Physics Topics are often described using mathematical equations, making them precise and quantifiable.
At what temperature do you consider hot or cold?
Heat is a form of energy. It makes a substance hotter. Heat cannot be seen by us. We can feel the heat by the ‘temperature effect’ it produces. When heat is given to a substance, its temperature increases and it becomes hotter. For example, when a utensil is kept on a gas burner, it gets heat, its temperature increases and it become hot.
On the other hand, when heat is removed from a substance, then its temperature decreases and it becomes cold. For example, when water is kept in a refrigerator, then heat gets removed from water, its temperature decreases and it becomes cold.
Hot and Cold
In our daily life, we come across a number of objects. Some of these objects are hot whereas other objects are cold. For example, tea is hot but ice is cold; soup is hot but ice cream is cold; the spoon kept in a cup of hot milk is hot whereas fruit juice is cold; a frying pan kept on a burning gas stove is hot but the handle of frying pan is cold. Even among the hot objects, some objects may be hotter than the others. Similarly, among the cold objects, some objects may be colder than the others. We usually decide whether an object is hot or cold just by touching it (provided it is not too hot or too cold!). Sometimes, however, our sense of touch is not reliable in telling us whether an object is really hot or cold. This point will become more clear from the following activity.
Take three bowls A, B and C. Put cold water in bowl A, hot water in bowl B and lukewarm water (slightly warm water) in bowl C (see Figure).
(i) We dip our left hand in cold water in bowl A and right hand in hot water in bowl B [see Figure (a)]. Keep the hands in this way for about 2 or 3 minutes.
(ii) Now take out the hands from bowls A and B and dip both the hands quickly in lukewarm water in bowl C [see Figure (b)].
We will find that the water in third bowl C does not feel equally warm to both the hands. To the left hand (which was earlier in cold water), this lukewarm water appears to be hot. But to the right hand (which was earlier in hot water), this lukewarm water appears to be cold. This, however, is impossible because the same water cannot be hot as well as cold at the same time. Actually, our sense of touch is misinforming us in this case.
From the above activity we conclude that we cannot depend on our sense of touch for estimating the hotness (or coldness) of an object. We need some reliable measure for estimating the hotness (or coldness) of an object. A reliable measure of the hotness (or coldness) of an object is its temperature. The temperature of an object is measured by using a device called thermometer. We will now discuss the temperature and its measurement in detail.
If we touch a utensil kept on a burning gas stove, it feels to be very hot. We say that the temperature of utensil is very high. On the other hand, if we touch a utensil placed in a refrigerator, it feels to be very cold. And we say that the temperature of utensil is very low. Thus, a hot object has a high temperature whereas a cold object has a low temperature. This gives us the following definition of temperature : The temperature of an object is the degree of hotness (or coldness) of the object. The temperature of an object tells us how hot or cold the object is. A high temperature of an object tells us that it is very hot whereas a low temperature of the object tells us that it is quite cold. For example, the temperature of boiling water is ‘hundred degrees Celsius’ (100°C), which is quite high. So, boiling water feels to be very hot. On the other hand, the temperature of melting ice is ‘zero degree Celsius’(0°C), which is quite low. So, ice feels to be very cold to touch.
Temperature is measured by using a device (or instrument) called thermometer. A thermometer has a scale marked on it which is used to read the temperature. The most common temperature scale marked on thermometers for measuring temperatures is the ‘Celsius scale’. So, the temperature is expressed in the unit of ‘degree Celsius’ (which is written in short form as °C). In the unit °C, the small circle at the top (°) denotes the term ‘degree’ and the letter C denotes Celsius. So, the unit °C is read as ‘degree Celsius’ or just ‘degree C’.
In the Celsius scale of temperature’, the temperature of melting ice is given a value of 0°C (zero degree Celsius) and the temperature of boiling water (or steam formed from it) is given a value of 100°C (hundred degrees Celsius). The length of a laboratory thermometer tube between 0° mark and 100° mark is divided into 100 equal divisions. Each such division is called a degree. Please note that the temperature of an object should always be stated with its unit: °C (degree Celsius).
