Heat
Introduction
- In science, a THEORY is an idea that is thought to explain a phenomenon that’s been observed. It can be altered, accepted or rejected.
- In science, a LAW can be proven and will always occur.
- Heat is a form of energy.
Caloric Theory
- The Caloric theory was developed in the 18th century and it stated that heat was an invisible fluid that travelled from place to place. This theory was disproved by Count Rumford after he shot a cannon and discovered that the heat was caused by friction.
Kinetic Theory of Matter
- All matter is made up of molecules
- The spaces in between the molecules are bigger than the molecules themselves
- They are held together by electric forces
- The particles are in constant motion
- The particles transfer energy in collisions
- When heat is added, molecules move faster
Changes in State
There are three states of which matter can take the form of:
Solid - Molecules are close together, but are still in constant motion as they vibrate.
Liquid - When heat is added to a solid, the molecules vibrate faster and faster until they break the bonds with each other. They become a liquid and move about freely, but are still in contact with each other.
Gas - When heat is added to a liquid the molecules gain more energy, move faster and spread even further apart. The particles seldom come in contact with each other.
Temperature and Heat Transfer
- The thermometer works by air colliding with the thermometer. If the air is warm, the alcohol in the thermometer will expand and rise.
- The most common scale to measure temperature is the Celsius scale.
- In science we use the Kelvin scale to measure temperature.
- 1ºK is equal to 1ºC
- 0ºK is the coldest temperature possible in the universe, equal to -273ºC.
The three ways to transfer heat are:
Conduction - This occurs through collisions and vibration of molecules in solids.
Ex. A hot element on a stove warms up a frying pan.
Convection - Heat is transferred by actual movement of the warmed matter in fluids (gases, liquids).
Ex. Air heated by a space heater rises and the cold air is pushed to the bottom which is then heated as well.
Radiation - No medium is required to transfer heat this way, as the energy travels in electromagnetic waves. Black and dull objects absorb radiation, while non-dark and shiny objects reflect it.
Ex. The sun’s rays bring heat to the earth.
Thermal Expansion
- When heat is added to something it will expand.
- If heat leaves something, it will contract.
- Water is an exception, as the volume of water increases as it cools. Water reaches its maximum density at 4ºC.
- When matter expands it does so in all three dimensions (length, width and volume).
- Specific heat capacity is the amount of heat required to raise the temperature of a substance that is 1 kg by 1ºC.
- Latent heat means hidden heat.
- When a substance, water in the form of ice, for example, is heated to reach its melting point of 0ºC, some of the water is still solid, though its temperature is 0ºC. Extra heat energy is required for the water to change state completely, and this is the latent heat (see diagram below).
- The same amount of heat that is needed to change water to steam, for example, is the same amount of heat that will be released when the steam turns back to water.
- Thermodynamics is the study of the relationships between heat and other forms of energy.
- Energy is described as any force that can move an object.
Ex. kinetic energy, gravity, nuclear energy, thermal energy, sound etc.
First Law of Thermodynamics
- The quantity of heat in a particular system is the same amount of total energy that entered the system.
Second Law of Thermodynamics
- Heat will always flow from a warm body to a cool body.
Third Law of Thermodynamics
- Absolute zero (-273ºC or 0ºK) can never be reached. (Throughout the entire universe there will always be a warmer object to heat up a cold object)
Zeroeth Law of Thermodynamics
- A body is said to be in thermal equilibrium if no heat exchange is taking place between it and its surroundings. (If all objects are the same temperature, no heat exchange will take place)
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