How Heat Moves
When you hold a cup of hot cocoa, you can feel its warmth. If you stand in the sunlight, you feel warmer. This feeling of warmth is caused by the transfer of thermal energy, which we call heat. Thermal energy is the total kinetic energy of the moving particles in a substance.
Heat is always transferred from a hotter object to a colder object. This transfer can happen in three different ways: conduction, convection, and radiation.
1. Conduction
Conduction is the transfer of heat through direct contact.
How it works: The fast-moving, high-energy particles in the hotter object bump into the slower-moving particles in the colder object. In these collisions, energy is transferred, making the slower particles move faster (and thus, the colder object gets warmer).
Where it happens: Conduction happens best in solids, where particles are packed tightly together.
Example: If you leave a metal spoon in a pot of hot soup, the handle of the spoon will eventually get hot. Heat is conducted from the soup, up the spoon, to your hand.
Conductors vs. Insulators:
A thermal conductor is a material that allows heat to pass through it easily. Metals are excellent conductors. This is why cooking pots are made of metal.
A thermal insulator is a material that does not allow heat to pass through it easily. Wood, plastic, and air are good insulators. This is why pot handles are often made of plastic or wood.
2. Convection
Convection is the transfer of heat through the movement of fluids (liquids or gases).
How it works: When a part of a fluid is heated, it expands, becomes less dense, and rises. The cooler, denser fluid around it sinks to take its place. This cooler fluid is then heated, and the process repeats. This circular movement is called a convection current.
Where it happens: Convection only happens in liquids and gases.
Example: Boiling water in a pot. The burner heats the water at the bottom, which rises. The cooler water at the top sinks, gets heated, and rises. This is also how a heater warms a room—it warms the air, which then circulates.
3. Radiation
Radiation is the transfer of heat through electromagnetic waves, such as infrared radiation.
How it works: Unlike conduction and convection, radiation does not require any matter (or medium) to travel through. It can travel through the vacuum of space. All objects give off some thermal radiation. The hotter an object is, the more radiation it emits.
Where it happens: It can happen anywhere, even in empty space.
Example: The heat you feel from the Sun traveled 93 million miles through the vacuum of space to reach you. The warmth you feel sitting next to a campfire is also mostly due to radiation.