JAMB Physics 1985

22 reviewed questions with answers and explanations.

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Question 1

Which of the following is NOT a fundamental S.I. unit?

  1. Metre
  2. Ampere
  3. Kelvin
  4. Second
  5. Radian
Answer and explanation

E: Radian

The radian measures plane angle and is a dimensionless derived unit. Metre, ampere, kelvin and second are SI base units.

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Question 6

If a car starts from rest and moves with a uniform acceleration of 10m s-2 for ten seconds, the distance it covers in the last one second of the motion is

  1. 95 m
  2. 100 m
  3. 500 m
  4. 905 m
  5. 1 000 m
Answer and explanation

A: 95 m

From rest, distance after t seconds is ½at². The final second covers ½ × 10 × (10² − 9²) = 95 m.

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Question 9

Which of the following is NOT a force?

  1. Friction
  2. Tension
  3. Upthrust
  4. Weight
  5. Impulse.
Answer and explanation

E: Impulse.

Impulse is the change in momentum, equal to force multiplied by time for a constant force. Its unit is N·s. Friction, tension, upthrust and weight are forces measured in newtons.

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Question 10

Two masses 40g and 60g respectively, are attached firmly to the ends of a light metre rule. The centre of gravity of the system is

  1. At the mid-point of the metre rule
  2. 40cm from the lighter mass
  3. 40cm from the heavier mass
  4. 60cm from the heavier mass
  5. indeterminate because the metre-rule is light.
Answer and explanation

C: 40cm from the heavier mass

Measure x from the 40 g mass. The centre of mass is x = (40 × 0 + 60 × 100)/(40 + 60) = 60 cm. It is therefore 40 cm from the heavier mass.

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Question 14

Two divers G and H are 20 m and 40 m below a lake surface. Their absolute pressures are P₁ and P₂ respectively. Atmospheric pressure equals 10 m of water. Find P₂/P₁ to two decimal places.

  1. 0.50
  2. 0.60
  3. 1.67
  4. 2.00
  5. 3.00
Answer and explanation

C: 1.67

Absolute pressure includes atmospheric pressure. Thus P₁ = ρg(10 + 20) and P₂ = ρg(10 + 40). Their ratio is 50/30 = 1.67 to two decimal places.

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Question 15

The areas of the effort and load pistons of a hydraulic press are 0.5m2 and 5m2 respectively. If a force F1 of 100N is applied on the effort piston, the force F2 on the load is

  1. 10 N
  2. 100 N
  3. 500 N
  4. 1 000 N
  5. 5 000 N
Answer and explanation

D: 1 000 N

Pascal’s principle gives equal pressure in the two pistons: F₁/A₁ = F₂/A₂. Therefore F₂ = 100 × 5/0.5 = 1000 N.

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Question 19

Equal masses of liquids P at 20 °C and Q at 80 °C have specific heat capacities 1.0 and 1.5 J/(kg·°C), respectively. They are mixed in a lagged calorimeter. Neglect heat loss and the calorimeter’s heat capacity. What is the equilibrium temperature?

  1. 44 °C
  2. 50 °C
  3. 56 °C
  4. 60 °C
  5. 70 °C
Answer and explanation

C: 56 °C

Heat lost by Q equals heat gained by P: 1.5(80 − T) = 1.0(T − 20). Hence 140 = 2.5T, giving T = 56 °C. Equal masses cancel from the equation.

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Question 21

In a gas experiment, the pressure of the gas is plotted against the reciprocal of the volume of the gas at a constant temperature. The unit of the slope of the resulting curve is

  1. Force
  2. Force/m
  3. Work
  4. Force/m3
  5. Energy/m2.
Answer and explanation

C: Work

Slope is pressure divided by reciprocal volume, so its units are pressure × volume: (N/m²) × m³ = N·m = J. These are the units of work.

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Question 22

Water shows anomalous behaviour

  1. Below 0 °C
  2. Between 0 °C and 4 °C
  3. At exactly 4 °C
  4. Between 4 °C and 100 °C
  5. Above 100 °C
Answer and explanation

B: Between 0 °C and 4 °C

Between 0 °C and 4 °C, water contracts as it warms and expands as it cools. Its density is greatest at about 4 °C. This reversal of ordinary thermal expansion is its anomalous behaviour.

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Question 25

Which statement about a pure liquid in equilibrium with its vapour is NOT correct?

  1. Boiling occurs when its saturated vapour pressure equals the external pressure.
  2. Both its boiling point and its saturated vapour pressure at a fixed temperature depend on external pressure.
  3. Its saturated vapour pressure rises as temperature increases.
  4. Its saturated vapour pressure is independent of available volume while both liquid and vapour remain present.
  5. Water can boil below 100 °C at high altitude.
Answer and explanation

B: Both its boiling point and its saturated vapour pressure at a fixed temperature depend on external pressure.

At a fixed temperature, a pure liquid’s saturated vapour pressure is determined by the liquid and its temperature. External pressure sets the boiling temperature by the condition that vapour pressure equals external pressure.

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Question 26

Which phenomenon cannot be explained by a simple kinetic molecular model of matter alone?

  1. Expansion
  2. Conduction
  3. Convection
  4. Radiation
  5. Evaporation.
Answer and explanation

D: Radiation

Radiation transfers energy by electromagnetic waves and can cross a vacuum. A simple kinetic molecular model alone does not describe this mechanism; electromagnetic theory is also needed.

