In this experiment, you will investigate the temperature distribution in a rectangular transparent tank of water when heated at one side. The tank is 30 cm ...
Question 1 Report
In this experiment, you will investigate the temperature distribution in a rectangular transparent tank of water when heated at one side. The tank is 30 cm long, 10 cm wide and 15 cm deep. You fills it with water at 18 °C. You places a flat electric heater against the left wall and records the temperature at six positions labelled A to F after 10 minutes. A is near the heater at the top, B is near the heater at the bottom, C is at the centre top, D is at the centre bottom, E is at the far wall top, and F is at the far wall bottom. Fig. 40.1 shows the arrangement and the positions of the thermometers. Table 40.1 shows the results.
| Position | Location | Temperature / °C |
|---|---|---|
| A | heater side, top | 42 |
| B | heater side, bottom | 28 |
| C | centre, top | 34 |
| D | centre, bottom | 22 |
| E | far side, top | 28 |
| F | far side, bottom | 20 |
Table 40.1
(a) Record the temperature at position A. [1]
(b) Record the temperature at position F. [1]
(c) Describe the pattern of temperature across the top of the tank (A to C to E). [1]
(d) Compare the temperatures at the top and bottom at each horizontal position. [1]
(e) Explain the temperature distribution using convection currents. [3]
(f) State two variables that should be controlled if the experiment were repeated with a different heater power. [2]
(g) Measure the length of the tank in Fig. 40.1. [1]
(h) State why you uses a transparent tank. [1]
Answer Details
(a) The temperature at position A (heater side, top) is 42 °C. [1]
(b) The temperature at position F (far side, bottom) is 20 °C. [1]
(c) Across the top of the tank, the temperature decreases from left (near the heater) to right (far side): A = 42 °C, then C = 34 °C, then E = 28 °C. [1]
This gradient exists because the warm water loses thermal energy as it travels away from the heat source along the top.
(d) At every horizontal position, the top is warmer than the bottom: A (42 °C) is greater than B (28 °C), C (34 °C) is greater than D (22 °C), and E (28 °C) is greater than F (20 °C). [1]
(e) Water near the heater is warmed, expands, becomes less dense, and rises to the top on the left side. [1] This warm water then flows along the top of the tank from left to right, gradually cooling as it transfers energy to the surroundings. [1] On the far side, the water has cooled enough to become denser, so it sinks. It then flows back along the bottom toward the heater, completing a convection loop. [1]
The temperature gradient across the top reflects the progressive cooling of the water as it moves further from the heater. The top-to-bottom difference at each position reflects the stratification caused by density: less dense warm water floats above denser cool water.
(f) Any two controlled variables from: same volume and starting temperature of water, same tank size and shape, same room temperature, same duration of heating. [1 each, max 2]
(g) Measure the horizontal distance from the left wall to the right wall of the tank outline in Fig. 40.1 using a ruler. The length is approximately 7.5 cm on the printed diagram (accept any reasonable measurement with a unit). [1]
(h) A transparent tank is used so that the positions of the thermometers can be seen, and dye could be added to visualise the convection currents directly. [1]
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