In this chapter, you will learn
- —Understand what heat is and how it flows
- —Learn the difference between heat and temperature
- —Explore the three modes of heat transfer: conduction, convection, and radiation
- —Understand how thermometers work and the different temperature scales
What is Heat?
Heat is a form of energy that flows from a hotter object to a colder object. When two objects at different temperatures come in contact, heat transfers from the warmer object to the cooler one until both reach the same temperature.
This flow of heat energy is what makes a hot cup of tea cool down when left on a table, or why an ice cube melts when you hold it in your hand.
Heat is measured in joules (J) in the SI system. In everyday life, we also use calories as a unit of heat.
Exam Tip
Always remember: Heat flows from HOT to COLD, never the other way around. This is a key point in CBSE exams.
Heat vs Temperature
Heat and temperature are related but different concepts:
Heat is the total energy of all the molecules in an object. It depends on the mass of the object, the type of material, and its temperature.
Temperature is the measure of how hot or cold an object is. It tells us about the average energy of the molecules.
For example, a bucket of warm water has more heat than a cup of hot water, even though the cup has a higher temperature. This is because the bucket contains more water molecules.
Exam Tip
When asked to differentiate between heat and temperature, always mention: Heat is energy (measured in joules), Temperature is degree of hotness (measured in °C or K).
Common Mistake
Students often write that heat and temperature are the same thing. Remember: They are related but NOT the same!
Modes of Heat Transfer
Heat can transfer from one place to another in three ways:
1. Conduction: Heat transfers through a solid material without the material itself moving. When you heat one end of a metal rod, the other end also becomes hot. This happens because molecules pass energy to their neighbors.
2. Convection: Heat transfers through liquids and gases by the actual movement of the heated material. When water is heated in a pot, the hot water rises and cold water sinks, creating a circular flow called convection currents.
3. Radiation: Heat transfers through electromagnetic waves without needing any medium. This is how heat from the Sun reaches the Earth through the vacuum of space.
Exam Tip
Remember the media: Conduction in SOLIDS, Convection in FLUIDS (liquids and gases), Radiation needs NO MEDIUM.
Conductors and Insulators
Conductors are materials that allow heat to pass through them easily. Metals like iron, copper, and aluminium are good conductors of heat. This is why cooking utensils are made of metal.
Insulators are materials that do not allow heat to pass through them easily. Wood, plastic, rubber, and air are good insulators. This is why handles of cooking utensils are made of plastic or wood.
Woollen clothes keep us warm in winter because wool traps air, and air is a poor conductor of heat (good insulator).
Exam Tip
CBSE often asks why handles are made of insulators - always connect the answer to 'prevents heat from reaching our hands'.