In this experiment, you will investigate how the temperature of water varies with height in a beaker heated from below. You fills a tall 500 ml beaker with ...
Question 1 Report
In this experiment, you will investigate how the temperature of water varies with height in a beaker heated from below. You fills a tall 500 ml beaker with water at room temperature (20 °C). You places five thermometers at heights of 2 cm, 5 cm, 8 cm, 11 cm and 14 cm above the base, holding them in position with a retort stand and clamps. You heats the base of the beaker with a Bunsen burner on a low flame and records all five temperatures after 4 minutes of heating. Fig. 37.1 shows the arrangement. Table 37.1 shows the results.
| Thermometer | Height above base / cm | Temperature after 4 min / °C |
|---|---|---|
| T1 | 2 | 52 |
| T2 | 5 | 41 |
| T3 | 8 | 35 |
| T4 | 11 | 30 |
| T5 | 14 | 26 |
Table 37.1
(a) Record the temperature at a height of 8 cm. [1]
(b) Record the temperature at the lowest position (T1). [1]
(c) Calculate the temperature difference between T1 and T5. [1]
(d) Describe the pattern shown by the results. [1]
(e) Explain, in terms of density, why the water near the base is hotter than the water near the top after only 4 minutes. [2]
(f) Suggest what the temperature readings would look like after a much longer time (e.g. 20 minutes). [1]
(g) State the process by which heat energy is transferred from the base to the top of the beaker. [1]
(h) Describe how you could improve the accuracy of the temperature readings. [1]
(i) State what would happen to the temperature distribution if the heater were placed at the top of the beaker. [2]
Answer Details
(a) The temperature at a height of 8 cm (thermometer T3) is 35 °C. [1]
(b) The temperature at the lowest position, T1 (2 cm above the base), is 52 °C. [1]
(c) Temperature difference between T1 and T5:
\[ \Delta T = T_1 - T_5 = 52 - 26 = 26\,{}^{\circ}\text{C} \]
The temperature difference is 26 °C. [1]
(d) The temperature decreases as the height above the base increases. [1] The hottest water is at the bottom (nearest the heat source) and the coolest is near the top.
(e) Water near the base is heated first; it expands and becomes less dense. [1] However, after only 4 minutes, the convection currents have not yet fully mixed the water throughout the beaker. The hottest water is still concentrated near the base because it takes time for the convection process to distribute thermal energy upward. [1]
In convection, warm less-dense water rises and is replaced by cooler denser water. This process takes time to reach a steady state where all the water is at a similar temperature.
(f) After a much longer time (e.g. 20 minutes), convection currents would thoroughly mix the water and all five thermometers would read approximately the same temperature, which would approach boiling point if heating continues. [1]
(g) Convection. [1]
(h) Wait for the thermometer readings to stabilise before recording, or use a digital thermometer with a faster response time. [1] Mercury or alcohol thermometers have thermal lag and may not show the true water temperature if read too quickly.
(i) If the heater were placed at the top:
- Only the water near the top would become hot, because warm water is less dense and stays at the top. [1]
- The water near the base would remain cool because convection cannot carry heat downward in a liquid. Cold, dense water stays at the bottom with no driving force to create circulation. [1]
This demonstrates that convection in liquids is only effective when heating is applied from below or at the side.
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