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Post-UTME Physics 2015 Objective Past Questions

All 20 questions from the Post-UTME Screening (Post-UTME) Physics 2015 Objective paper, with the correct answer and a full explanation for each. Free, no signup needed.

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Physics 2015 Objective — Question 1

Use the diagram above (an AC source, f=150Hz, connected to an inductor L=2H) to answer this question. From the diagram above, calculate the reactance of the inductor.

  • A. 400 ohm
  • B. 360 ohm
  • C. 250 ohm
  • D. A

Physics 2015 Objective — Question 2

What happens when excited electrons in an atom return to lower energy levels?

  • A. They absorb photons
  • B. They emit protons
  • C. They emit photonsCorrect
  • D. They absorb neutrons

Explanation

When an excited electron falls to a lower energy level, it emits a photon carrying the energy difference.

Physics 2015 Objective — Question 3

A radioactive material has a half-life of 15 minutes. What fraction of the initial mass of the material will be left after one hour?

  • A. 1/12
  • B. 1/4
  • C. 1/16Correct
  • D. 1/8

Explanation

One hour = 60 minutes = 4 half-lives. Fraction remaining = (1/2)^4 = 1/16.

Physics 2015 Objective — Question 4

In an A.C circuit, the alternating current across a resistor is 5 sin 50 pi t. The root mean square value of the current is

  • A. 5/sqrt(2) A
  • B. sqrt(2)/5 A
  • C. sqrt(2)/2 A
  • D. 5sqrt(2)/2 ACorrect

Explanation

Irms = Ipeak/sqrt(2) = 5/sqrt(2) = 5sqrt(2)/2 A.

Physics 2015 Objective — Question 5

The work function of a certain metal is 2.1 eV. Calculate the minimum energy required to remove an electron from the metal surface. [1eV = 1.6 x 10-19C]

  • A. 3.36 x 10^-19JCorrect
  • B. 4.20 x 10^-19J
  • C. 5.20 x 10^-19J
  • D. 6.36 x 10^-19J

Explanation

Energy = 2.1 x 1.6x10^-19 = 3.36x10^-19J.

Physics 2015 Objective — Question 6

An instrument that uses colour change as a thermometric property for temperature measurement is called

  • A. mercury thermometer
  • B. resistance thermometer
  • C. radiation pyrometerCorrect
  • D. crystal spectrometer.

Explanation

A (optical) radiation pyrometer measures temperature using the colour/intensity of radiation emitted by a hot object.

Physics 2015 Objective — Question 7

The maximum strain which a steel wire can withstand is 3 x 10-6. Calculate the stress on the wire. [Young's modulus for steel = 1.8 x 10^11Nm-2]

  • A. 2.7 x 10^5Nm-2
  • B. 8.3 x 10^-18Nm-2
  • C. 5.4 x 10^5Nm-2Correct
  • D. 1.7 x 10^-17Nm-2

Explanation

Stress = strain x Young's modulus = 3x10^-6 x 1.8x10^11 = 5.4x10^5 Nm-2.

Physics 2015 Objective — Question 8

The force between the molecules of the same substance is

  • A. cohesive forceCorrect
  • B. the force due to gravity
  • C. the force due to osmotic pressure
  • D. adhesive force.

Explanation

The force of attraction between molecules of the same substance is called cohesive force.

Physics 2015 Objective — Question 9

A quantity of liquid has a volume of 80cm3 at 15C. If its cubic expansivity is 10-3K-1, calculate its volume at 45C.

  • A. 82cm3Correct
  • B. 75cm3
  • C. 85cm3
  • D. 78cm3

Explanation

Change in volume = V x gamma x deltaT = 80 x 10^-3 x 30 = 2.4cm3. New volume = 80+2.4 = 82cm3.

Physics 2015 Objective — Question 10

20g of water at 2C is added to 35g of water at 65C. Calculate the final steady temperature of the mixture.

