2019 A/L Physics Past Paper · Question 06 (English Medium)

Answer: (3) By using a mercury-glass thermometer with a large mercury bulb, the range of measurements can be increased.

What the question asks

We want:

Which of the five statements about thermometers is not correct.

The five statements are about the thermometric property, the bulb wall, the bulb size, two thermometers, and the contact angle.

Relevant theory

In a mercury thermometer the mercury is in the bulb.

When it gets hotter, it expands.

The extra mercury is pushed up the thin bore.

The rise for each degree is the sensitivity:

$$s = \dfrac{\gamma V}{\pi r^2}$$

A bigger bulb $V$ gives a bigger $s$.

The stem has a fixed length $L$. So the highest temperature that can be read is:

$$\Delta\theta_{max} = \dfrac{L}{s}$$

A bigger $s$ gives a smaller range.

The length of a column of mercury in a glass capillary tube which has a uniform cross section, is the thermometric property of this thermometer.

Reading the options

Each statement is compared with how a thermometer works.

(1) A measurable property is needed

Correct

Without a property that changes with temperature, nothing can be measured

(2) The bulb has a thin wall

Heat goes in fast, so the reading follows quickly

(3) A bigger bulb gives a bigger range

Incorrect

A bigger bulb means a bigger $s$. Then $\dfrac{L}{s}$ is smaller

(4) Two thermometers can read differently

Thermometric properties do not all change in the same way

(5) A large contact angle is an advantage

Mercury does not wet the glass, so it does not stick to the wall

$0.57$

$s = \frac{100 \times 0.00018}{\pi \times 0.1^2}$

$262$

$\Delta\theta_{max} = \frac{150}{0.57}$

$2.29$

$s = \frac{400 \times 0.00018}{\pi \times 0.1^2}$

$65$

$\Delta\theta_{max} = \frac{150}{2.29}$

Now compare with the options.

Statements 1, 2, 4 and 5 are correct.

Only statement 3 is not.

A bulb four times bigger cuts the range from $262$ °C to $65$ °C.

Why the other options fail

This statement is correct. A thermometer needs a measurable property.

This statement is correct. A thin wall gives a quick response.

This statement is correct. Thermometric properties do not all change alike.

This statement is correct. Mercury does not wet the glass.

Conclusion

$\Delta\theta_{max} = \frac{L\,\pi r^2}{\gamma V}$

A bigger bulb is more sensitive.

But the stem has the same length. So the range is smaller.

So the statement that is not correct is (3).

Thinking that a bigger bulb makes a better thermometer in every way.

A bigger bulb is more sensitive. But its range is smaller. It also responds more slowly.