What is Heat and Temperature - It's Introduction, Their Types, Atomic Level Explanation, Examples

 

Heat:

Heat is the transfer of kinetic energy from one medium or object to another, or from an energy source to a medium or object. Such energy transfer can occur in three ways: radiation, conduction, and convection. The standard unit of heat is the calorie (cal), which is the amount of energy transfer required to raise the temperature of one gram of pure liquid water by one degree Celsius, provided the water temperature is higher than the freezing point and lower than the boiling point. Note that, heat transfer is transfer of energy, without doing work, and without passing material to the system. You can call this energy what you like. You can call it heat energy or thermal energy. Inside the system it takes the form of kinetic energy of the particles, and field energy of the fields. Once it has arrived in any given system, it is best to call it internal energy.

What happens at atomic level:

Heat is the process of energy transfer from a high-temperature system to a low-temperature system, caused by a temperature difference. This transfer happens primarily through particle collisions. What happens to atoms and their energy during heat. Atoms are always in motion. At the atomic level, atoms and molecules are never completely still. They are constantly vibrating, rotating, and in gases or liquids, they moves around. This motion is a form of kinetic energy.

Imagine region of fast-moving atoms with high kinetic energy, high temperature is in contact with a region of slow-moving atoms with low kinetic energy, low temperature. A fast moving atom from the hot side collides with a slow moving atom from the cold side. During this collision, kinetic energy is transferred from the fast moving atom to the slow moving one. The atom that was once fast moving loses kinetic energy and slows down. The atom that was once slow moving gains kinetic energy and speeds up. This microscopic event, repeated trillions of times per second across the contact area, is the flow of heat.

What happens to the atoms? The atoms in the hot object gradually slow down and lose kinetic energy, their temperature decreases. The atoms in the cold object gradually speed up and gain kinetic energy, their temperature increases. This continues until all atoms have the same average kinetic energy. This is thermal equilibrium, meaning no more net heat flow.

Real life analogy:

Imagine a playground full of children. Temperature is like how fast the average child is running around. A high temperature object like boiling water is a playground where all the kids are sprinting, jumping, and crashing into each other. A low temperature object like ice is a playground where the kids are mostly standing still or shuffling slowly. Heat is what happens when the fast kids bump into the slow kids. When you put an ice cube means slow kids in your hand means fast kids, the fast kids in your hand constantly bump into the slow kids in the ice. This transfers energy. The fast kids slow down a little i.e. your hand feels colder, and the slow kids start moving faster i.e. the ice melts. This flow of energy from the fast movers to the slow movers is heat.
Replace kids with atoms and molecules. Everything is made of them, and they are always jiggling and vibrating. More vibration means higher temperature and less vibration means lower temperature. Heat is the energy that gets transferred from the fast-vibrating molecules to the slow-vibrating molecules when they interact.

You don't "have" heat. You have thermal energy. You only feel or measure heat when that energy is moving. A hot cup of coffee has a lot of thermal energy. The heat is what flows from the cup into your cold hands. Heat always flows from hot to cold, never the other way around on its own. It's nature's way of trying to even things out. It wants everything to be the same temperature, a state called thermal equilibrium.

Heat is the total amount of thermal energy being transferred. It's an extensive property, a huge pot of boiling water has far more heat i.e. thermal energy to give away than a tiny cup of boiling water, even though they are at the same temperature.


Heat moves in three ways: Radiation, conduction, and convection.

Radiation: 

It happens when heat moves as energy waves, called infrared waves, directly from its source to something else. This is how the heat from the Sun gets to Earth.  In fact, all hot things radiate heat to cooler things. When the heat waves hits the cooler thing, they make the molecules of the cooler object speed up.  When the molecules of that object speed up, the object becomes hotter. The electromagnetic spectrum includes all kinds of energy that can travel in waves, including light, heat, x-rays, radio waves, ultraviolet waves and microwaves. All of these kinds of waves contain a lot of energy. Also, they can all travel through deep space. That's why we can see sunlight from stars billions of light years away.
The light from them radiates to us.


