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 Physics Paper 1, MAy/June. 2009
 Questions: 1 2 3 4 5 6 Main

Weakness/Remedies
Strength

Question 2

(a)  Diagram

(i)You are provided with a measuring cylinder, two different tins labeled C and D, thermometer and other necessary materials.

(ii)Use the measuring cylinder provided to measure 100 cm3 of water and pour it into the tin labeled C(iii)Heat the water in the tin almost to boiling (90 oC).

(iv)Remove the tin and place it on a cork or wooden stand.

(v)Insert the thermometer into the tin and record the temperature of the water every minute starting from 85 oC until the temperature falls to 60 oC.

(vi)Repeat the experiment with the tin labeled D using exactly the same volume of water and temperature range.

(viii)    On the same graph sheet and using the same axes and scales, plot two graphs of temperature on the vertical axis and time on the horizontal axis from the readings obtained using tins C and D.
(ix)       Label the graphs appropriately as C and D to correspond with the tins used.
(x)       From each graph, read off the time taken to cool from 85 oC to 65 oC.

(xi)State two precautions taken to ensure accurate results.

(b)  (i)  Explain how heat losses by radiation and convection are minimized in a vacuum flask.
(ii)   State four factors which affect the rate of evaporation of a liquid in an open container.

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Observation

This question was very popular and well attempted by many candidates.  Good scales were chosen and good graphs plotted.  Candidates also provided correct answers to the short response questions.  However, few candidates mixed up the recording of temperatures for the two tins thereby graphs could not be labeled.  Many candidates were unable to correctly deduce the time taken for the temperature to fall from 85 oC to 65 oC.  Few candidates could not specifically mention how heat loss by radiation is minimized in a vacuum flask.

In part (a) the tasks required of the candidates included:

1. Reading and recording values of temperature;

(Trend; As t increasesθcand θDincrease)

1. Recording their data in a composite table showing t, θc and θD;
2. Distinguishing graph axes, selecting reasonable scales, plotting at least four points correctly from each set of temperaturesθcandθDon the same axes.;
3. Drawing two smooth curves and indicating on each graph temperature of 85 oC and 65 oC;
4. Reading and recording the time taken to cool from 85 oC and 65 oC on each graph;
1. Stating the precautions to ensure accurate results as follows:

-     Liquid gently stirred;
-     Parallax error avoided in reading the thermometer;
-     Splashing of water avoided.

The expected responses to part (b) were as follows:

(b)(i)    How heat losses are minimized in a vacuum flask.

1. Vacuum prevents convection and conduction OR
2. Insulating cork/stopper minimizes  convection

(ii)   Factors which affect the rate of evaporation of a liquid in an open container:
-     Temperature

• Draught/wind
• Nature of liquid
• Humidity
• Area of liquid surface exposed