  • A. 63.9C
  • B. 56.0C
  • C. 42.1CCorrect
  • D. 40.6C

Explanation

Heat balance: 20(Tf-2)=35(65-Tf). Solving gives Tf approximately 42.1C.

Physics 2015 Objective — Question 11

The instrument used for measuring the specific gravity of a liquid is

  • A. barometer
  • B. manometer
  • C. hygrometer
  • D. hydrometer.Correct

Explanation

A hydrometer measures the specific gravity (relative density) of a liquid.

Physics 2015 Objective — Question 12

Convectional current is a consequence of the fact that

  • A. no material medium is necessary for the process
  • B. at thermal equilibrium, temperature is constant
  • C. the current is due to charge movement
  • D. density is temperature-dependent.Correct

Explanation

Convection currents arise because a fluid's density changes with temperature, causing warmer (less dense) fluid to rise and cooler (denser) fluid to sink.

Physics 2015 Objective — Question 13

A Pyrex glass can be used to boil water because it

  • A. has a low coefficient of expansionCorrect
  • B. has a high coefficient of expansion
  • C. is a good conductor of heat
  • D. is a bad conductor of heat.

Explanation

Pyrex glass has a low coefficient of thermal expansion, making it resistant to cracking under rapid temperature changes.

Physics 2015 Objective — Question 14

The pressure of 760cm3 of gas initially at 120cmHg is changed at constant temperature to 76cm Hg. What is its new volume?

  • A. 481
  • B. 700
  • C. 1200Correct
  • D. 12

Explanation

By Boyle's law, P1V1=P2V2: 120x760=76xV2, giving V2=1200cm3.

Physics 2015 Objective — Question 15

The dimensions of a rectangular glass block are 4cm by 10cm by 16cm. Calculate the ratio of the maximum pressure to the minimum pressure which the block can exert when placed on a horizontal surface.

  • A. 4:1Correct
  • B. 2:1
  • C. 1:1
  • D. 4:3

Explanation

Max pressure occurs with the smallest face down (4x10=40cm2); min pressure with the largest face down (10x16=160cm2). Ratio = 160/40 = 4:1.

Physics 2015 Objective — Question 16

When a liquid in a container is evaporating, the temperature of the bulk liquid

  • A. decreasesCorrect
  • B. initially decreases but later increases
  • C. does not change
  • D. increases.

Explanation

Evaporation removes the most energetic molecules, lowering the average kinetic energy (and thus temperature) of the remaining liquid.

Physics 2015 Objective — Question 17

The upper and lower fixed points on a certain thermometer are marked 155 and -5 respectively. Which mark on the thermometer corresponds to 80C?

  • A. 125.0
  • B. 123.0Correct
  • C. 9.7
  • D. 2.6

Explanation

Reading = lower + (80/100) x (upper - lower) = -5 + 0.8x160 = -5+128 = 123.0.

Physics 2015 Objective — Question 18

Sounds produced by bats are of

  • A. high frequencies
  • B. ultra-high frequencies
  • C. low frequencies
  • D. very high frequencies.Correct

Explanation

Bats produce and detect very high (ultrasonic) frequency sounds for echolocation.

Physics 2015 Objective — Question 19

An object is placed 20 cm in front of a plane mirror. If the mirror is moved 5 cm towards the object, what is the distance between the image and the object?

  • A. 30 cmCorrect
  • B. 15 cm
  • C. 45 cm
  • D. 25 cm

Explanation

New object distance = 20-5=15cm, so image distance (behind mirror) is also 15cm. Total distance between image and object = 15+15 = 30cm.

Physics 2015 Objective — Question 20

If the diagram of the motion of a wave is at right angles to the direction of particle vibration, the wave is

  • A. sound
  • B. longitudinal
  • C. transverseCorrect
  • D. electromagnetic.

Explanation

A wave whose direction of travel is at right angles to the direction of particle vibration is, by definition, a transverse wave.

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