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Question 27

In order to find the depth of the sea. A ship sends out a sound wave and receives an echo after one second. If the velocity of sound in water is 1500m/s, what is the depth of the sea?

  1. 0.75km
  2. 1.50km
  3. 2.20km
  4. 3.00km
  5. 3.75km
Answer and explanation

A: 0.75km

The echo time includes travel down to the seabed and back. Depth = vt/2 = 1500 × 1/2 = 750 m = 0.75 km.

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Question 28

When a sound wave passes from air into water its

  1. Speed and frequency increase but its wavelength remains the same.
  2. Speed and wavelength increase but its frequency remains the same
  3. Speed decreases.
  4. Speed remains the same but its frequency and wavelength change.
  5. Speed increases but its frequency and wavelength decrease.
Answer and explanation

B: Speed and wavelength increase but its frequency remains the same

Sound travels faster in water than in air. Its frequency stays fixed by the source as it crosses the boundary. Since λ = v/f, the wavelength increases with speed.

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Question 29

If the fundamental frequency of a closed pipe organ on a day when the speed of sound is 340m s-1 is 170 Hz, then the length of the pipe is

  1. 50cm
  2. 70cm
  3. 100cm
  4. 150cm
  5. 200cm
Answer and explanation

A: 50cm

For the fundamental of a pipe closed at one end, L = λ/4 = v/(4f). Thus L = 340/(4 × 170) = 0.50 m = 50 cm, neglecting end correction.

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Question 30

An object is placed 15cm in front of a concave mirror of radius 40cm. The image formed is

  1. Virtual and 60cm behind the mirror
  2. Real and 60cm in front of the mirror
  3. Virtual and at infinity
  4. Real and at infinity
  5. Virtual and 40cm from the mirror.
Answer and explanation

A: Virtual and 60cm behind the mirror

The focal length is R/2 = 20 cm. With real distances positive, 1/v = 1/20 − 1/15 = −1/60. Thus the image is virtual, 60 cm behind the mirror; the object lies inside the focus.

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Question 31

A 45° triangular glass prism can be used as a reflector of light because

  1. Refraction never takes place in such a prism
  2. The angle of reflection equals the angle of incidence
  3. The refractive index of glass is less than 1
  4. It is transparent
  5. The critical angle for glass is less than 45°
Answer and explanation

E: The critical angle for glass is less than 45°

In a right-angled 45° glass prism, a ray entering normally at a short face reaches the long face at 45°. If the glass–air critical angle is smaller, total internal reflection occurs there.

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Question 33

Two rays from a point under a flat water surface lie in the same vertical plane, symmetrically about the vertical. Their angle in water is 60°. Find their angle in air to one decimal place. Take the refractive index of water as 4/3.

  1. 41.8°
  2. 44.1°
  3. 60.0°
  4. 83.6°
  5. 120.0°
Answer and explanation

D: 83.6°

Each ray is 30° from the normal in water. Snell’s law gives sin r = (4/3)sin 30° = 2/3, so r = 41.81°. The emergent rays are separated by 2r = 83.6° to one decimal place.

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Question 39

Which of the following will convert a milliammeter to a voltmeter?

  1. Low series resistance
  2. Low parallel resistance
  3. High series resistance
  4. High parallel resistance
  5. Parallel resistance equal to milliameter resistance.
Answer and explanation

C: High series resistance

A high resistance in series limits the current through the meter. The resulting combination measures a larger voltage because V = I(Rseries + Rmeter).

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Question 42

An electric kettle connected to 240 V produces 6.0 × 10⁵ J of heat in 5 minutes. Find its resistance.

  1. 14.4 ohms
  2. 28.8 ohms
  3. 144 ohms
  4. 288 ohms
  5. 2 880 ohms
Answer and explanation

B: 28.8 ohms

Five minutes is 300 s. Power = E/t = 600000/300 = 2000 W. Using P = V²/R gives R = 240²/2000 = 28.8 Ω.

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Question 43

On which of the following does the operation of a moving coil ammeter depend? I Electromagnetic induction. II Force on a current-carrying conductor in a magnetic field. III Magnetic effect of an electric current.

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

D: II and III

A current-carrying coil experiences magnetic forces that produce a turning effect in the meter’s magnetic field. These are the magnetic effect of current and the force on a current-carrying conductor, so II and III apply.

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Question 44

A 10 Ω resistor and a 5 Ω resistor are connected in parallel, with total current I entering their common junction. The 5 Ω resistor dissipates 40 W. What power does the 10 Ω resistor dissipate?

  1. 10 W
  2. 20 W
  3. 40 W
  4. 80 W
  5. 100 W
Answer and explanation

B: 20 W

Parallel resistors have the same voltage across them. From the 5 Ω branch, V² = PR = 40 × 5 = 200 V². The power in the 10 Ω branch is therefore V²/10 = 200/10 = 20 W.

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Question 47

Which of the following statements about electrolysis is NOT correct?

  1. The substances in solution in the electrolyte become ionized.
  2. Ions are electrically charged and are attracted towards electrodes when a potential difference is applied.
  3. Ions may be discharged at the electrodes, forming bubbles or deposits.
  4. The mass deposited depends upon the length of time for which current flows.
  5. The mass of gas set free or metal deposited is proportional to the square of the current.
Answer and explanation

E: The mass of gas set free or metal deposited is proportional to the square of the current.

Faraday’s law gives deposited mass m = MIt/(nF). For the same material and duration, mass is proportional to current, not to its square.

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