🔶 1. Introduction
We often feel if something is hot or cold by touching it. But our sense of touch is not always reliable. For example, the same water can feel different to different people. So we need a proper instrument — a Thermometer — to measure temperature accurately.
🔶 2. What is Temperature?
- Temperature is the measure of how hot or cold an object is
- Hotter object = Higher temperature
- Colder object = Lower temperature
- SI unit of temperature = Kelvin (K)
🔶 3. Types of Thermometers
✅ A) Clinical Thermometer
- Used to measure human body temperature
- Range = 35°C to 42°C
- Normal body temperature = 37.0°C = 98.6°F
- Earlier used mercury — now digital ones are common
- Mercury is toxic, so digital is safer
How to use:
- Wash before and after use
- Do not hold from the bulb
- Read at eye level
✅ B) Digital Thermometer
- Uses electronic heat sensors to measure temperature
- No mercury — completely safe
- Easy to read numbers on screen
- Very accurate and fast
✅ C) Non-Contact Thermometer
- Measures temperature using infrared radiation
- No physical touch needed
- Widely used during COVID-19 pandemic
- Very safe and hygienic
✅ D) Laboratory Thermometer
- Used in science experiments and labs
- Range = –10°C to 110°C (wider range)
- Uses alcohol or mercury inside
How to use:
- Hold it vertically
- Bulb should not touch the container walls
- Read at eye level while still in the liquid
- Handle carefully — do not shake
Why not use for body temperature?
- Its range is too wide for the narrow body temperature range
- Not convenient to use inside mouth or armpit
- Not precise enough for human body readings
🔶 4. Temperature Scales
🌡️ A) Celsius Scale (°C)
- Freezing point of water = 0°C
- Boiling point of water = 100°C
- Most commonly used in daily life
🌡️ B) Fahrenheit Scale (°F)
- Freezing point of water = 32°F
- Boiling point of water = 212°F
- Normal body temperature = 98.6°F
- Becoming less common in science
🌡️ C) Kelvin Scale (K)
- SI unit of temperature
- No degree sign used with K
- Formula: K = °C + 273.15
- 0 K = Absolute Zero (lowest temperature possible)
- Used in scientific and research work
📝 Writing Rules:
- Scale names start with Capital Letter (Celsius, Fahrenheit, Kelvin)
- Symbols are °C, °F, K
- Never write °K — just write K
🔶 5. Comparison Table
| Feature | Clinical Thermometer | Laboratory Thermometer |
|---|---|---|
| Used for | Body temperature | Experiments |
| Range | 35°C – 42°C | –10°C to 110°C |
| Type | Digital/Mercury | Mercury/Alcohol |
| Safety | Safer (digital) | Handle carefully |
🔶 6. Important Keywords
| Term | Meaning |
|---|---|
| Temperature | Measure of hotness or coldness |
| Thermometer | Instrument to measure temperature |
| Clinical Thermometer | Measures body temperature |
| Laboratory Thermometer | Used in experiments |
| Celsius (°C) | Common temperature scale |
| Fahrenheit (°F) | Scale used mainly in USA |
| Kelvin (K) | SI unit of temperature |
| Absolute Zero | 0 K — lowest possible temperature |
| Digital Thermometer | Uses electronic sensors, no mercury |
| Non-Contact Thermometer | Measures using infrared, no touch |
✅ NCERT Questions & Answers
Q1. What is the normal temperature of a healthy human body? Ans. Normal temperature = 37.0°C or 98.6°F
Q2. What is the SI unit of temperature? Ans. The SI unit of temperature is Kelvin (K)
Q3. Name two types of thermometers. Ans.
- Clinical Thermometer
- Laboratory Thermometer
Q4. Why is a laboratory thermometer not used to measure body temperature? Ans. Because:
- Its range (–10°C to 110°C) is too wide for body temperature
- It is not convenient to use inside the mouth or armpit
- It is not precise for the narrow human body temperature range (35°C–42°C)
Q5. Someone says she has a fever of 101 degrees — is it Celsius or Fahrenheit? Ans. It is Fahrenheit (°F). Human body temperature normally stays between 35°C to 42°C. In Fahrenheit, this range is 95°F to 107.8°F. So 101°F is a fever temperature, but 101°C would be impossible for a living human body.
Q6. Can a clinical thermometer measure boiling water temperature? Ans. No. Boiling water is at 100°C, but a clinical thermometer only goes up to 42°C. So it cannot measure boiling water temperature — and it would get damaged.
Q7. What is absolute zero? Ans. Absolute zero is 0 Kelvin (0 K) — it is the lowest possible temperature that can exist anywhere in the universe.
🔴 MCQ Practice Questions
1. Which instrument measures temperature? a) Barometer b) Thermometer ✅ c) Hygrometer d) Ammeter
2. Normal body temperature is: a) 37 K b) 37°C ✅ c) 37°F d) 100°C
3. Range of laboratory thermometer: a) 35°C–42°C b) 0°C–100°C c) –10°C to 110°C ✅ d) 32°F–212°F
4. SI unit of temperature: a) Celsius b) Fahrenheit c) Kelvin ✅ d) Joule
5. Non-contact thermometer works using: a) Mercury b) Alcohol c) Infrared radiation ✅ d) Water
6. 0 K is called: a) Freezing point b) Melting point c) Absolute Zero ✅ d) Normal zero
7. Which thermometer is safest for home use? a) Mercury b) Laboratory c) Digital ✅ d) Alcohol
⭐ Points to Remember
- ✅ Sense of touch is not reliable for measuring temperature
- ✅ Normal body temperature = 37°C = 98.6°F
- ✅ Clinical thermometer range = 35°C to 42°C
- ✅ Lab thermometer range = –10°C to 110°C
- ✅ SI unit = Kelvin (K)
- ✅ Absolute Zero = 0 K (lowest temperature possible)
- ✅ Mercury is toxic — digital thermometers are safer
- ✅ Conversion formula: K = °C + 273.15
- ✅ Non-contact thermometer uses infrared — no touch needed
- ✅ Do NOT write °K — just write K