JAMB Physics 1991

24 reviewed questions with answers and explanations.

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Question 4

An object moves with uniform speed round a circle. Its acceleration has

  1. Constant magnitude and constant direction.
  2. Constant magnitude and varying direction.
  3. Varying magnitude and constant direction
  4. Varying magnitude and varying direction.
Answer and explanation

B: Constant magnitude and varying direction.

Centripetal acceleration has magnitude v²/r. Both speed and radius are constant, so this magnitude stays constant. Its direction continually changes as it points from the moving object toward the centre.

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Question 10

A 20 kg load is pulled at constant speed along a plane inclined at 30° to the horizontal. If the plane’s efficiency is 75%, find the pulling force parallel to the plane to one decimal place. Use g = 10 m/s².

  1. 13.3 N
  2. 73.5 N
  3. 133.3 N
  4. 533.2 N
Answer and explanation

C: 133.3 N

Efficiency is useful work divided by input work. For motion through distance s along the slope, 0.75 = mg(s sin 30°)/(Fs). Thus F = 20 × 10 × 0.5/0.75 = 133.3 N to one decimal place.

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Question 11

The spiral spring of a spring balance is 25.0cm long when 5N hangs on it and 30.0cm long, when the weight is 10N. What is the length of the spring if the weight is 3N assuming Hooke’s Law is obeyed?

  1. 15.0 cm
  2. 17.0 cm
  3. 20.0 cm
  4. 23.0cm
Answer and explanation

D: 23.0cm

An extra 5 N extends the spring by 5 cm, giving compliance 1 cm/N. Its unloaded length is 25 − 5 = 20 cm. With 3 N attached, its length is 20 + 3 = 23 cm.

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Question 12

A stone gives an apparent mass reading of 15.0 g when fully immersed in water and 10.0 g when fully immersed in a liquid of relative density 2.0. Neglect air buoyancy. Find its mass in air.

  1. 5.0g
  2. 12.0g
  3. 20.0g
  4. 25.0g
Answer and explanation

C: 20.0g

Let b be the mass-equivalent buoyancy in water and m the true mass. Then m − b = 15 g and m − 2b = 10 g. Subtracting gives b = 5 g, so m = 20 g.

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Question 14

In which of the following is surface tension important?

  1. The floating of a ship in water
  2. The floating of a dry needle in water
  3. The floating of a balloon in air
  4. The diffusion of a sugar solution across membrane.
Answer and explanation

B: The floating of a dry needle in water

Surface tension can support a carefully placed dry needle at the water surface. A ship and a balloon are mainly supported by buoyancy, while membrane diffusion is driven by concentration differences.

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Question 17

At normal atmospheric pressure, how much more heat is released by 4 kg of steam at 100 °C than by 4 kg of water at 100 °C when each ends as water at 80 °C? Use latent heat 2260000 J/kg and water specific heat 4200 J/(kg·K).

  1. 4,200J
  2. 2,260,000J
  3. 9,040,000J
  4. 9,380,000J
Answer and explanation

C: 9,040,000J

Both samples release 4 × 4200 × 20 = 336000 J as water cools from 100 °C to 80 °C. The steam also releases 4 × 2260000 = 9040000 J on condensing. That latent heat is the difference.

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Question 19

How long does a 750 W heater take to raise 1 kg of water from 20 °C to 50 °C, neglecting heat losses? Use specific heat capacity 4200 J/(kg·K).

  1. 84s
  2. 112s
  3. 168s
  4. 280s
Answer and explanation

C: 168s

Required heat is mcΔT = 1 × 4200 × (50 − 20) = 126000 J. At 750 J/s, the heater needs 126000/750 = 168 s if no heat is lost.

