Physics Past Questions | West African Examinations Council (WAEC)

Question 1

Two magnets are used to magnetize a soft iron bar PQ as illustrated in the diagram given. The correct polarity at P and Q respectively

Options

A) S and N

B) N and N

C) S and S

D) N and S

The correct answer is D.

Question 2

In storing magnets, keepers are used to

Options

A) reduced self-demagnetization

B) cancel the effect of the earth's magnetic field

C) protect the magnet from stray electric field

D) increase the strangth of the magnets

The correct answer is A.

Explanation:

A magnet keeper, also known historically as an armature, is a paramagnetic bar made from soft iron or steel, which is placed across the poles of a permanent magnet to help preserve the strength of the magnet by completing the magnetic circuit.

Question 3

A $10\mu C$

charge moves with a velocity of $1.0\times {10}^{5}m{s}^{-1}$ at right angle to a uniform magnetic field of flux density $5.0\times {10}^{-4}T.$

Calculate the force on the charge.

Options

A) $5.0\times {10}^{-5}N$

B) $5.0\times {10}^{-4}N$

C) $5.0\times {10}^{5}N$

D) $5.0\times {10}^{6}N$

The correct answer is B.

Explanation:

$F=qvB$

$=10\times {10}^{-6}\times 1.0\times {10}^5\times {10}^{-4}$
$=5.0\times {10}^{-4}N$

Question 4

Which of the following statements is not correct about steel and soft iron?

Options

A) steel is more magnetized than soft iron

B) permanent magnets are usually made of steel

C) soft iron is more readily magnetized than steel

D) soft iron more readily loses its magnetism than steel

The correct answer is A.

Question 5

What determines the polarity at the ends of an electromagnet? The

Options

A) magnitude of the current passing through the wire

B) material of the core of the magnet

C) material of the coil

D) direction of current in the wire

The correct answer is D.

Question 6

How many beta particle(s) are emitted in the radioactive decay of ${}_{79}^{198}Au{\leftarrow }_{80}^{198}Hg$

?

Options

A) 1

B) 2

C) 3

D) 5

The correct answer is A.

Explanation:

A beta particle forms when a neutron changes into a proton and a high-energy electron. The proton stays in the nucleus but the electron leaves the atom as a beta particle. When a nucleus emits a beta particle, these changes happen:

• the mass number stays the same
• the atomic number increases by 1
• the nuclear charge increases by 1

NB: Usually gold is created from platinum, which has one less proton than gold, or from mercury, which has one more proton than gold. Bombarding a platinum or mercury nucleus with neutrons can knock off an neutron or add on a neutron, which through natural radioactive decay can lead to gold.

Question 7

The phenomenon by which two light atomic nuclear combine to form a heavy nuclide with the release of energy is known as

Options

A) radioactivity

B) nuclear fusion

C) nuclear fission

D) chain reaction

The correct answer is B.

Explanation:

Nuclear fusion is a nuclear reaction in which atomic nuclei of low atomic number fuse to form a heavier nucleus with the release of energy.

Question 8

In a p-type semiconductor, the

Options

A) number of holes are equal to the number of electrons

B) electrical resistivity increases

C) electrons are the majority charge carriers

D) holes are the majority charge carriers

The correct answer is D.

Explanation:

In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers. P-type semiconductors are created by doping an intrinsic semiconductor with acceptor impurities. A common p-type dopant for silicon is boron or gallium.

Question 9

Let Δx be the uncertainty in the measurements of position and Δp the uncertainty in measurement of momentum. The uncertainty principle relation is given as

Options

A) Δx . Δp = h

B) Δx . Δp ≤ h

C) Δx . Δp ≥h

D) Δx . Δp > h

The correct answer is C.

Explanation:

Uncertainty principle, also called Heisenberg uncertainty principle or indeterminacy principle, statement, articulated (1927) by the German physicist Werner Heisenberg, that the position and the velocity of an object cannot both be measured exactly, at the same time, even in theory. The very concepts of exact position and exact velocity together, in fact, have no meaning in nature.

Question 10

Two objects of masses 80kg and 50kg are separated by a distance of 0.2m. If the gravitational constant is 6.6 x 10^-11Nm kg, calculate the gravitational attraction between them.

Options

A) 4.9 × 10^-9N

B) 1.3 × 10^-6N

C) 6.6 × 10^-8N

D) 6.6 × 10^-6N

E) 2.6 × 10^-9N

The correct answer is D.

