In this topic you will learn how heat travels and affects matter.

Matter is made up of particles that are always moving and have energy. Heat is the movement of energy from warmer to cooler objects. Heat always flows from the object with more energy to the object with less energy. An insulator is a material that does not transfer heat very well. The opposite of an insulator is a conductor. A conductor is a material that transfers heat well.

The more energy an object has, the faster its particles move. The particles of a warm object cause the particles of a cool object touching it to move faster. Eventually the particles making up both objects move at the same speed. Heat can be transferred by the flow of a liquid or a gas, called convection. When gas or liquid particles are heated, they spread out. The heated gas or liquid becomes less dense and rises. Cooler, denser air or liquid is pulled down by gravity. In conduction, the transfer of energy is caused by one particle bumping into another. Fast-moving particles bump into other particles and transfer some of their energy.

Heat from the Sun travels through space in light waves. When these waves reach Earth, some of their energy is absorbed, which causes particles to move faster. You feel the energy as heat. Radiation is the transfer of heat through space. Most types of matter expand, or get larger, when they are heated.

Adding energy makes the particles that make up matter move faster. Faster-moving particles bounce off one another with more energy and they take up more space. The opposite happens when energy is taken away. When cooled, most matter contracts or shrinks.

Heat is not the same as temperature. Temperature is a measure of how hot or cold something is. Temperature is measured with an instrument called a thermometer. All animals need heat to stay warm and alive. Animals that depend on heat from their environment are called cold-blooded animals. Animals that keep nearly the same temperature all the time are called warm-blooded. They get their heat energy from the food they eat.

Understanding that heat is a form of energy and understanding how it is transferred has allowed us to find ways to keep warm or cool.