Hot OR Cold :-
In scientific terms, hot and cold describe the presence or absence of thermal energy and the movement of particles:
Hot: Refers to high thermal energy. When a substance is hot, its atoms and molecules are moving or vibrating rapidly with high kinetic energy.
Cold: Science views cold not as a distinct form of energy, but simply as the absence or lack of heat. When a substance is cold, its molecules are moving slowly and possess low kinetic energy.
Key Rule of Thermodynamics: Heat energy always flows naturally from a hotter object to a colder one until both reach the same temperature (thermal equilibrium).
Temperature and Thermometers
1. What is Temperature?
Temperature is the measure of how hot or cold an object or substance is.
A cup of hot tea has a higher temperature.
Ice has a lower temperature.
Temperature tells us the degree of hotness or coldness.
2. What is a Thermometer?
A thermometer is an instrument used to measure temperature.
The temperature is usually measured in:
Degree Celsius (°C)
Degree Fahrenheit (°F)
Kelvin (K)
3. Common Types of Thermometers
Clinical thermometer
Used to measure human body temperature.
Normal human body temperature is approximately 37°C.
It generally has a range of about 35°C to 42°C.
Laboratory thermometer
Used to measure the temperature of liquids and other substances in laboratories.
It generally has a wider range than a clinical thermometer.
Digital thermometer:-
Shows temperature on a digital display.
It is easy to read and does not use mercury.
Infrared thermometer
Measures temperature without direct contact.
It detects infrared radiation emitted by an object.
Celsius Scale:-
The Celsius scale is a temperature scale in which:
0°C → Freezing point of water
100°C → Boiling point of water
The interval between 0°C and 100°C is divided into 100 equal parts.
Each division represents 1°C (one degree Celsius).
It is represented by the symbol °C.
Examples:
Ice melts at 0°C.
Normal room temperature is about 20–25°C.
Water boils at 100°C (at normal atmospheric pressure).
Normal Body Temperature and Its Variations
1. Standard Normal Body Temperature
The universally accepted average normal body temperature is:
- 98.6°F (37°C)
However, normal body temperature is a range rather than a single fixed number. For a healthy adult, normal body temperature typically ranges between:
- 97°F to 99°F (36.1°C to 37.2°C)
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2. Variations by Measurement Method (Measurement Sites)
Body temperature varies depending on where on the body it is measured:
- Oral (Mouth): 97.6°F – 99.6°F (36.4°C – 37.6°C) — Standard benchmark reference.
- Rectal / Tympanic (Ear): 98.6°F – 100.4°F (37.0°C – 38.0°C) — Reads about 0.5°F to 1°F higher than oral (closest to core body temp).
- Axillary (Armpit): 96.6°F – 98.0°F (35.9°C – 36.7°C) — Reads about 0.5°F to 1°F lower than oral.
- Temporal (Forehead / Skin Scanner): 97.0°F – 98.8°F (36.1°C – 37.1°C) — Fast, but sensitive to ambient air temperature.
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3. Key Factors Causing Normal Temperature Variations
- Time of Day (Circadian Rhythm):
* Lowest in the early morning (around 4:00 AM – 6:00 AM) due to decreased metabolic rate during sleep.
* Highest in the late afternoon and evening (around 4:00 PM – 8:00 PM), rising by up to 1°F to 1.5°F.
- Age:
* Infants and Young Children: Have higher average body temperatures (often up to 99.5°F / 37.5°C) due to higher surface-area-to-mass ratio and active metabolism.
* Older Adults: Tend to have lower baseline body temperatures (often 96.8°F – 97.7°F / 36.0°C – 36.5°C) and reduced ability to regulate heat.
- Physical Activity & Exercise:
* Strenuous exercise increases muscle activity, which can temporarily raise body temperature up to 101°F – 103°F (38.3°C – 39.4°C) safely.
- Hormonal Factors (Gender & Menstrual Cycle):
* Women's body temperature rises by about 0.5°F – 1.0°F (0.3°C – 0.6°C) after ovulation due to increased progesterone levels.
- Environmental Factors & Clothing:
* Hot weather, high humidity, or heavy bundling can temporarily raise body temperature; extreme cold can lower it.
- Food, Drink, and Smoking:
* Eating a meal increases metabolic rate (diet-induced thermogenesis).
* Consuming hot or cold drinks, or smoking immediately before an oral test, can temporarily alter oral temperature readings.
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4. Clinical Temperature Ranges (Reference Benchmarks)
- Hypothermia: Core body temperature below 95.0°F (35.0°C)
- Normal Range: 97.0°F – 99.0°F (36.1°C – 37.2°C)
- Low-grade Fever: 99.5°F – 100.3°F (37.5°C – 37.9°C)
- High / Clinical Fever (Pyrexia): 100.4°F (38.0°C) or higher
- High Fever: 103.0°F (39.4°C) or higher
- Hyperpyrexia (Medical Emergency): Above 105.8°F (41.0°C)
Fahrenheit Scale and Kelvin Scale (SI Unit of Temperature)
Fahrenheit Scale:
The Fahrenheit scale is a temperature scale used to measure how hot or cold an object is. It is represented by the symbol °F.