Explanation:

F = $\frac{G{m}_1{m}_2}{r^2}=\frac{6.6\times {10}^{-11}\times 80\times 50}{0.2\times 0.2}$

= 6.6 × 10^-6N

Question 11

Two capacitors C₁ and C₂ are connected as shown in the diagram. The capacitance C₂ is twice C₁ when the key is opened the energy stored up in C₁ is W. If the key is later closed and the system is allowed to attain electrical equilibrium, the total energy stored in the system will be

Options

A) $\frac{1}{2}$

W

B) $\frac{2}{3}$

W

C) W

D) 2W

E) 3W

The correct answer is E.

Explanation:

E = $\frac{1}{2}C{r}^2=\frac{1}{2}(C-1+C_2)V^2=\left(3C_1\right)V^2$

= $3\left(\frac{1}{2}{C}_1{v}^2\right)$

= 3W

Question 12

In which of the points labelled A, B, C, D and E on the conductor shown would electric charge tend to concentrate most?

Options

A) A

B) B

C) C

D) D

E) E

The correct answer is E.

Explanation:

As a result of point action, charges are concentrated on the area in the diagram that is the most narrow. E is the point that is the most narrow.

Explanation provided by 

Question 13

Which of the following is stored by dry Leclanche cell?

Options

A) chemical energy

B) nuclear energy

C) solar energy

D) heat energy

E) electrical energy

The correct answer is E.

Explanation:

The Leclanché cell is an electric battery invented and patented by the French scientist Georges Leclanché in 1866. The battery contained a conducting solution (electrolyte) of ammonium chloride, a cathode (positive terminal) of carbon, a depolarizer of manganese dioxide (oxidizer), and an anode (negative terminal) of zinc (reductant). The chemistry of this cell was later successfully adapted to manufacture a dry cell.

Question 14

Using the data in the circuit illustrated above, calculate the value of R

Options

A) 0.02 $\Omega$

B) 0.05 $\Omega$

C) 5.00 $\Omega$

D) 20.00 $\Omega$

E) 50.00 $\Omega$

The correct answer is E.

Explanation:

R = $\frac{V}{T}=\frac{2v}{40\times {10}^{-3}}$

= 50.00 $\Omega$

Question 15

A 90W immersion heater is used to supply energy for 5 minutes. The energy supplied is used to completely melt 160g of a solid at its melting point. Calculate the specific latent heat of the solid.

Options

A) 2.81 Jg^-1

B) 6.25 Jg^-1

C) 8.89 Jg^-1

D) 168.75 Jg^-1

E) 5333.33 Jg^-1

The correct answer is D.

Explanation:

Pt = mL
90 x 5 x 60 = 160 x L
L = $\frac{27000}{160}$

= 168.75 Jg^-1

Question 16

In the circuit above R is a resistor whose resistance increases with increases with increase in temperature L₁ and L₂ are identical lamps. If the temperature of R increases

Options

A) L₁ becomes brighter and L₂ becomes dimmer

B) L₁ becomes brighter and L₂ does not change

C) L₂ becomes dimmer and L₁ does not change

D) L₁ becomes dimmer and L₂ does not change

E) L₁ and L₂ becomes brighter

The correct answer is A.

Question 17

A house is supplied with a 240V a.c mains. To operate a door bell rated at 8V, a transformer is used if the number of turns in the primary coil of the transformer is 900. Calculate the number of turns in the secondary coil of the transformer

Options

A) 30

B) 240

C) 248

D) 450

E) 1248

The correct answer is A.

Explanation:

$\frac{E_p}{E_s}=\frac{n_p}{n_s}$

n_s = 30

Question 18

The diagram above shows a current carrying wire between the poles of a magnet. In which direction would the wire tend to move?

Options

A) into the paper

B) out of the paper

C) towargs the north pole of the magnet

D) towards the south pole of the magnet

E) towards the top of the pages

Question 19

The correct answer is B.

Explanation:

Towards the south pole of the magnet

Explanation provided by Henry Ugo

Question 1

An induction coil is generally used to

Options

A) rectify an alternating current

B) produce a large input voltage

C) smoothen a pulsating direct current

D) modulate an incoming signal

E) produce a large output voltage

The correct answer is E.

Explanation:

An induction coil or "spark coil" is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current (DC) supply.

Question 20

The unit of stress is

Options

A) Nm

B) N

C) Nm^-2

D) Nm²

E) Nm¹

The correct answer is C.

Explanation:

The SI unit of stress is the newton per square meter, which is called the pascal.

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