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Question 20

For a pure liquid in equilibrium with its vapour, the saturated vapour pressure increases as the

  1. Volume of the liquid increases at fixed temperature
  2. Volume of the liquid decreases at fixed temperature, with liquid still present
  3. Temperature of the liquid increases
  4. Temperature of the liquid decreases
Answer and explanation

C: Temperature of the liquid increases

Saturated vapour pressure rises with temperature. Warmer liquid molecules have greater energy, and equilibrium is reached at a higher vapour pressure. At a fixed temperature it does not depend on the liquid amount while both phases remain.

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Question 21

The absolute temperature of a perfect gas is proportional to the average

  1. Potential energy of the molecules
  2. Separation between the molecules
  3. Translational kinetic energy of the molecules
  4. Velocity of the molecules
Answer and explanation

C: Translational kinetic energy of the molecules

For an ideal gas, the mean translational kinetic energy per molecule is 3kT/2. It is therefore directly proportional to the absolute temperature T.

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Question 24

Which statement is true of light waves and sound waves in air?

  1. They both transmit energy
  2. They both need a material medium for propagation
  3. They are both transverse waves
  4. Their speeds in air are equal
Answer and explanation

A: They both transmit energy

Both light and sound transfer energy. Light can travel through vacuum, while sound needs matter. In air, sound is longitudinal and much slower than light.

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Question 25

The image in pin-hole camera is

  1. Erect and formed by refraction through a lens
  2. Virtual and formed by dispersion
  3. Erect and gets sharper as the hole becomes larger
  4. Inverted and formed by the light from each point traveling in a straight line.
Answer and explanation

D: Inverted and formed by the light from each point traveling in a straight line.

Rays from the top of an object pass through the small hole to the bottom of the screen, and rays from the bottom reach the top. Straight-line propagation therefore produces an inverted real image.

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Question 26

A plane mirror is rotated through a small angle Q in the plane of incidence while the incident ray remains fixed. Through what angle does the reflected ray rotate?

  1. ½Q
  2. Q
  3. 2Q
  4. 3Q
Answer and explanation

C: 2Q

With the incident ray fixed, rotating the mirror also rotates its normal by Q. The equal-angle reflection law makes the reflected direction change by twice that angle, or 2Q.

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Question 28

In a ray diagram for a thin converging lens, a ray that is not parallel to the optic axis but passes through the optic center will

  1. Pass through undeviated
  2. Pass through the center of curvature after refraction
  3. Emerge parallel to the principal axis
  4. Pass through the principal focus after refraction.
Answer and explanation

A: Pass through undeviated

In the thin-lens approximation, a ray through the optical centre is treated as undeviated. The small lateral displacement through a real lens is neglected in the ray diagram.

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Question 29

Which of the following correctly describes the image of an object, 4cm from a diverging lens of focal length –12cm?

  1. The image is virtual, 3cm in front of the lens
  2. The image is real, 6cm behind the lens
  3. The image is virtual, 6cm in front of the lens
  4. The image is real, 3cm in front of the lens.
Answer and explanation

A: The image is virtual, 3cm in front of the lens

Using 1/f = 1/u + 1/v gives 1/v = −1/12 − 1/4 = −1/3, so v = −3 cm. The negative image distance means a virtual image on the same side as the object.

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Question 30

Two tuning forks of frequencies 256Hz and 260 Hz are sounded close to each other. What is the frequency of the beats produced?

  1. 2 Hz
  2. 4 Hz
  3. 8 Hz
  4. 258 Hz
Answer and explanation

B: 4 Hz

Beats arise from alternating reinforcement and cancellation of nearby frequencies. Their frequency is the absolute difference: |260 − 256| = 4 Hz.

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Question 33

Which of the following statements is CORRECT about the earth’s magnetic field?

  1. The angle of dip is the angle which a free suspended magnet makes with the vertical
  2. The angle of declination is the angle between the magnetic meridian and the geographic meridian
  3. The angle of declination is the angle which a magnetic compass makes with the magnetic meridian.
  4. The angle of inclination is the difference between the angle of dip and the angle of declination.
Answer and explanation

B: The angle of declination is the angle between the magnetic meridian and the geographic meridian

Magnetic declination is the horizontal angle between geographic north and magnetic north. Dip, or inclination, is instead the angle of the magnetic field relative to the horizontal.

