towards the north pole of the magnet
  2. Always away from the south pole of the magnet
  3. Along the line joining the point to the neutral point
  4. In the direction the north pole of a compass needle would point.
Answer and explanation

D: In the direction the north pole of a compass needle would point.

The direction of a magnetic field is defined as the direction in which the north-seeking pole of a small compass points at that location.

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Question 45

When two parallel wires carry currents in opposite directions, the force on either wire is

  1. Away from the other wire
  2. Zero, because the currents cancel each other
  3. Twice as much as when the currents are in the same direction
  4. Towards the other wire.
Answer and explanation

A: Away from the other wire

Parallel conductors carrying currents in opposite directions repel. Each wire’s magnetic field exerts a force on the current in the other wire, directed away from it.

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Question 48

Which statement is true of the ordinary single-photon photoelectric effect?

  1. It cannot occur in liquids
  2. The maximum emitted-electron energy is independent of the surface work function
  3. The maximum emitted-electron energy depends on the incident wavelength
  4. At fixed wavelength, greater light intensity necessarily gives greater maximum emitted-electron energy
Answer and explanation

C: The maximum emitted-electron energy depends on the incident wavelength

In the ordinary single-photon photoelectric effect, maximum kinetic energy is Kmax = hc/λ − φ. It depends on wavelength and work function. Increasing intensity at fixed wavelength chiefly increases the number emitted.

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Question 49

Isotopes are nuclei which have

  1. The same number of neutrons and electrons
  2. Equal number of electrons and protons
  3. The same atomic number but different number of neutrons
  4. The same number of total particles.
Answer and explanation

C: The same atomic number but different number of neutrons

Isotopes have the same atomic number, meaning the same number of protons, but different neutron numbers. Their mass numbers therefore differ.

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