1.0 Understanding Heat and Temperature
We often use the words "heat" and "temperature" interchangeably in daily conversation, but in Physics, they represent two very different concepts. Heat is a form of energy that flows, while Temperature is a measure of how "hot" or "cold" something is.
Heat vs. Temperature
- Heat: It is the total internal energy of the molecules in a substance. It always flows from a body at a higher temperature to a body at a lower temperature.
SI Unit: Joule (J) | Common Unit: Calorie (cal) - Temperature: It is the degree of hotness or coldness of a body. It determines the direction of flow of heat.
SI Unit: Kelvin (K) | Common Unit: Degree Celsius (°C)
1.1 Units of Heat and Their Relation
One calorie is defined as the amount of heat energy required to raise the temperature of 1 gram of water by 1°C.
Mechanical Equivalent of Heat
$$1\,cal \approx 4.186\,J$$
For numerical simplicity, we often use $1\,cal = 4.2\,J$.
1.2 Effects of Heat
When a substance absorbs heat energy, it can lead to several changes:
- Change in Temperature: The body becomes hotter.
- Change in Size: Most substances expand on heating (Thermal Expansion).
- Change in State: Ice melting into water or water boiling into steam.
- Chemical Change: Some substances may decompose or react when heated.
Always be careful with kcal (kilocalorie) vs cal (calorie).
$1\,kcal = 1000\,cal = 4200\,J$. Check the units provided in your numericals before starting the calculation.
Convert 500 calories of heat energy into Joules. (Take $1\,cal = 4.2\,J$)
Solution:
1. Given: Heat in calories = $500\,cal$
2. Conversion Factor: $1\,cal = 4.2\,J$
3. Calculation: $500 \times 4.2 = 2100\,J$
Final Answer: 500 calories is equal to $2100\,Joules$.
The Kelvin scale is called the absolute scale of temperature because 0 K (Absolute Zero) is the lowest possible temperature where all molecular motion theoretically stops!
2.0 Thermal Expansion
When most substances are heated, they increase in size. This phenomenon is known as Thermal Expansion. It happens because heat energy causes the molecules of the substance to vibrate more vigorously and move further apart.
2.1 Expansion in Solids
In solids, molecules are tightly packed, so the expansion is relatively small. It occurs in three ways:
- Linear Expansion: Increase in length.
- Superficial Expansion: Increase in area.
- Cubical Expansion: Increase in volume.
The Bimetallic Strip
A Bimetallic Strip consists of two different metal strips (like brass and iron) riveted together. Since different metals expand by different amounts for the same rise in temperature, the strip bends when heated. This property is used in thermostats, fire alarms, and electric irons.
2.2 Expansion in Liquids and Gases
Liquids expand more than solids, and gases expand the most for the same amount of heat. This is because the intermolecular forces are weakest in gases, allowing molecules to move apart easily.
Order of Expansion
Gases > Liquids > Solids
For the same rise in temperature, gases show the maximum thermal expansion.
Railway tracks are never laid as one continuous long rail. Small gaps are left between successive rails to allow for expansion during hot summers. Without these gaps, the tracks would buckle (bend), leading to train accidents.
Why does a thick glass tumbler often crack when boiling water is poured into it?
Solution:
1. Glass is a poor conductor of heat.
2. When hot water is poured, the inner surface of the glass expands suddenly.
3. However, the outer surface remains cool and does not expand at the same time.
4. This uneven expansion creates a thermal stress that cracks the glass.
The Eiffel Tower can be about 15 centimeters taller during the summer than in the winter because the iron structure expands due to the heat of the sun!
3.0 Measurement of Temperature
To measure temperature accurately, we use a device called a Thermometer. It works on the principle of liquid expansion, where a liquid (usually Mercury or Alcohol) expands uniformly when heated and contracts when cooled.
3.1 Why use Mercury?
Mercury is the preferred thermometric liquid because:
- It is a shining, opaque metal, making it easy to read.
- It does not stick to the glass walls of the capillary tube.
- It is a good conductor of heat and responds quickly.
- It has a wide temperature range (Freezes at -39°C, Boils at 357°C).
3.2 Clinical vs. Laboratory Thermometers
There are significant differences in how these two types of thermometers are constructed and used:
| Feature | Clinical Thermometer | Laboratory Thermometer |
|---|---|---|
| Range | 35°C to 42°C (94°F to 108°F) | -10°C to 110°C |
| Kink | Present (Constriction) | Absent |
| Usage | To measure body temperature | To measure temp in experiments |
The "Kink" Secret
The Kink prevents the mercury level from falling back into the bulb instantly. This allows the doctor to read the temperature accurately even after the thermometer is removed from the patient's mouth.
Never wash a clinical thermometer with boiling hot water. The expansion of mercury will exceed the 42°C limit and cause the thermometer to burst.
Why is a laboratory thermometer not used to measure human body temperature?
Solution:
1. Laboratory thermometers do not have a kink.
2. As soon as it is removed from the body, the mercury level starts falling rapidly due to contact with the cooler air.
3. This makes it impossible to get an accurate reading of the body temperature.
The Normal Human Body Temperature is 37°C or 98.6°F. If your temperature goes above this, it is commonly known as a fever!
4.0 Methods of Heat Transfer
Heat energy is always on the move, seeking out cooler areas. There are three distinct ways in which heat can travel from one place to another: Conduction, Convection, and Radiation.
4.1 Conduction
Conduction is the process of heat transfer in solids where energy is passed from molecule to molecule through vibrations, without the actual movement of the molecules from their positions.
- Conductors: Materials that allow heat to pass through easily (e.g., Copper, Iron, Aluminum).
- Insulators: Materials that do not allow heat to pass through (e.g., Wood, Plastic, Air).
4.2 Convection
Convection is the mode of heat transfer in fluids (liquids and gases). Here, the molecules themselves move, carrying heat energy with them. Hotter, less dense fluids rise, while cooler, denser fluids sink, creating a Convection Current.
4.3 Radiation
Radiation is the fastest method of heat transfer. It does not require any medium and can travel through a vacuum. Heat from the Sun reaches the Earth through radiation in the form of infrared waves.
Quick Comparison
Conduction: Needs a Solid medium.
Convection: Needs a Fluid (Liquid/Gas) medium.
Radiation: Works in a Vacuum (No medium).
These are natural convection currents. During the day, the land heats up faster than the sea. Hot air over land rises, and cool air from the sea blows in (Sea Breeze). At night, the process reverses (Land Breeze).
Why are the handles of cooking utensils made of wood or ebonite?
Solution:
1. Utensils are made of metals which are good conductors of heat to cook food quickly.
2. Wood and ebonite are insulators (poor conductors).
3. They do not allow heat to flow from the utensil to our hands, preventing burns.
A Thermos Flask (Vacuum Flask) minimizes heat transfer by all three methods! The vacuum prevents conduction and convection, while the silvered inner walls reflect heat back to prevent radiation.