JAMB Physics 2000

18 reviewed questions with answers and explanations.

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Question 3

In the simple model of dry sliding friction, which statements are approximately correct? I. Friction is independent of apparent contact area. II. It depends on the nature of the surfaces. III. It depends on sliding speed. IV. It is directly proportional to normal reaction.

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

A: I, II and IV

In the simple dry-sliding-friction model, F =μN. The coefficient depends on the contacting materials; force is approximately independent of apparent area and, over the model’s range, sliding speed.

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Question 4

A ship of unchanged mass sails from salt water into less-dense fresh water and remains afloat. The fraction of its volume above the water surface will

  1. Remain the same
  2. Increase
  3. Decrease
  4. Increase then decrease.
Answer and explanation

C: Decrease

A floating ship displaces water whose weight equals its own. Fresh water is less dense than salt water, so a larger submerged volume is needed. The volume remaining above the surface decreases.

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Question 7

For a body moving at constant speed around a circle, which statements are true? I. Its velocity is constant. II. The net work done on it is zero. III. It has constant acceleration away from the centre. IV. The centripetal force points toward the centre.

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

D: II and IV

At constant speed the kinetic energy is unchanged, so the net work is zero. Velocity still changes direction, and the net centripetal force points toward the centre. Therefore II and IV are correct.

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Question 8

A particle has velocity v =10 +2 t² in m/s, with t in seconds. Find its instantaneous acceleration at t =5 s.

  1. 10 m/s²
  2. 15 m/s²
  3. 20 m/s²
  4. 60 m/s²
Answer and explanation

C: 20 m/s²

Instantaneous acceleration is the derivative of velocity: a =dv/dt =4 t. At t =5 s this is a =4 ×5 =20 m/s². The constant 10 contributes no acceleration.

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Question 9

A force acts parallel to velocity. If both force and speed are halved, with direction unchanged, the mechanical power is

  1. Doubled
  2. Constant
  3. Reduced to a quarter
  4. Reduced by half.
Answer and explanation

C: Reduced to a quarter

Mechanical power is P =Fv for force parallel to motion. Halving both quantities gives P_new =(F/2)(v/2) =P/4.

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Question 10

The velocity ratio of a machine is 5 its efficiency is 75%. What effort would be needed to lift a load of 150 N with the machine?

  1. 50 N
  2. 40 N
  3. 30 N
  4. 20 N
Answer and explanation

B: 40 N

Efficiency equals mechanical advantage divided by velocity ratio. Thus MA =0.75 ×5 =3.75, and effort =load/MA =150/3.75 =40 N.

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Question 21

In a traditional mercury-and-alcohol Six maximum-minimum thermometer, P labels the sensing reservoir, Q the liquid in the bottom U-bend, and R the liquid above Q in the opposite limb, below the expansion space. Identify P, Q and R.

  1. Alcohol, mercury and alcohol
  2. Air, alcohol and mercury
  3. Mercury, alcohol and mercury
  4. Air, mercury and alcohol.
Answer and explanation

A: Alcohol, mercury and alcohol

In the traditional mercury-and-alcohol Six thermometer, alcohol in the sensing side expands and displaces the mercury in the U-bend. The liquid above the mercury on the other side is also alcohol; the expansion space is farther up in its bulb.

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Question 29

White light passes obliquely from air into a transparent medium with normal dispersion. Three refracted rays a, b and c lie in order from closest to farthest from the normal. Which colours could a, b and c represent?

  1. Blue, yellow and red
  2. Green, red and blue
  3. Red, green and blue
  4. Yellow, blue and red
Answer and explanation

A: Blue, yellow and red

In a normally dispersive medium, blue light has a higher refractive index than yellow or red, so it bends closest to the normal. Yellow is intermediate and red is farthest from the normal.

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Question 30

Capacitors P =5 μF and Q =10 μF are connected in parallel across a 20 V d.c. source. Find their stored charge magnitudes, P then Q.

  1. 200 μC and 100 μC
  2. 100 μC and 200 μC
  3. 4 μC and 2 μC
  4. 2 μC and 4 μC
Answer and explanation

B: 100 μC and 200 μC

Parallel capacitors each have the full 20 V. Using Q =CV gives Q_P =5 ×20 =100 μC and Q_Q =10 ×20 =200 μC.

