11.2 – Heat and Temperature

Heat is a form of energy and like other forms of energy it can be measured in joules (J). Heat and temperature are not quite the same thing. The temperature of a substance is a measure of how hot or cold it is. It is usually measured in degrees Celsius (°C) using a thermometer.

In many cases, substances with more heat energy have a higher temperature, but this may not always be the case. For example, on a cold day, a gas heater may provide a small bedroom with 1000 J of energy and increase the rooms temperature to 26°C. The same gas heater may provide a large living room with 1000 J of energy but increase the rooms temperature to only 23°C. Both areas have the same amount of heat energy, but the bedroom has a higher temperature because the energy is heating a smaller volume of air.

Why temperature changes

Heat energy flows from a hotter object to a cold one. When an object receives heat energy, its particles move faster, and its temperature rises. When an object loses heat energy to another object, its particles move more slowly. And its temperature drops.

Heat continues to flow from hotter objects to colder objects until their temperatures are equal. The movement of heat then stops. Heat never flows from colder to hotter objects.

For example, if a cup of hot chocolate is left sitting on a bench, it cools down. The fast-moving particles in the hot chocolate loses some of their energy to the air near the cup. The hot chocolate keeps cooling. Until it reaches room temperature. If children juice is left sitting on a bench, it warms up. The particles in the juice gained some energy from the warmer air around the glass. If left out of the fridge, the juice warms up until it reaches room temperature.

Thermometers

When an object absorbs heat, its particles move faster. The faster the particles move, the more space they take up. As the particles take up more and more space, the object expands.

A thermometer works because the substance inside it takes up more space when it is heated. The substance used in most modern thermometers is alcohol dyed red to make it easy to see.

Most glass thermometers measure temperatures to +/-1°C, but digital thermometers are more accurate and can often measure temperature to plus +/-0.1°C. Digital thermometers are commonly used by doctors to check whether a patient has a high temperature. You might use a digital thermometer in your experiment if you need to measure small temperature changes with great accuracy.

How heat is transferred

Heat moves from a region of high temperature to a region of low temperature through one or more of the following processes, conduction, convection, and radiation. Heat transfer by conduction occurs mainly in solids. Well convection occurs generally in liquids and gases. Radiation can occur through any space, even in a vacuum.

Heat transfer by conduction

The metals used to make pots and pans are very good conductors of heat. This helps to ensure that the heat from the flame or hot plate is spread evenly.

Heat travels by conduction. When fast moving particles collide with other particles nearby, making them move faster. Heat can travel by conduction. Through objects or from one object to another, such as from a cook top to a saucepan.

Heat travels by conduction at different speeds depending on the type of material and its state of matter. Heat travels more quickly in solids than in liquids or gases because the conduction occurs more quickly when the particles in an object are closer together. Gases are the poorest conductors because the particles that they are made of a far apart solids are usually very good conductors of heat. Because the particles in them are packed closely together, although not all solids conduct heat well. Metals are generally good conductors of heat and electricity, while non-metals such as glass, plastic and wood do not conduct as well. Metals that conduct heat and electricity poorly are called insulators.

Heat transfer by convection

Have you ever noticed that in summer the air in a two-storey house is warmer upstairs than downstairs? You may have heard the saying that hot air rises. In the previous section you read that heat travels by conduction faster in solids because the particles are more closely packed together. The transfer of heat by conduction in liquids and gases is not very efficient. Instead, heat travels through liquids and gases by convection.

Convection heaters work in this mode of heat transfer. The heater Blows out warm air, which then rises. As the warm air rises, it transfers some of its energy to the surrounding, causing the air to cool and so begin to fall. This flow of warm air up and cool air down creates A circular current quarter convection current. The same pattern can be seen in liquids.

Coastal sea breezes

Sea breezes are often created by convection currents along the coastline. As land along the coast warms up during the day, warm air rises. This warm air cools as it rises above the sea. Cool air then moves to two. Cool air then moves in to replace the warm air over the land, causing a circular convection current. At night, the sea temperature is higher than the temperature on land, so convection currents move in the opposite direction.

Heat transfer by radiation

The sun provides energy to the Earth. Without heat from the sun, the Earth would be far too cold for humans to live on. Heat from the sun must travel through space to reach the Earth. The heat does not travel by conduction or convection because there are two few particles in space to transfer the energy between the sun and the Earth. Instead, heat from the sun reaches the Earth by radiation.

Heat that travels by radiation is called radiant heat. Radiant heat travels very quickly because it does not rely on the movement of particles to move energy from one place to another. The heat from the sun only takes about 8 minutes to reach Earth.

Detecting heat using infrared scanners

All objects, including the human body, radiates some heat. The human body usually radiates more heat than the environment around it. Infrared scanners detect the radiant hate coming from the human body. That's why infrared scanners a useful for finding people lost at sea, in bushland or even buried under a collapsed building.

Insulation

Slowing down the flow of heat is the key to keeping drinks cool in the summer and warm in the winter. On a hot day, heat flows. From the hot environment to a cold drink until they have both at the same temperature. On a cold day, heat flows from a warm drink to the cooler environment until their temperatures are equal.

Heat does not travel into or out of thermos flasks very easily. For this reason, thermos flask can be used to keep cold drinks cold and warm drinks warm. A thermos flask has a number of features that slow heat transfer by conduction, convection, and radiation.

Insulating your body

The temperature of the human body is about 37°C. When the air temperature is much less than this, heat moves from your body to the environment. In very hot weather, heat moves from the environment to your body.

Unless your body is touching a very hot or very cold object, you won't gain or lose heat by conduction very easily. Most of the movement of hate neo buddy happens by convection and radiation.

Clouds keep you warm in winter by stopping your body heat from escaping.

Fabrics made from natural fibres, like wolves, are good insulators of heat. This is because natural fibres contain only very small pockets of air. This prevents convection currents forming and carrying heat away.

Air can flow easily through thin material, keeping you cool in summer. Loose fitting clothes allow more convection currents to form. The convection currents help heat to escape from your body.