Thermal Energy and Systems
You might recall thermal energy is the energy due to the motion of particles that make up an object. This energy makes up the total energy of a system. A system is a collection of objects that interact in some way. For example, imagine you are roasting a marshmallow on a poker in a campfire on a crisp fall day. There are multiple systems interacting at the same time within an even larger system. These include the movement of thermal energy from the campfire into the air, heating you up on the cool day, or the movement of thermal energy from the campfire to the marshmallow. Does all thermal energy stay within a system, or does it move in and out of a system? Let's find out!

Thermal Energy Movement
Suppose you are measuring the temperature of a refrigerator. The refrigerator and the contents inside it are a system. Systems can be used to model the movement of energy. A closed system is a system that does not exchange matter or energy with the environment. This means thermal energy is only moving within the system and does not leave the system. Some situations can be thought of as closed systems, such as thermal energy transfers inside a refrigerator because the refrigerator is insulated from open air.

Recall that thermal energy is the total energy of a system. Thermal energy is not
the same as temperature, but can you track thermal energy using temperature?

In reality, there are no closed systems. The refrigerator is attached to an electrical
outlet. Every physical system transfers some energy to or from its environment.
Due to this there is a transfer of thermal energy into and out of the system.
However, a refrigerator with all its contents at the same temperature can
represent a closed system. An open system is a system that exchanges matter or
energy with the environment. An example of an open system can be the campfire
on a crisp fall day.

Thermal Energy Transfer
Have you ever gotten into a car on a hot summer day? You can guess that the inside of the car is hot even before you touch the door handle. You open the door and hot air seems to pour out of the car. When you touch the metal safety-belt buckle, it is hot. How is thermal energy transferred between objects? Thermal energy is transferred in three ways—by radiation, conduction, and convection.

Conduction
Suppose it's a hot day and you have a cold glass of lemonade. The lemonade has a lower temperature than the surrounding air. Therefore, the particles that make up the lemonade have less kinetic energy than the particles that make up the air. When particles with different kinetic energies collide, the particles with higher kinetic energy transfer energy to particles with lower kinetic energy.

The particles that make up the air collide with—and transfer kinetic energy to—the particles that make up the lemonade. As a result, the average kinetic energy, or
temperature, of the particles that make up the lemonade increases. Since kinetic energy is being transferred, thermal energy is being transferred. The transfer of
thermal energy by collisions between particles of matter is called conduction. Conduction continues until the thermal energy of all particles in contact is equal
and has reached thermal equilibrium

Recall the campfire situation. As you sit near the fire with your marshmallow poker roasting your marshmallow, thermal energy is transferred up the poker and to your hand. The thermal energy has left the campfire system and entered the poker system. Thermal energy moves via conduction within an object or from object to object. In this case, thermal energy moves from the campfire to the poker and then from the poker to your hand

Convection
When you heat a pot of water on the stove, the burner heats the pot by conduction. The pot heats the water by conduction. As the water heats up, i rises. As it rises, it begins to cool. Once it has cooled, it sinks back down to the bottom of the pot where it heats back up again. It cycles through the whole process over and over. This process is called convection. Convection is the transfer of thermal energy by the movement of particles from one part of a material to another. Convection only occurs in fluids, such as water, air, magma, and maple syrup. Convection cannot occur in solids because particles in solids cannot flow.

Think about a beaker sitting on a ring stand with a burner under it. Thermal energy is transferred into the system from the burner. Itis transferred from the burner to the beaker, and then from the beaker to the water. Thermal expansion occurs in water near the bottom of the beaker. Heating increases the water's volume, making it less dense. At the same time, water molecules at the water's surface transfer thermal energy to the air—thermal energy s transferred out of the system. This causes cooling and thermal contraction of the water on the surface. The denser water at the surface sinks to the bottom, forcing the less-dense water upward. This cycle continues until all the water in the beaker is at the same temperature.

An air fryer is a countertop electrical device that cooks food. The device cooks through convection. A heating element in the machine warms the air and a fan circulates air evenly around the food to cook it from all angles. An air fryer has a compact design that is energy efficient.

Radiation
Some energy transfers without touching an object through radiation. Radiation is the transfer of thermal energy from one material to another by electromagnetic waves. All matter, including the Sun, fire, you, and even ice, transfers thermal energy by radiation. Warm objects emit more radiation than cold objects do. For example, when you place your hands near a fire, you can more easily feel the transfer of thermal energy by radiation than when you place your hands near a block of ice.

A thermogram is an image created by a technology that measures the radiation
given off by objects. Objects giving off more radiation are shown in white, reds,
and yellows, while cooler objects are shown with blues, purples, and black. The
thermogram scale designates white as the warmest areas and black as the
coldest. Thermograms are used in the medical field for a variety of uses such as
cancer detection and allergy detection. Thermograms are also used in veterinary
Medicine.

Radiation transfers thermal energy through solids, liquids, and gases. All of these
states of matter have atoms that are in constant contact with each other.
However, thermal energy from the Sun can only travel to Earth by radiation. This
is because space is a vacuum. Itis an area that contains lttle or no mater. Since
there is little matter in space, thermal energy cannot transfer by conduction