JAMB Physics 1984

28 reviewed questions with answers and explanations.

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Question 1

The distance travelled by a particle starting from rest is plotted against the square of the time elapsed from the commencement of motion. The resulting graph is linear. The slope of this graph is a measure of

  1. Initial displacement
  2. Initial velocity
  3. Acceleration
  4. Half the acceleration
  5. Half the initial velocity
Answer and explanation

D: Half the acceleration

Starting from rest with constant acceleration, s = ½at². A graph of s against t² therefore has slope ½a. Equivalently, if s = kt², differentiating twice gives a = 2k.

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Question 2

A uniform metre rule PT is horizontal and pivoted at R, its 70 cm mark. A downward force of 0.1 N acts at Q, the 60 cm mark, and a downward force of 0.4 N acts at S, the 85 cm mark. The rule is in equilibrium under these forces, its weight and the pivot reaction. What is its weight?

  1. 0.25 N
  2. 0.30 N
  3. 0.35 N
  4. 0.50 N
  5. 0.56 N
Answer and explanation

A: 0.25 N

The uniform rule’s weight W acts at 50 cm, 20 cm left of the pivot. Taking moments about 70 cm gives 0.4 × 15 = 0.1 × 10 + W × 20, with distances in centimetres. Thus W = (6 − 1)/20 = 0.25 N.

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Question 3

For which quantities are the dimensions ML²T⁻² correct? I. Moment of a force. II. Work. III. Acceleration.

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

D: I and II

Force has dimensions MLT⁻². Both moment of force and work multiply force by a distance, giving ML²T⁻². Acceleration has dimensions LT⁻², so I and II are correct.

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Question 4

What volume of alcohol of density 8.4 × 10² kg/m³ has the same mass as 4.2 m³ of petrol of density 7.2 × 10² kg/m³?

  1. 1.4 m³
  2. 3.6 m³
  3. 4.9 m³
  4. 5.0 m³
  5. 5.8 m³
Answer and explanation

B: 3.6 m³

The petrol mass is 720 × 4.2 = 3024 kg. For alcohol of density 840 kg/m³, volume = mass/density = 3024/840 = 3.6 m³.

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Question 5

For correcting long sight defects in the human eye we require a

  1. Converging lens
  2. Diverging lens
  3. Microscope
  4. Periscope
  5. Plain glass sheet.
Answer and explanation

A: Converging lens

A long-sighted eye does not converge light from close objects sufficiently. A converging lens adds focusing power so that the image forms on the retina.

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Question 6

For a concave mirror to form a real diminished image, the object must be placed

  1. Behind the mirror
  2. Between the mirror and its focus
  3. Between the focus and the centre of curvature
  4. At the centre of curvature
  5. At a distance greater than the radius of curvature
Answer and explanation

E: At a distance greater than the radius of curvature

A concave mirror forms a real, smaller image when the object is beyond its centre of curvature. The image then lies between the focus and the centre of curvature.

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Question 7

The unit of quantity of electricity (electric charge) is called

  1. The ampere
  2. The volt
  3. The coulomb
  4. The ammeter
  5. Electromotive force.
Answer and explanation

C: The coulomb

Electric charge is measured in coulombs. One coulomb is the charge transferred by a current of one ampere flowing for one second: Q = It.

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Question 8

The resistance of a wire depends on

  1. The length of the wire
  2. The diameter of the wire
  3. The temperature of the wire
  4. The resistivity of the wire
  5. All of the above.
Answer and explanation

E: All of the above.

For a uniform wire, R = ρL/A. Resistance depends on its length, cross-sectional area (and therefore diameter), and resistivity. Temperature also affects resistivity, so all four factors matter.

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Question 10

Two cells, each of emf 1.5 V and internal resistance 2 Ω, are connected in parallel with matching polarities. What current flows through an external 1 Ω resistor?

  1. 0.75 A
  2. 1.5 A
  3. 0.5 A
  4. 1.0 A
  5. 0.6 A
Answer and explanation

A: 0.75 A

Identical cells in parallel retain emf 1.5 V, while their combined internal resistance is 2/2 = 1 Ω. Including the external resistor, I = 1.5/(1 + 1) = 0.75 A.

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Question 11

Which component is NOT contained in a zinc-carbon dry cell?

  1. Carbon rod
  2. Paste containing manganese dioxide
  3. Paste containing ammonium chloride
  4. Zinc case
  5. Copper rod
Answer and explanation

E: Copper rod

A zinc-carbon dry cell contains a carbon rod, a zinc casing and a paste containing ammonium chloride and manganese dioxide. A copper rod is not one of these components.

