## Physics 2020 Past Question | WAEC

46. Which of the following units is not fundamental?

**A.**Metre**B.**Kilogram**C.**Joule**D.**Candela

**Correct Option: Answer is C**

**47. **The vacuum between the double walls of a thermos flask reduces heat loss through

**A.**conduction and radiation**B.**convection and conduction**C.**radiation only**D.**conduction only

**Correct Option: Answer is B**

**48. **In photoelectric effect, the number of electrode emitted per second from a metallic surface is proportional to the

**A.**intensity of the incident radiation**B.**frequency of the incident radiation**C.**energy of the incident radiation**D.**work function of the metal.

**Correct Option: Answer is A**

**49. **When two cells of negligible internal resistances and equal e.m.f. denoted by E1and E2are connected in parallel, combined e.m.f. E is given by

**A.**E = E1 + E2**A.**E = E1/E2**B.**1/E=1/E1+1/E2**C.**E = E1 = E2

**Correct Option: Answer is A**

**50. **Which of the following statement about viscosity is not true? it

**A.**depends on areas of the surfaces in contact**B.**occurs in fluids**C.**is independent of the relative velocity of the layers**D.**depends on the tangential force between the layers

**Correct Option: Answer is A**

### PHYSICS THEORY PAST QUESTIONS 2020 WITH SOLUTION

- An elastic material of length 3m is to be stretched to reduce an extension three times its original length. Calculate the force required to produce the extension

**Solution & Explanation:**

Calculation of the force needed to produce extension of material

F = Ke

= 988.3 x 3 x 3

= 8840.74N

**2. **In a solar panel for heat supply, state the function of each of the following parts: (a) metal flat plate; (b) thermal insulator: (c) tubes.

**Solution & Explanation:**

** **

(a) Function of metal flat plate of solar panel

– Receives and absorbs solar radiation to produce heat

(b) Function of internal insulator of solar panel

– Minimises heat loss

(c) Function of tubes of solar panel

– Helps to circulate the heat

**3. **(a) In the design of an optical fibre, what type of material is most suitable for the design of the core?

(b) State one condition necessary to confine signals to the core of an optical fibre.

**Solution & Explanation:**

(a) Type of material most suitable for the design of the core of an optical fibre

– Glass/plasties

(b) Condition necessary confine signal to the core of an optical fibre

– The refractive index of the core must be greater than that of the cladding,

**4. **The velocity v, of a wave in a stretched string, depends on the tension T, in the spring and the mass per unit length of the spring. Obtain an expression for v in terms of T and u, using the method of dimensions.

**Solution & Explanation:**

**5. **A satellite launched with velocity VEEjust escapes the earth’s gravitational attraction. Given that the radius of the earth is R, show that VE= √20R [g = 10ms^{−2}

**Solution & Explanation: **

**6. **A bullet is fired from a gun at 30oto the horizontal. The bullet remains in flight for 25s before touching the ground. Calculating the velocity of projection. [g = 10ms^{−2}]

### Solution & Explanation:

**7. **State three properties of lasers that make them preferable to ordinary light beam**.**

**Solution & Explanation:**

Travels a very long distance without losing its intensity

- Has a very narrow optical bandwidth.
- Gives more intense light/power
- Is monochromatic

**8. **(a)i) Define a torque.

(ii)State three factors that determine a torque

(b)(i) Define free fall.

(ii) A body is thrown vertically upwards from the top of a tower 40.0m high with a velocity of 10.0ms^{−1}. Calculate the time taken for the body to reach the ground. [g.= 10.0ms^{−2}]

- c) A cube of wood of side 8.0cm, floats at the interface between oil and water with 2.0cm of its surface below the interface as shown in the diagram below. Given that the relative densities of oil and water are 0.72 and 1.00 respectively, calculate the mass of the wood

**Solution & Explanation:**

(a)i) Definition of a torque

– The turning effect of force about an axis/pivot

(ii) Factors that determine a torque

– perpendicular distance of the force from the pivot

– direction of the force.