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Question 36

The function of the system of granulated carbon mixed with manganese (IV) oxide in a Leclanche cell is to

  1. Increase the emf of the cell to 2.0 V
  2. Prevent local action in the cell
  3. Prevent polarization in the cell
  4. Make the cell black and hence a good radiator
Answer and explanation

C: Prevent polarization in the cell

Manganese(IV) oxide is reduced in the cell’s cathode reaction and serves as the depolarizer. Carbon improves electrical conduction in the mixture, helping the cell deliver current.

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Question 41

A resistive device dissipates 1500 W and has resistance 375 Ω. What current does it draw?

  1. 0.10 A
  2. 2.00 A
  3. 4.00 A
  4. 77.5 A.
Answer and explanation

B: 2.00 A

For a resistive device, P = I²R. Hence I = √(P/R) = √(1500/375) = √4 = 2.00 A.

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Question 42

To convert an alternating current dynamo into a direct current dynamo, the

  1. Number of turns in the coil is increased
  2. Strength of the field magnet is increased
  3. Slip rings are replaced with a split-ring commutator
  4. Coil is wound on a soft-iron armature
Answer and explanation

C: Slip rings are replaced with a split-ring commutator

A split-ring commutator reverses the coil’s connection to the external circuit every half-turn. The external current then flows in one direction, giving pulsating direct current rather than alternating current.

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Question 44

The electrochemical equivalent of platinum is 5.0 × 10⁻⁷ kg/C. How long must a 100 A current flow to deposit 1.0 kg of platinum, to one decimal place in hours?

  1. 5.6 hours
  2. 56 hours
  3. 1.4 × 10⁴ hours
  4. 2.0 × 10⁴ hours
Answer and explanation

A: 5.6 hours

Using m = ZIt, time is 1/(5.0 × 10⁻⁷ × 100) = 20000 s. Dividing by 3600 gives 5.56 hours, or 5.6 hours to one decimal place.

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Question 45

Which statements are true of isotopes of an element? I. Their neutral atoms have the same electron configuration and very similar chemical properties. II. They can be separated using differences in physical properties such as mass. III. Their nuclei have the same number of protons.

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

D: I, II and III.

Isotopes have the same proton number. Their neutral atoms have the same electron configuration and very similar chemistry. Their different masses allow separation by physical methods such as mass separation.

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Question 47

A 4.0 g sample of a radioactive isotope has a half-life of 10 days. How long does it take for 3.5 g of the original isotope to decay?

  1. 1¼ days
  2. 8¾ days
  3. 30 days
  4. 80 days.
Answer and explanation

C: 30 days

If 3.5 g decays,0.5 g of the original radioactive isotope remains. The sequence is 4 →2 →1 →0.5 g: three half-lives. Therefore the time is 3 ×10 =30 days.

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Question 48

In the energy-releasing proton-proton fusion chain, four protons, each of rest mass Mp, are converted into a helium nucleus X of rest mass Mx, along with other reaction products. Which relation is correct?

  1. 4 Mp > Mx
  2. 4 Mp = Mx
  3. 4 Mp < Mx
  4. Mp = Mx
Answer and explanation

A: 4 Mp > Mx

The proton-proton fusion chain produces a helium nucleus with less rest mass than the four initial protons. The rest-mass difference supplies the released energy and other reaction products, so 4 Mp > Mx.

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Question 49

Which statements are true for alpha decay? I. Mass number decreases by four. II. Atomic number decreases by two. III. Mass number does not change.

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

A: I and II only

An alpha particle contains two protons and two neutrons. Its emission reduces the parent’s mass number by 4 and atomic number by 2. Thus I and II are true, while III is false.

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