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Question 12

Which of the following can be described as high tension transmission?

  1. High resistance and low voltage
  2. Low current and high voltage
  3. High current and low voltage
  4. High voltage and zero current
  5. High current and low resistance.
Answer and explanation

B: Low current and high voltage

High-tension transmission means high-voltage transmission. For the same power, using a high voltage permits a low current and reduces I²R heating losses in the conductors.

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Question 13

All heat generated in a 5 Ω resistor by a 2 A current flowing for 30 seconds evaporates 5 g of liquid at its boiling point. What is the specific latent heat of the liquid?

  1. 120 J
  2. 60 J/g
  3. 120 J/g
  4. 1500 J
  5. 1500 J/g
Answer and explanation

C: 120 J/g

The heat supplied is I²Rt = 2² × 5 × 30 = 600 J. Specific latent heat is energy per unit mass: L = 600/5 = 120 J/g.

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Question 14

When a standing wave occurs in an air column, the distance between a node and its nearest antinode is equal to

  1. One-quarter of the wavelength
  2. One-half of the wavelength
  3. The wavelength
  4. Twice the wavelength
  5. Four-times the wavelength.
Answer and explanation

A: One-quarter of the wavelength

In a standing wave, adjacent nodes are half a wavelength apart. An antinode is midway between them, so a node and its nearest antinode are one-quarter wavelength apart.

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Question 15

Which statements are NOT correct? I. At fixed temperature and composition, pressure changes do not affect the speed of sound in air treated as an ideal gas. II. The speed of sound in air increases with temperature. III. The quality of a note depends only on its frequency.

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

C: III only

At fixed temperature and composition, pressure changes do not change the ideal-gas speed of sound. Sound speed increases with temperature. The quality or timbre of a note depends on its mixture of frequencies and their relative amplitudes, so only III is incorrect.

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Question 16

Two identical tuning forks have natural frequency 256 Hz. One is loaded, and 4 beats per second are heard when they sound together. What is the frequency of the loaded fork?

  1. 260 Hz
  2. 252 Hz
  3. 248 Hz
  4. 264 Hz
  5. 258 Hz
Answer and explanation

B: 252 Hz

Adding a small load to a tuning fork lowers its frequency. The beat frequency is the difference between the two frequencies, so the loaded fork vibrates at 256 − 4 = 252 Hz.

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Question 17

Dew point is the temperature at which water vapour in the atmosphere

  1. Turns into steam
  2. Solidifies into ice pellets
  3. First condenses into liquid form
  4. Is just sufficient to cause cooling
  5. Has a relative humidity of fifty percent
Answer and explanation

C: First condenses into liquid form

The dew point is the temperature to which air must be cooled, without changing its water-vapour content, to become saturated and start forming liquid droplets.

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Question 18

The lower (0 °C) and upper (100 °C) fixed points of a mercury-in-glass thermometer are 210 mm apart. In a room, the mercury ends 49 mm above the lower point. What is the room temperature to one decimal place?

  1. 55.3 °C
  2. 23.3 °C
  3. 49.0 °C
  4. 16.1 °C
  5. 76.7 °C
Answer and explanation

B: 23.3 °C

The 210 mm interval represents 100 °C. A rise of 49 mm therefore corresponds to (49/210) × 100 = 23.33… °C, or 23.3 °C to one decimal place.

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Question 19

If a solid changes directly into a gas when heat is applied the process is called

  1. Vaporization
  2. Evaporation
  3. Sublimation
  4. Ionization
  5. Conversion.
Answer and explanation

C: Sublimation

Sublimation is the direct change from solid to gas without passing through a liquid phase. Dry ice is a familiar example.

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Question 27

Heat is supplied at 100 W to 1.0 × 10⁻² kg of liquid for 20 seconds. Its temperature rises by 5 °C. Assuming the supplied heat warms the liquid, what is its specific heat capacity?

  1. 2.0 × 10² J/(kg·K)
  2. 2.0 × 10² J/kg
  3. 4.0 × 10⁴ J/(kg·K)
  4. 4.0 × 10⁴ J/kg
  5. 8.4 × 10³ J/(kg·K)
Answer and explanation

C: 4.0 × 10⁴ J/(kg·K)

Heat supplied is Pt = 100 × 20 = 2000 J. The temperature rise is 5 K, so c = Q/(mΔT) = 2000/(0.01 × 5) = 4.0 × 10⁴ J/(kg·K).

