# Waec Gce 2021 physics answers

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(1a)
Power; = f*d/t = MLT^-² * L/T =ML²T^-³

(1b)
Force; = m*a =MLT^-²

(1c)
Pressure; F/A = MLT^-²/L² =ML^¹T^²
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(2)
(i)Boron
(ii)Aluminium
(iii)Gallium
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(3a)
(i)Iron
(ii)nickel

(3b)
(i)Magnets are used in magnetic compass, doorbells, refrigerators.
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(4)
w = 2π/T
V/R = 2π/T
R/V = T/2π
R = VT/2π
= 8×10³×5×10³/2×3.14
= 6.37×10⁶m
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(5)
Given :
e= 5.00×10-³m
P.E= 50J
Using; P.E = 1/2ke²
50 = 1/2×k×(5×10-³)²
k = 50×2/(5×10-³)²
= 50×2/25×10-⁶
k = 4×10⁶N/m

(11ai)
(i)Lines of force Start only on positive charges and end only on negative charges.
(ii)Lines of force do not cross each other

(11aii) (11bi)
The surface density of a conductor is the quantity of say electric charge per unit area flowing through it.

(11bii)
The opposition to the flow of alternating current altered by a capacitor is known as capacitive reaction ie Xc

(11ci)
From ; Xc =1/2πfc, when frequency is increased, Xc drops. Consequently, from I = v/Xc, current increases

(11cii)
Vt = VmSin (wt-90°)
If It = Im Sin wt
Voltage lags by 90°

(11ciii)
Xc = 1/2πfc
:. C = 1/2πfXc = 1/2*3.14*100*500
= 1/314000
= 3.185µF

(11d)
A stator and armature
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10a)
(i)Long sight
(ii)Short sight

(10bi)
For the man to see objects clearly at 25.0cm from the eye, the image must appear to be at 60.0cm.
Hence ie U = 25cm, V = -60cm
1/f = 1/v + 1/u = 1/-60 + 1/25
1/f = 1/25 – 1/60
1/f = 12-5/300 = 7/300
f = 300/7cm = 42.86
He need a convex lens at focal length 42.86cm(He is long sighted)

(10bii)
Power = 100/f = 100)42.86
= 2.34

(10ci)
Dispersion of light is the phenomenon of light at different wavelengths being separated by retraction by different currents when white light is passed through a glass prism

(1cii)
An echo us a sound heard after the reflection of sound waves from a plane surface

(10di)
V = 2d/t
ie d = Vt/2
= 2800×0.5/2
= 700meters

(10dii)
When white is passed through a triangular glass prism, an elongated coloured pitch of light is obtained on a screen placed behind the prism. The coloured pattern as known as spectrum of white light, and it contains red, orange, yellow, green, blue, indigo and violet

(10diii)
(i)Pitch
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(9ai)
Constant-Volume gas thermometer
(i)Thermometric substance –> Gas
(ii)Physical property –> Change of gas pressure at constant volume with temperature

(9aii)
Thermocouple thermometer
(i) Substance –> Two dissimilar metals
(ii) Property –> Change in electric potential between two metal junctions at different temperature

(9aiii)
Liquid in glasses
(i) Substance –> Mercury or Alcohol
(ii) Property –> Change in volume of liquid with temperature

(9bi)
(i)Smaller in size and can therefore be used to measure temperature nearly at a point
(ii)More sensitive

(9bii)

(9ci)
Linear espansivity of a substance is defined as the increase in length per unit length per degree rise in temperature

(9cii)
∆l = L1α∆θ
= 0.3×1.2×10-⁵×50
= 18×10-⁵
= 0.00018m of 0.018cm

(9d)
Heater draws current, I = P/v = 2750/240
= 11.46A
Since this current exceeds the current breaker rated current of 10A, the circuit breakers trios off- line become inactive

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