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Question 31

Which basic two-lens instrument is best constructed using only converging lenses of focal lengths 500 cm and 5 cm?

  1. Compound microscope
  2. Terrestrial telescope
  3. Astronomical telescope
  4. Galileo’s telescope
Answer and explanation

C: Astronomical telescope

A refracting astronomical telescope uses a long-focal-length converging objective and a short-focal-length converging eyepiece. The 500 cm and 5 cm lenses fit that arrangement.

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Question 33

A cell of emf 2 V and internal resistance 1 Ω supplies 3 Ω and 2 Ω resistors in series. Ideal voltmeters V₁ and V₂ measure across the 3 Ω and 2 Ω resistors respectively. Find their readings.

  1. 1 V and 1/3 V
  2. 1/3 V and 1 V
  3. 1/3 V and 2/3 V
  4. 1 V and 2/3 V
Answer and explanation

D: 1 V and 2/3 V

Total resistance is 3 +2 +1 =6 Ω including the cell. Current is 2/6 =1/3 A, giving V₁ =(1/3) ×3 =1 V and V₂ =(1/3) ×2 =2/3 V.

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Question 36

An ideal 2 H inductor is connected to a sinusoidal supply of frequency 50/π Hz. Find its reactance.

  1. 200 Ω
  2. 50 Ω
  3. 100/π Ω
  4. 25/π Ω
Answer and explanation

A: 200 Ω

Inductive reactance is X_L =2πfL. With f =50/π Hz and L =2 H, X_L =2π ×(50/π) ×2 =200 Ω.

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Question 37

A radioactive sample has 1/64 of its original undecayed atoms remaining after 120 years. Find its half-life.

  1. 2 years
  2. 10 years
  3. 20 years
  4. 24 years
Answer and explanation

C: 20 years

The remaining fraction 1/64 equals(1/2)⁶, so six half-lives have elapsed. The half-life is 120/6 =20 years.

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Question 39

In a pure intrinsic semiconductor in thermal equilibrium, the number of conduction-band electrons is

  1. Equal to the number of holes in the valence band
  2. Greater than the number of holes in the valence band
  3. Less than the number of holes in the valence band
  4. Twice the number of holes in the valence band.
Answer and explanation

A: Equal to the number of holes in the valence band

In an intrinsic semiconductor, thermal excitation of one valence electron into the conduction band leaves one hole behind. Thus conduction-electron and valence-hole populations are equal.

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Question 41

Photons of energy 5 eV strike a metal with work function 3 eV. Find the stopping potential magnitude.

  1. 15.0 V
  2. 8.0 V
  3. 2.0 V
  4. 1.7 V
Answer and explanation

C: 2.0 V

The maximum photoelectron energy is photon energy minus work function:5 −3 =2 eV. A stopping potential of 2 V removes 2 eV from an electron, bringing the fastest electrons to rest.

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Question 43

A 200-turn coil has the magnetic flux through each turn changing at 0.08 Wb/s. Find the induced emf magnitude.

  1. 1.6 V
  2. 16.0 V
  3. 25.0 V
  4. 250.0 V
Answer and explanation

B: 16.0 V

Faraday’s law gives emf magnitude N|dΦ/dt|. With 200 turns and a flux change of 0.08 Wb/s through each turn, the induced emf is 200 ×0.08 =16 V.

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Question 45

In a basic mains-device wiring diagram, P is in the live lead, Q in the neutral lead and S in the protective-earth lead. Which marked position is appropriate for the series fuse?

  1. S
  2. Q
  3. P
  4. Between P and Q
Answer and explanation

C: P

The fuse belongs in series with the live conductor. If it opens, it disconnects the device from the live supply. A fuse in the neutral can leave the device live, while the protective earth must remain continuous.

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Question 49

A generator delivers 3.0 kW at its 1.5 kV output terminals through cables whose total resistance is 20 Ω. Find the resistive power loss in those cables.

  1. 0.1 W
  2. 10.0 W
  3. 40.0 W
  4. 80.0 W
Answer and explanation

D: 80.0 W

The line current is I =P/V =3000/1500 =2 A. The total cable loss is I²R =2² ×20 =80 W.

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