H EAT E NERGY Made by Ms. Priya

WHAT IS HEAT? Heat is an important kind of energy. Heat from the Sun makes life on Earth possible. Heat makes your home warm. Heat cooks your food. You use more heat than any other kind of energy. 2

W HAT I S H EAT? Things that make heat are called heat sources. The Sun is a heat source. Fire is a heat source. Melted rock and metal inside Earth are sources of heat. Heat sources make other things hot. 3

WHY DO THINGS GET HOT? Things get hot because tiny bits of matter called atoms and molecules move around. 4

WHY DO THINGS GET HOT? Atoms and molecules are always moving around. They move around slowly in something that is cold. They move around quickly in something that is hot. Heat energy comes from moving atoms and molecules. 5

HOW DOES HEAT MOVE? Heat energy can move in three ways. These three ways are called Conduction Convection Radiation 6

C ONDUCTION Conduction is the transfer of heat through particles/molecules in solid. It carries heat through solid. The frying pan heats the egg and bacon through conduction. 7

C ONVECTION Convection heats liquids and gases. Convection happens only in liquids and gases. A radiator heats air in a room by convection. Air near the radiator gets hot. Hot air rises. Cooler air replaces the heated air. This air gets hot and also rises. The air keeps going around and around from cooler to hotter. The way the air moves is called a convection current. 8

R ADIATION 9

Radiation can send heat across empty space. This is how heat from the Sun travels to Earth. Energy that travels by radiation is called electromagnetic radiation. Radiation moves heat energy in the form of waves. 10

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E XPANSION AND C ONTRACTION They move around slowly in something that is cold. They move around quickly in something that is hot. 14

E XPANSION AND C ONTRACTION Substances expand or get bigger when they are heated up. They contract or get smaller when they are cooled down. Heat expansion or contraction can happen to solids, liquids, and gases. Expansion or contraction due to changes in temperature is not permanent (objects expand when heated then contract when cooled). The number of atoms and the mass of the atoms do not change with changes in temperature. 15

W HAT DO THE PARTICLES DO ? When substances expand or contract, their particles stay the same size. It is the space between the particles that changes: The particles in a solid vibrate more when it is heated, and take up more space. The particles in a liquid move around each other more when it is heated, and take up more space. The particles in a gas move more quickly in all directions when it is heated, and take up more space. 16

E XPANSION As the temperature of a substance increases, the average distance between the atoms/molecules/particles of the substance increases, causing the substance to expand. For example: Thermometers work because the liquid inside them expands and rises up the tube when it gets hotter. For example : bridges expand in the summer heat and need special joints to stop them bending out of shape. 17

18 (a)The expansion joints narrow when bridge segments expand in hot weather. (b) Expansion joints in a bridge separate when the side-by- side segments of a bridge contract in cold weather. Expansion joints allow steel and concrete to expand without cracking.

CONTRACTION As the temperature of a substance decreases, the average distance between the atoms/molecules decreases, causing the substance to contract. 19

EXPANSION AND CONTRACTION OF SOLIDS People who design structures such as buildings and bridges are very careful about the materials they use. They choose materials that will not break when temperatures change. A few examples of solids that expand and contract are metal frames around windows hot-water pipes in homes railroads car engines metal lids on jar 20

EXPANSION AND CONTRACTION OF GASES Gases that are heated will expand in their containers. The energy of the gas particles will increase. The particles of warm gases hit the walls of the container more often. They also hit the walls harder. If the walls of the container are flexible, the walls expand. If the walls are not flexible and cannot expand, then the pressure inside the container increases. Examples of gases that expand and contract are air in tires air in a soccer ball air in balloons 21

IF MATERIALS DID NOT EXPAND AND CONTRACT THEN ……… When cars and bicycles travel, their tires rub on the road. This makes the tires get hot. The air inside the tires will expand. If the tires are too full, they could burst. Sports balls such as soccer balls are also filled with air. If you leave a soccer ball out in the cold, the air inside the ball will contract. The ball will get soft and it will become unusable 22