Conduction: 

It is another way that heat moves. Heat is a form of energy, and when it comes into contact with matter like anything that you can touch physically, it makes the atoms and molecules move. Once atoms or molecules are moving, they collide with other atoms or molecules, making them move too. These, then bump into other molecules and make them move, too. In this way, the heat is transferred through matter. Conduction is what makes the handle of a pot hot when only the bottom of the pot is touching the stove. The heat from the burner starts the molecules in contact with the burner start to move. Those molecules bump against others in the pot, which bump others, until all the molecules in the pot, including in the handle, are moving. When someone touches the pot handle, they feel the heat. The heat has moved from the burner to the cook's hand through conduction. Conduction is an important way that heat travels in space, but only within a spacecraft. Since there is very little matter in deep space, heat cannot leave a spacecraft by conduction.

Convection: 

It happens when a substance that can flow, like water or air is heated in the presence of gravity. When air or water is in the presence of gravity, the gravity pulls all of it down. The bottom of the air or water becomes denser because it is pulled down and also pushed down by the weight of the molecules on top of it. When there is heat at the bottom of this air or water, the air or water molecules in contact with the heat start to move, and the molecules spread apart. The heated air or water becomes less dense. It rises up until it gets to air or water with the same density as it has and when it gets there, it pushes the air or water that was there out of the way. At the same time, new air or water fills the space that was vacated when the heated molecules rose up. The air or water that gets pushed out of the way falls down. This sets up a circular motion. Air or water is heated at the bottom, travels to the top, cools, gets denser, falls, is heated again and the whole cycle starts again. Convection does not occur in space because there is no gravity. Ovens work by convection. The heating coils at the bottom of the oven heat the air which climbs to the top, cools slightly, and falls down again. 

Temperature:

Temperature is a measurement of how hot or cold something is, how fast the molecules are moving on average. It's an intensive property, a single spark from a fire is at the same high temperature as the fire itself. Temperature is a measure of how fast the atoms and molecules in something are vibrating or moving. It's not the amount of energy, but the intensity of it. Temperature is directly proportional to the average kinetic energy of a system's particles. Temperature is an average property. A single atom cannot have a temperature; temperature is a statistical measure of the energy of a vast collection of particles.

What happens at atomic level:

What happens to atoms and their energy during temperature. Atoms are always in motion. At the atomic level, atoms and molecules are never completely still. They are constantly vibrating, rotating, and in gases or liquids, they moves around. This motion is a form of kinetic energy.

In low temperature or in cold objects, the atoms and molecules vibrate and move relatively slowly. Each particle has a low amount of kinetic energy. The average of all these low energies is also low temperature. While in a hot object or in hot temperature, the atoms and molecules vibrate and move extremely rapidly and violently. Each particle has a high amount of kinetic energy. The average of all these high energies is also high temperature. As the amplitude or size and frequency or speed of an atom's vibration in a solid increases, temperature increases. In a liquid or gas, the atoms/molecules simply move through space at higher speeds.

Real life analogy:

Imagine our playground from the heat explanation. Heat was the transfer of energy when a fast-running kid i.e from the hot object, bumped into a slow-walking kid i.e. from the cold object. Temperature is the average speed of the kids on a single playground. A high temperature means, on average, every kid is running around frantically. A low temperature means, on average, every kid is just standing still or shuffling slowly.

A spark from a fire has a very high temperature, its molecules are vibrating extremely fast. But it contains a very small amount of total heat energy because it's so tiny. It can't warm up a room. A bathtub full of warm water has a much lower temperature than the spark, its molecules are vibrating slower. But it contains a massive amount of total heat energy. It can warm up the entire bathroom. Temperature is the intensity. Heat is the total amount of that energetic motion.

An iceberg is at a very low temperature (0°C). Its molecules are moving very slowly. However, because it is so incredibly massive, it holds a gigantic amount of thermal energy. This is why it takes so long to melt an iceberg. you have to transfer a huge amount of heat into it to speed up all of its molecules. Temperature tells you the direction that heat will flow. Your hand is at 37°C. A cup of coffee you holding might be at 80°C. Because the coffee has a higher temperature, its molecules are, on average, vibrating much faster than the molecules in your hand. Therefore, you know that heat will flow from the coffee (high temperature) into your hand (lower temperature). The coffee will transfer its energetic motion to your hand until both are at the same temperature.