Ch. 7

Temperature & its Measurement

Highly Dense Study Notes / Science Study Sheet

Wrong measurements are worse than no measurements at all.

— Anna Mani (Indian Meteorologist)

7.1 Hot or Cold?

We cannot always rely on our sense of touch to judge temperature.

Activity 7.1 experiment:
If you dip one hand in warm water and the other in cold water, then place both in tap water:
  • Warm-conditioned hand feels Cold
  • Cold-conditioned hand feels Warm
Conclusion: Our touch is relative, not absolute!

7.2 Defining Temperature

Temperature: A reliable measure of the hotness or coldness of a body.

Thermometer: A physical device used to measure temperature.

SI Unit of Temperature:

kelvin (K)

Name starts lowercase (kelvin) but the symbol is a capital K. No degree sign (°) is used with Kelvin!

Three Common Scales

  • Celsius Scale °C
  • Fahrenheit Scale °F
  • Kelvin Scale K

Grammatical Rules for Scales:

  • Scale Names capitalized: Celsius, Fahrenheit, Kelvin.
  • Unit Names lowercase: degree celsius, degree fahrenheit, kelvin.
  • Always leave a space between number and unit (e.g., 37 °C, NOT 37°C).

7.3.1 Clinical Thermometer

Designed only for human body temperature.

NORMAL BODY TEMP 37.0 °C or 98.6 °F
MEASUREMENT RANGE 35 °C → 42 °C

Digital Thermometers: Built with heat sensors; do not use mercury. Runs on batteries. Safer and easier to read.

Non-contact (Infrared) Thermometers: Measure temp from a distance using infrared radiation. Crucial during COVID-19 pandemic to curb transmission.

Mercury Alert: Traditional glass thermometers contain mercury (extremely toxic, hazardous when broken, very hard to dispose).

Scale Conversions

Converting Celsius to Kelvin Scale:

Standard Equation K = °C + 273.15 Example: 0 °C = 273.15 K
Think About It:

Can we use a clinical thermometer for boiling water or ice?
No! Boiling water (~100 °C) and melting ice (~0 °C) fall completely outside the clinical range (35 °C to 42 °C).

7.3.2 Laboratory Thermometer

Used for general laboratory experiments.

TYPICAL TEMPERATURE RANGE -10 °C → 110 °C

Filled with alcohol (colored red) or mercury.

CRITICAL WARNING

Unlike clinical thermometers, the liquid level falls instantly when removed from the test substance. Therefore:

Always read laboratory thermometer readings while the bulb is still fully immersed in the liquid!

Usage Precautions

CLINICAL:
  • Wash tip with soap/water before/after use.
  • Ensure digital display is reset.
  • Place deep under tongue, close mouth.
  • Wait for beep/flash.
LABORATORY:
  • Keep thermometer held completely vertical.
  • Bulb must NOT touch container bottom or sides.
  • Read with eye level aligned with liquid top.

Curious Science Facts

Altitude effect on Boiling:

Water boils at lower temperatures at high altitudes. In Shillong, water boils at ~98 °C instead of the standard 100 °C due to lower atmospheric pressure!

Anna Mani (1918 - 2001):

Known as the "Weather Woman of India". She designed & manufactured multiple advanced weather monitoring instruments, and championed wind and solar energy studies in India.

SI Unit: kelvin (K)
Body Normal: 37 °C / 98.6 °F
Lab Range: -10 °C to 110 °C
Melting/Boiling Temp: Constant!