– magnitude of the force

(b)(i) Definition of free fall:

– The vertical motion of a body only under the influence of gravity/its own weight

(ii) Calculation of the time taken by a body thrown vertically upward to reach the ground

S = 10t – 1/210t^{2}

-40 = 10t – 1/210t^{2 }x 10 t^{2}

(t + 2)(t – 4) = 0

Time to fall to the ground, t = 4s

(c) Calculation of the mass of wood:

– Mass of wood = upthrust due to water displace + upthrust due to oil displaced

= 1000 x 2 x 8 x 8 x 10 + 720 x 6 x 8 x 8 x 10

= 0.404kg

**9. **(a)Explain resonance frequency as applied in RLC series Circuit.

(ii) Sketch a diagram to illustrate the variation of frequency, f, with the resistance, R, the capacitive reactance, Xc and the inductive reactance XLL, in RLC series circuit.

(iii) Using the diagram drawn in (a)(ii) state whether the current in the circuit leads, lags or is in phase with the supply voltage when: (α) f = foo; (β) f < foo ; γfoo; when foo is the resonant frequency.

b)(i) Define mutual inductance.

(i) The coil of an electric generator has 500 turns and 8.0cm diameter. If it rotates in a magnetic field of density 0.25T, calculate the angular speed when its peak voltage is 480V. [π = 3.142].

**Solution & Explanation:**

(a)i) Explanation of resonance frequency as applied to RLC series circuit

– It is the frequency of oscillation of an RLC series circuit when the capacitive reactance is equal to the inductive reactance. The impedance is equal to the resistance /the impedance is minimum.

(ii) A sketched graph

(iii) αα Relationship between current and voltage when f = foo;

Current is in phase with the supply voltage (β)

– Relationship when current and voltage when f = foo;

Current lags the Supply voltage (γ)

– Relationship between current and voltage when f > foo;

Current leads the supply voltage.

(b) (i) Definition of mutual inductance:

The ratio of the induced emf in one coil/circuit to the time rate of change of current in the other coil. OR The production of e.m.f in a circuit as a result of the change in the magnetic flux/magnetic circuit in an adjacent circuit linked to it

.

(ii) Calculation of angular speed of the coil of an electric generator: E = ωBAN

480 = ω x 0.25 x π x (8.0×10−228.0×10−22) x 500 = ω = 763.8 rad/s

(c)(i) Explanation of eddy current:

The current is induced and flows within a metal block when there is a change in magnetic flux linking the block

**10. **

- a) (i) Define Optical angle.

(ii) Explain two conditions necessary for total internal reflection to occur.

(iii) List three practical applications of total internal reflection.

(b) State two effects of refraction.

(c)(i) Define progressive waves.

(ii) A plane progressive wave is represented by the equation y = 0.5 sin(1000πr = 100πλ/17) where y is in millimetres, t in seconds and x in metres. Calculate the: (α) frequency of the wave; (β)of the wave; (γ) speed of the wave

**Solution & Explanation:**

** **

(a) (i) The angle of incidence in the optically denser medium for which the angle of refraction in the less dense medium is 90^{o}

(ii) Conditions necessary for total internal reflection to occur:

– Light rays must travel from a dense medium to a less dense medium.

– Angle of incidence in the dense medium must be greater than the critical angle.

iii) Practical applications of total internal reflection

– Prism periscopes.

– Prism Binoculars.

– Fibre optics

– Fish eye view.

– Transmission of radio signals

(b) Effects of refraction

– straight objects placed in liquid appear bent (on the interface or boundary)

– mirages.

– dispersion of white light/formation of rainbow.

– apparent displacement of objects placed in liquids/apparent shadows of pools base.

(c)(i) Definition of progressive waves: A disturbance which travels through a medium that enables energy to be transferred from its source to another (without the particles of the medium transferred). OR Transfer of energy and momentum from the source of disturbances.