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# 2.6 Heat and Energy Transfer

K

Krish Gupta

Daniella Garcia-Loos

61ย resources
See Units

## What is Heat?

Heat is the transmitted thermal energy from one object to another.
The units for heat are joules since it's a form of energy transfer and the symbol we will use is Q.
Q will be positive if heat is added to the system and negative if taken away
Heat is a form of energy that is transferred from one body to another as a result of a difference in temperature. Here are some key points about heat in thermodynamics:
1. Heat is a measure of the total energy of the particles in a body, including both the kinetic energy of their motion and the potential energy of their interactions.
2. Heat is measured in units of energy, such as joules or calories.
3. Heat is a form of energy that is transferred from one body to another as a result of a difference in temperature.
4. Heat can be transferred by conduction, convection, or radiation.
5. The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another. Heat is a form of energy that can be converted into other forms, such as work or electrical energy

## Heat Transfer Methods

Now letโs look at the 3 ways this process actually occurs.
1. Conduction: You place your hand on a hot car and feel the thermal energy transfer to your hand. The process by which this happens is known as conduction. The agitates and energized molecules in the hot metal car collide with the atoms of your hand and make them vibrate faster transferring thermal energy. Microscopically, itโs simply the transfer of kinetic energy between particles during collisions. Basically when two things in contact have different temperatures there will be a transfer of thermal energy due to atomic interactions๐ค
2. Convection: The most classic example of convection is the air around a candle. As the candle warms us the air around itself making it less dense and leading to a buoyant force causing it to rise. Convection is driven by gravity (which causes rising and falling of fluids) while conduction is driven by molecular collisions. This process that causes motion of a fluid is called convection. The convection currents you learned about in your Biology or Environmental Science class are a result of this same effect ๐จ
3. Radiation: This effect is the toughest of the 3 to visualize. While conduction comes from molecular interactions, convection is driven by gravity, radiation results from electro-magnetic interactions creating different frequencies. Sunโs rays warming up your body is a perfect example of heat transfer due to radiation. Radiation is a very important topic in modern physics and higher level nuclear physics so we will come back to this topic again later
Here are some key things to remember about heat transfer methods:
1. Conduction is the transfer of heat through a solid or stationary fluid by molecular collisions. It occurs when two bodies are in direct contact with each other, such as when a metal spoon is placed in a hot cup of coffee.
2. Convection is the transfer of heat by the movement of a fluid, such as air or water. It occurs when a fluid is heated and becomes less dense, causing it to rise and be replaced by cooler fluid.
3. Radiation is the transfer of heat through electromagnetic waves, such as light or infrared radiation. It does not require a medium to travel through and can occur even in a vacuum.
4. The rate of heat transfer depends on the temperature difference between the two bodies, the properties of the materials involved, and the distance between the bodies.

Q. Classify what type of transfer is taking place:

1. Ice melting in your hand ๐ง
2. Sunlight ๐
3. Hair straighter heating your hair ๐โโ๏ธ
4. Warm air rising in a room ๐จ
5. Microwave oven ๐ฅ
6. Hot air balloon ๐
7. X rays ๐ฅผ
8. Walking on hot sand with bare feet ๐
9. A heat sensor detects body heat ๐ก
10. Burning a marshmallow over a fire ๐ฅ
A. A good way to tell if conduction is at work is to see if two things are in contact. For convection check if there is movement of fluids based on density. Radiation can be hard to detect but look for electromagnetic waves or fast moving particles. Usually if you don't see conduction or convection applying to the situation at all, go with radiation.
1. Conduction โ
3. Conduction โ
4. Convection โ
6. Convection โ
8. Conduction โ
Example Problem #1:
Imagine that you have two metal blocks, one at a temperature of 100 degrees Celsius and the other at a temperature of 50 degrees Celsius. The blocks are in contact with each other, and there is no heat loss to the environment.
1. Predict the direction of heat transfer between the two blocks.
2. Explain your prediction in terms of the interactions between the atoms in the two blocks.
1. Heat will flow from the block at 100 degrees Celsius to the block at 50 degrees Celsius.
2. At higher temperatures, the atoms in a body have more kinetic energy, meaning that they are moving faster and have a higher average velocity. When two bodies are in contact with each other, the atoms in the hotter body will transfer some of their kinetic energy to the atoms in the cooler body through collisions. As a result, the temperature of the hotter body will decrease and the temperature of the cooler body will increase. In this case, the block at 100 degrees Celsius has a higher temperature than the block at 50 degrees Celsius, so heat will flow from the hotter block to the cooler block.
Example Problem #2:
Imagine that you have two containers of gas, one at a temperature of 50 degrees Celsius and the other at a temperature of 30 degrees Celsius. The containers are separated by a thin wall, and the gases are allowed to interact with each other through the wall.
1. Predict which direction the energy will flow between the two containers of gas.
2. Explain your prediction in terms of the interactions between the gas molecules at the microscopic level.
1. The energy will flow from the container with the higher temperature (50 degrees Celsius) to the container with the lower temperature (30 degrees Celsius).
2. This is because the gas molecules in the container with the higher temperature have a higher average kinetic energy than the gas molecules in the container with the lower temperature. When the gases interact with each other through the wall, the high-energy gas molecules will collide with the low-energy gas molecules, transferring some of their energy to the low-energy molecules. This will cause the average kinetic energy of the gas molecules in the container with the lower temperature to increase, resulting in an overall transfer of energy from the high-temperature container to the low-temperature container.
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