| Question 12(a) State the principle of conservation of energy.
(b) State three effects of heat on a substance.
(c) Explain why when the bulb of thermometer is dipped into ice-cold water the
mercury level first rises before falling.
(d) Water of volume 200 cm3 in an aluminum container at 20o C is cooled to – 10oC
when put in a freezer.1. Calculate the quantity of heat energy extracted from the water.2. If heat is extracted at a rate of 120 Js-1 by the freezer, calculate the time taken to produce this ice.3. State a reason why the calculated time will be less than the actual time.(Density of water = 1000 kg m-3; specific heat capacity of water = 4200 J kg-1; specific latent heat of fusion of ice = 336000\j kg-1; specific heat capacity of ice = 2100J kg-1 K-1)
|Commentsa) This was well answered by most candidates however, few candidates omitted the phrase “ in a closed system or “isolated system” in their statement.(b) The three effects of heat on a substance fell an easy prey to most candidates.Performance was satisfactory.
formula m = v eluded most of the candidates.
Also applying the formula Q = mc appropriately was another problem due to their poor understanding of the concepts of specific heat capacity and latent heat.
The expected answers are:
(a) In an isolated / closed system, the total amount of energy is (always) constant (although energy may be transformed from one form to another)
(d)(i) m = x V
(ii) Q = Pt or P = Q
(iii) Why the calculated time is less than the actual time
e.g (No consideration is given to)
| Question 13
(i) principal focus for a diverging lens;
(ii) principal axis.
(b) State a situation under which the speed of light can change.
(c) The separation between the lenses of an astronomical telescope in normal adjustment is 35.0 cm and the angular magnification is 8 Calculate the focal lengths of the lenses.The lenses separation are adjusted to produce the final image 30 cm from the eye piece lens. Calculate the new separation of the lensesSketch a ray diagram to illustrate the image formed in 13(c)(ii)above.
|(a) Most respondent candidates were not able to define correctly principal focus for a
diverging lens and principal axis.(b) Relatively few candidates succeeded in stating the required situation under which
the speed of light can change. Many of them failed to realize that the speed
changes as light travels from one medium to another of different optical densities.(c) Most candidates messed up with their responses on this numerical on astronomical telescope in normal adjustment. This state of affairs could readily be ascribed to the effect of rote-learning as candidates reactions showed lack of proper understanding of the concept. The sketching of the ray diagram was a mirage.The expected answers are:(a)(i) This is a point on the principal axis to which all rays parallel and close to the
principal axis appear to diverge after refraction / passing through the lens. (ii) This is a line joining the centre of curvature (principal focus) of the lens to
the optical centre.
(b) When the light travels from one medium to another of different refractive
(c) Angular magnification= fo
fo + fe = 35
8fe + fe = 35
9fe = 35
fe = 3.89 cm
fo = 35 – 3.89
= 31.11 cm
1 = 1 + 1
fe u v
1 = 1 + 1
3.89 u -30 1 = 1 + 1 = 30 + 3.89
u 3.89 30 116.7
u = 116.7
New separation of lenses = 3.44 + 31.11 = 34.55cm
Parallel rays incident on objective lens
|(a) Define magnetic flux.(b) Sketch a diagram to illustrate magnetic lines of force for a bar magnet placed in a uniform earth’s magnetic field with its north pole pointing north.Indicate the neutral points.(c) State two ways in which the earth’s magnetic field is important to life on the earth surface.(d) List two applications of electromagnets(e) In a certain house, three ceiling fans each of 80 W, an air-conditioner rated 1500 W are operated for 6 hours eachday. Seven lamps each rated 40 W are switched on for 10 hours each day. The home theatre 100 W and television set 80 W are also operated each day for 5 hours after which they are kept in the standby mode. In the standby mode, the power consumption is 5% of the power rating. Calculate the:
1. Total energy consumed in the house for 30 days in kWh;
2. Cost of operating all the appliances for 30 days at N10.00 per kWh.
|(a) The description of magnetic flux posed some problem to candidates who failed to see it as basically a collection of (imaginary) lines of force in a magnetic field.(b)This part called for a sketch of a given magnetic field. It was poorly reacted to by candidates. Some sketches were rough while some did not indicate the correct direction of the arrows on the magnetic lines.(c)Most candidates were able to state ways in which magnetic field is important to life on earth. Performance was fairly alright.(d)Listing of applications of electromagnets was well answered by virtually all the respondent candidates. Performance was good.(e)The numerical part was poorly tacked most respondent candidates mishandled the standby mode part of the question. Performance was poor.
The expected responses are:
(b)correct shape of lines of force correct arrows / directions two neutral points P correctly indicated
(c)Importance of earth’s magnetic field to life on earth e.g
(d) Application of electromagnetic e.g
1. Energy consumed in a day = 14.311 kWh
Energy consumed in 30 days = 14.311 x 30
(ii) Cost of operating for 30 days
= N 4293.30
| Question 151. Explain radioactivity under the following headings:2. meaning;3. types.1. State two:
2. advantages of nuclear fusion over nuclear fission;
3. methods of detection of radioactivity.
1. A nuclear fusion equation for generation of power is shown below
1. An electron of mass 9.1 x 10-31 kg moves with a speed of 4.8 x 108 ms-1 in a vacuum. Calculate the wavelength of the associated wave.
[h = 6.6 x 10-34 Js]
1. Many candidates gave a fair attempt at this question as they found it very tractable. However, few candidates were still using the word “spontaneous” in defining radioactivity.
1. Advantages of nuclear fusion over nuclear fission was poorly attempted. However, they performed well in their attempt to state two methods of detecting radioactivity.
|.In the calculation of the energy released in the reaction, some candidates made themselves creditably equal to the required task. However, few candidates found the conversion of the unified atomic mass to kilogram difficult..Very few candidates did the correct thing while most of other respondent candidates derailed by starting their responses on a wrong note; i.e with wrong formula. Generally, performance was fair.The expected responses are:(a)(i) Radioactivity is the decay/disintegration of the nucleus of an unstable
nucleus/atom (of an element) with the release of any one, two or all of alpha, beta and gamma radiations. (ii) Types of radioactivity
(b)(i) Advantages of nuclear fusion over nuclear fission
(ii) Methods of detecting radioactivity
(c) m = mass defect = (mass of products ) – (mass of reactants)
Mass defect = 0.00577u
(d) λ = h
= 1.51 x 10-12 m
ALL QUESTIONS AND COMMENTS PROVIDED BY WAEC