Thermal Energy
Thermal energy is the sum of the kinetic energies and potential energies of the particles in a substance. Atoms and molecules are in constant motion. Motion may be random, translational, or rotational. The higher the degree of motion, the higher the kinetic energy. In the diagram below, gases have more motion and higher kinetic energy than solids.
The internal energy of a substance can be changed by:
Heating it (or taking heat from it)
Doing work on the object (or having the object do work)
Heat vs Temperature
Temperature is defined as a measure of the average random kinetic energy of the particles in a substance. The higher the temperature, the higher the kinetic energy of the particles. The total kinetic energy of ALL the particles in a sample of matter is the heat energy of the sample.
Temperature is a way of describing how hot or cold an object is and is measured in degrees Celsius. Heat is a form of energy measured in Joules. The more heat transferred to an object, the more it’s temperature will rise. So heat and temperature are related but they are not the same.
The heat capacity of a substance is the amount of heat required to raise the temperature of a material by 1 degrees Celsius. Specific heat capacity is the amount of heat energy needed to raise 1 kg of a substance by 1 degrees Celsius.
To calculate heat capacity:
Energy Transferred (J) = Specific heat capacity (J/kg) x mass (kg) x temperature change (°C)
E = C x M x ΔT
where ΔT = new temperature — old temperature
Specific Heat Capacity = Energy transferred / (mass x temperature change)
Some objects can have a low temperature but have a very large amount of thermal energy. For example, when comparing the temperature and heat of a teacup vs an iceberg, the temperature of the teacup is greater than the temperature of the iceberg because the particles in the teacup are moving significantly quicker. However, the total heat of the iceberg is greater than the total heat of the teacup because there are many more particles in the iceberg, meaning that the sum of all the heat is actually greater inside of it. In other words, the iceberg has higher thermal energy and the teacup has low thermal energy.
Heat Transfer
Heat is only able to be transferred from hot to cold. For example, when someone opens the door on a cold winter night, it is not actually the cold air being let into the house but it is the hot air from inside the house being let out into the atmosphere. Since there is then less heat inside of the house, the overall temperature inside goes down, thus why it appears colder inside afterwards.
There are three ways in which heat may be transferred:
- Conduction:
Conduction is the first way in which heat may be transferred. Conduction, or touching something hot allows for the kinetic energy to move around quickly. Conduction is the transfer of heat between substances that are in direct contact with each other. The better the conductor, the more rapidly heat will be transferred. Metal is a good conduction of heat. Conduction occurs when a substance is heated, particles will gain more energy, and vibrate more. These molecules then bump into nearby particles and transfer some of their energy to them. This then continues and passes the energy from the hot end down to the colder end of the substance.
2. Radiation
The second way for heat to be transferred is through radiation, or transferring heat without contact. Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object as is the case with conduction and convection. Heat can be transmitted through empty space by thermal radiation often called infrared radiation. This is a type of electromagnetic radiation . No mass is exchanged and no medium is required in the process of radiation. Examples of radiation is the heat from the sun, or heat released from the filament of a light bulb.
3. Convection:
The third type of transfer was convection. Convection, by definition is “the movement caused within a fluid by the tendency of hotter and therefore less dense material to rise, and colder, denser material to sink under the influence of gravity, which consequently results in transfer of heat”. What this means is that thermal energy is transferred from hot places to cold places by convection. Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. Cooler liquid or gas then takes the place of the warmer areas which have risen higher. This results in a continuous circulation pattern. An example of convection is in the atmosphere. The earth’s surface is warmed by the sun, the warm air rises and cool air moves in.
Cooking
Thermal energy is a main component of cooking and is necessary in order for us to eat food and ingest nutrients properly. Cooking involves the heat transfer of an appliance to the food that needs to be cooked. The process of cooking breaks down some of the food’s fibers and plant cell walls, making them easier to consume and making it easier for the body to digest and absorb nutrients. Some foods also have harmful bacteria that can harm those who ingest them raw. A prime example of this is raw chicken. Raw chicken contains the presence of bacteria such as salmonella. When this is ingested, humans can experience all sorts of different diseases such as food poisoning and typhoid fever. Cooking food also allows the harmful bacteria to be killed by high temperatures. Although some foods are better left raw, such as broccoli, most foods are much better off being cooked!
There are many different methods of cooking that use the different kinds of heat transfer in order to cook food. Here are some few.
Boiling — Boiling involves cooking food in boiling water or other water based liquids such as stock or milk. This process involves convection of liquids in order to transfer heat
Baking — Baking involves cooking food that uses prolonged dry heat, normally in an ove, but also in hot ashes, or on hot stones. This process mainly involves radiation, but fans within the oven will increase cooking times via convection of the air
Broiling — Broiling involves exposing food directly to radiant heat. In this process, food is turned during the process so that the food is cooked one side at a time
Deep Frying — Deep frying involves the use of heat and oil, instead of water. Cooking in oil has many advantages in comparison to cooking in boiling water. The main advantage is that oil is able to heat up much quicker than water due to oil having a lower specific heat capacity than water. This also allows oil to reach much higher cooking temperatures.
Microwaving — Microwaving food is able to occur because the water molecules inside the microwave absorb the radiation of the microwave, which then heat up and heat the surrounding food
As you can see, there are so many different ways to cook food, and while some methods have advantages over others, they all produce the same thing: cooked food.