Measuring Temperature : Thermometers
A thermometer is a device for measuring the temperature of an object. There are two common types of thermometers : Laboratory thermometer and Clinical thermometer. Both these thermometers are based on the Celsius scale of temperature. The laboratory thermometer as well as clinical thermometer are mercury thermometers. When heat is supplied to the thermometer bulb containing mercury (by the hot body whose temperature is to be measured), the mercury expands and rises in the glass tube of thermometer. This fact is used in measuring the temperature of the hot body (or hot object). We will now describe the construction and working of a laboratory thermometer and a clinical thermometer in detail, one by one. Let us discuss the laboratory thermometer first.
A laboratory thermometer is used for measuring the temperature in a science laboratory. A laboratory thermometer is made up of a long glass tube T having a thin bore in it (see Figure). There is a glass bulb B containing mercury which is joined at the lower end of the glass tube. The top end of glass tube is sealed (after removing air). The whole length of thermometer glass tube is graduated (or calibrated) in degrees Celsius. The graduations marked on the tube of a commonly used laboratory thermometer are from, – 10°C to 110°C (minus ten degree C to hundred ten degree C). So, a common laboratory thermometer can measure temperatures from, -10°C to 110°C.
This is called the range of thermometer. Thus, the range of a laboratory thermometer is generally from, -10°C to 110°C (see Figure). This means that a common laboratory thermometer can measure a lowest temperature of, —10°C and a highest temperature of 110°C. If we look at the temperature markings of say, 0°C and 10°C on the thermometer tube, we will find that there are 10 small divisions in between them (see Figure). Now, since 10 small divisions on this laboratory thermometer tube represent a temperature of 10 degrees Celsius, therefore, 1 small division on the laboratory thermometer will represent a temperature of 1 degree Celsius (or 1°C). We can see a thin silvery thread of mercury in the narrow glass
tube of the thermometer. The upper end of this mercury thread (or column) tells us the temperature of the object in which the thermometer bulb is placed.
We will now describe how temperature is measured by using a laboratory thermometer. We will measure the temperature of hot water kept in a beaker.
Take a laboratory thermometer. Hold the thermometer from its glass tube and immerse the bulb of thermometer in hot water taken in a beaker (see Figure). The bulb of thermometer should not touch the sides or the bottom of the beaker. We will see the shining thread of mercury moving up in the thermometer tube. After some time the mercury thread will stop rising and stand at one place. So, wait till the mercury thread becomes steady. We then read the temperature on thermometer tube which corresponds to the top of mercury thread. This will give us the temperature of hot water.
For example, in Figure, the thermometer shows that the temperature of hot water taken in the beaker is 80°C. Please note that whenever we measure the temperature of hot water (or anything else), we should wait until the mercury stops rising in the thermometer tube. We should note down the temperature reading by keeping the thermometer bulb still immersed in hot water. This is because if the thermometer bulb is taken out of hot water, its mercury thread will start falling. This will give a wrong reading for the temperature of hot water. Another point to be noted is that if we put the thermometer bulb in a beaker containing cold water, then the mercury of thermometer bulb will contract (by losing heat) and the level of mercury thread in the thermometer tube will fall showing a lower temperature for cold water.
Precautions in Using a Laboratory Thermometer
- The laboratory thermometer should be held vertically (or upright) while measuring temperature. It should not be tilted.
- The thermometer bulb should be surrounded from all sides by the substance whose temperature is to be measured.
- The thermometer bulb should not touch the sides or bottom of the container in which the substance is taken.
- Read the thermometer while its bulb is still in touch with the substance whose temperature is being measured.
- Read the thermometer by keeping the level of mercury along the line of sight.
- Do not hold the thermometer by the bulb.
- Handle the thermometer with care. It can break if hit against any hard object.
A laboratory thermometer cannot be used to measure the human body temperature. This is because as soon as we take out the bulb of the laboratory thermometer from our mouth, the level of mercury in its tube will start falling quickly (due to cooling of its bulb by air).
This will give a wrong value of the body temperature. It is clear that for measuring the body temperature, we should use a thermometer in which the mercury level does not fall even when its bulb is removed from the mouth. A clinical thermometer is such a thermometer. Before we describe the clinical thermometer, we should know the meaning of the term ‘kink’. The kink is a narrow and sharp bend (or curve) in the bore of a clinical thermometer tube. Kink is also known as constriction. The clinical thermometer is also called Doctor’s thermometer.