The freezing point of water is 32°F.
The boiling point of water is 212°F at normal atmospheric pressure.
The interval between these two points is divided into 180 equal divisions.
The Fahrenheit scale is commonly used in the United States.
Kelvin Scale:
The Kelvin scale is the standard temperature scale used in science. Its unit is called the kelvin, represented by the symbol K.
The SI unit of temperature is kelvin (K).
The freezing point of water is approximately 273 K.
The boiling point of water is approximately 373 K at normal atmospheric pressure.
The lowest possible temperature is called absolute zero, which is 0 K or −273.15°C.
The kelvin symbol is written without a degree sign. For example, 300 K, not 300°K.
Conversion between Celsius and Kelvin:
Kelvin temperature = Celsius temperature + 273.15
For Class 6, we usually use the approximate formula:
K = °C + 273
Example: Convert 25°C into kelvin.
K = 25 + 273 = 298 K
Conversion between Celsius and Fahrenheit:
°F = (°C × 9/5) + 32
Example: Convert 0°C into Fahrenheit.
°F = (0 × 9/5) + 32 = 32°F
Laboratory Thermometer — Range and Smallest Value
A laboratory thermometer is an instrument used to measure the temperature of substances during scientific experiments in a laboratory.
Main Features:
It usually consists of a glass tube containing a liquid such as coloured alcohol or mercury.
It has a bulb at one end that contains the temperature-sensitive liquid.
The liquid expands when heated and contracts when cooled.
The level of the liquid moves along a graduated scale to indicate the temperature.
Range of a Laboratory Thermometer:
A commonly used school laboratory thermometer has a range of −10°C to 110°C.
This means it can measure temperatures from −10°C up to 110°C.
The range may vary depending on the thermometer.
Range of a Laboratory Thermometer:
A commonly used school laboratory thermometer has a range of −10°C to 110°C.
This means it can measure temperatures from −10°C up to 110°C.
The range may vary depending on the thermometer.
Smallest Value (Least Count):
The smallest value or least count of a thermometer is the smallest temperature difference that it can measure.
The least count of a typical school laboratory thermometer is 1°C.
For example, if the liquid level indicates 35°C, the thermometer measures the temperature as 35°C.
Correct Way of Measuring Temperature
1. Use a suitable thermometer to measure temperature.
2. Hold the thermometer by its stem, not by the bulb.
3. Ensure that the bulb of the thermometer is properly placed in contact with the substance whose temperature is to be measured.
4. While measuring the temperature of a liquid, the bulb should be fully immersed in the liquid but must not touch the bottom or sides of the container.
5. Wait until the thermometer reading becomes steady.
6. Read the thermometer at eye level to avoid errors.
7. Record the temperature along with its unit, usually degrees Celsius (°C).
Important: A laboratory thermometer should not be used to measure human body temperature. A suitable clinical or digital thermometer should be used instead.
Temperature of Boiling Water and Ice
Temperature of boiling water:
- Pure water boils at 100°C at normal atmospheric pressure.
- During boiling, water changes from a liquid into water vapour.
- The temperature remains constant at the boiling point while the water is boiling under constant pressure.
Temperature of ice:
- Pure ice melts at 0°C at normal atmospheric pressure.
- At 0°C, ice changes into liquid water.
- During melting, the temperature remains constant until all the ice has melted, under normal conditions.
Important Points:
- The melting point of ice is 0°C.
- The boiling point of water is 100°C.
- These values apply at normal atmospheric pressure.
Air Temperature:-
Air temperature is the measure of how hot or cold the surrounding air is.
- Air temperature is commonly measured using a thermometer.
- It is usually expressed in degrees Celsius (°C).
- Weather stations measure air temperature using thermometers placed in a shaded, well-ventilated shelter.
- A thermometer should not be placed in direct sunlight when measuring the temperature of the surrounding air, as direct sunlight can heat the instrument and give an inaccurate reading.
- Air temperature changes throughout the day and from season to season.
- It is generally measured away from direct heat sources and hot surfaces to obtain a representative reading.
Importance of measuring air temperature:
1. It helps in predicting weather conditions.
2. It helps us understand daily and seasonal changes in weather.
3. It is useful in agriculture, transport and planning outdoor activities.
Know a Scientist — Anna Mani
Anna Mani (1918–2001) was a famous Indian physicist and meteorologist, known as the “Weather Woman of India.”
* She worked at the India Meteorological Department (IMD).
* She developed instruments to measure weather conditions such as wind speed, rainfall and solar radiation.
* She contributed to research on solar and wind energy.
* Her work helped improve weather observations in India.
Remember: Anna Mani made important contributions to meteorology and renewable energy in India.
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