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Question 35

Between terminals X and Y, a 4 μF capacitor connects X to an intermediate node. From that node to Y are three parallel branches containing a 4 μF, a 3 μF and a 2 μF capacitor respectively. What is the effective capacitance between X and Y?

  1. 3/4 μF
  2. 2 10/13 μF
  3. 12 μF
  4. 4 12/13 μF
  5. 13 μF
Answer and explanation

B: 2 10/13 μF

The three parallel capacitors add to 4 + 3 + 2 = 9 μF. This 9 μF combination is in series with the first 4 μF capacitor, giving C = (4 × 9)/(4 + 9) = 36/13 μF = 2 10/13 μF.

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Question 37

In a model atmosphere, sea-level pressure is 10⁵ N/m², density is uniform at 1 kg/m³, and g = 10 m/s². What is the height of this atmosphere?

  1. 100 m
  2. 1000 m
  3. 10 000 m
  4. 100 000 m
  5. 1000 000 m
Answer and explanation

C: 10 000 m

For the stated uniform-density model, p = ρgh. Hence h = 10⁵/(1 × 10) = 10000 m. This is the height of the simplified model atmosphere.

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Question 38

Which of the following is a correct explanation of the inertia of a body?

  1. Ability to overcome the earth’s gravity
  2. Reluctance to stop moving
  3. Readiness to start moving
  4. Reluctance to start moving and its readiness to stop moving once it has begun to move
  5. Reluctance to start moving and its reluctance to stop moving once it has begun to move.
Answer and explanation

E: Reluctance to start moving and its reluctance to stop moving once it has begun to move.

Inertia is resistance to a change in velocity. A body tends to stay at rest or continue moving with constant velocity unless a resultant external force acts.

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Question 39

A water pump at Kainji Dam lifts 1000 kg of water through a vertical height of 10 m in 10 s. Taking g = 10 m/s², what is its useful lifting power?

  1. 1.0 kW
  2. 10.0 kW
  3. 12.5 kW
  4. 15.0 kW
  5. 20.0 kW
Answer and explanation

B: 10.0 kW

The gain in gravitational potential energy is mgh = 1000 × 10 × 10 = 100000 J. Dividing by 10 s gives 10000 W = 10 kW of useful lifting power.

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Question 40

What is the apparent colour of a RED SHIRT when viewed in PURE green light?

  1. Red
  2. Green
  3. Yellow
  4. Black
  5. Blue
Answer and explanation

D: Black

In the ideal colour model, a red shirt reflects red light and absorbs green light. Under pure green illumination there is no red light to reflect, so it appears black.

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Question 41

Which list gives the colours of the visible spectrum in order from longest to shortest wavelength?

  1. Blue, red, green. Yellow, indigo, violet, orange.
  2. Red, orange, yellow, green, blue, indigo, violet.
  3. Red, orange, yellow, indigo, green, blue, violet.
  4. Indigo, green, blue, violet, yellow, red, orange.
  5. Yellow, blue, green, violet, orange, indigo, red.
Answer and explanation

B: Red, orange, yellow, green, blue, indigo, violet.

From longest to shortest visible wavelength, the usual order is red, orange, yellow, green, blue, indigo and violet. Frequency increases in the reverse direction to wavelength.

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Question 42

Cathode rays are

  1. High-energy electromagnetic waves
  2. Protons
  3. Streams of electrons
  4. Neutrons
  5. Radio waves
Answer and explanation

C: Streams of electrons

Cathode rays are streams of electrons. They carry negative charge and can be deflected by electric and magnetic fields.

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Question 45

Radio waves travel at 3 × 10⁸ m/s. A station broadcasts at 800 kHz. What is the wavelength?

  1. 375.0 m
  2. 267.0 m
  3. 240.0 m
  4. 37.5 m
  5. 26.7 m
Answer and explanation

A: 375.0 m

Convert 800 kHz to 800000 Hz. Wavelength λ = v/f = (3 × 10⁸)/800000 = 375 m.

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Question 47

What does it cost to run five 50 W lamps and four 100 W lamps for 10 hours at a tariff of 2 kobo per kWh?

  1. ₦0.65
  2. ₦0.13
  3. ₦3.90
  4. ₦39.00
  5. ₦234.00
Answer and explanation

B: ₦0.13

Total power is 5 × 50 + 4 × 100 = 650 W = 0.65 kW. In 10 hours the lamps use 6.5 kWh. At the stated rate, the cost is 6.5 × 2 = 13 kobo = ₦0.13.

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