Heat and Temperature — Class 7 Science (Samacheer Kalvi)
TN State Board (Samacheer Kalvi) Class 7 Science, Term 2 — Chapter 1. Measuring how hot or cold things are.
1. About this chapter
This chapter covers heat and temperature, the temperature scales, how a thermometer works, and the clinical and digital thermometers.
2. Heat and temperature
- Temperature is the measure of the degree of hotness or coldness of a body. Heat is a form of energy that flows from a hotter body to a colder one.
- The international (SI) unit of temperature is the kelvin (K); other scales are Celsius (°C) and Fahrenheit (°F).
3. The thermometer
- A thermometer measures temperature. When its bulb touches a hot object, the liquid inside expands and rises up the tube.
- Mercury is used in laboratory thermometers because it expands uniformly and is a liquid at room temperature.
- On the Celsius scale, the lower fixed point is the melting point of ice (0 °C) and the upper fixed point is the boiling point of water (100 °C).
4. Clinical and digital thermometers
- A clinical thermometer measures body temperature; the temperature of a healthy person is 37 °C. Its range is small — about 35 °C to 42 °C (94 °F to 108 °F).
- A kink in the clinical thermometer stops the mercury flowing back into the bulb, so the reading can be noted after removal.
- A digital thermometer is widely used because of its high accuracy and it does not use mercury.
- The temperature of boiling water cannot be measured by a clinical or a digital thermometer (their range is too low).
5. Worked examples
Example 1. What is the SI unit of temperature? The kelvin (K).
Example 2. What is the body temperature of a healthy person? 37 °C.
Example 3. Why is there a kink in a clinical thermometer? To stop the mercury from flowing back so the temperature can be read after removal.
6. Book-back questions (Samacheer Kalvi)
I. Choose the correct answer
- The international unit of temperature is — (a) Celsius / (b) kelvin. Ans: (b) kelvin.
- The body temperature of a healthy person is — (a) 37 °C / (b) 47 °C. Ans: (a) 37 °C.
- The lower fixed point on the Celsius scale is the — (a) melting point of ice / (b) boiling point of water. Ans: (a) melting point of ice.
- Mercury is used in thermometers because it — (a) is cheap / (b) expands uniformly. Ans: (b) expands uniformly.
- The thermometer with high accuracy is the — (a) digital thermometer / (b) clinical thermometer. Ans: (a) digital thermometer.
II. Fill in the blanks 6. Digital thermometers do not use mercury. 7. A doctor uses a clinical thermometer to measure human body temperature. 8. At room temperature, mercury is in the liquid state.
III. Answer briefly 9. What is temperature? — The measure of the degree of hotness or coldness of a body. 10. Why can't a clinical thermometer measure the temperature of boiling water? — Its range (35–42 °C) is too low.
7. Common mistakes
- Mistake: Saying heat and temperature are the same. Fix: Temperature measures hotness/coldness; heat is the energy that flows.
- Mistake: Using a clinical thermometer for boiling water. Fix: Its range is only 35–42 °C, far below 100 °C.
- Mistake: Forgetting the SI unit of temperature. Fix: The SI unit is the kelvin (K).
8. Quick revision
- Term 2 · Ch 1 · heat and temperature.
- Temperature = degree of hotness/coldness; SI unit kelvin; also °C and °F.
- Thermometer: liquid expands on heating; mercury expands uniformly; Celsius lower point = melting point of ice (0 °C).
- Clinical thermometer: 35–42 °C, kink stops mercury return, body temp 37 °C; digital = high accuracy, no mercury; neither measures boiling water.
