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May 22, 2019 · Thermal Engineering. Heat in Thermodynamics. While internal energy refers to the total energy of all the molecules within the object, heat is the amount of energy flowing from one body to another spontaneously due to their temperature difference. Heat is a form of energy, but it is energy in transit. Heat is not a property of a system.
Mar 6, 2024 · Heat is the thermal energy transfer between systems or bodies due to a temperature difference. Thermal energy, in turn, is the kinetic energy of vibrating and colliding particles. Heat occurs spontaneously from a hotter body to a colder one.
- Overview
- Key points
- Heat in thermodynamics
- Relationship between heat and temperature
- Zeroth law of thermodynamics: Defining thermal equilibrium
- Heat capacity: Converting between heat and change in temperature
- Calculating q using the heat capacity
- Example problem: Cooling a cup of tea
- 1. Calculate the mass of the substance
- 2. Calculate the change in temperature, ΔT
What heat means in thermodynamics, and how we can calculate heat using the heat capacity.
•Heat, q , is thermal energy transferred from a hotter system to a cooler system that are in contact.
•Temperature is a measure of the average kinetic energy of the atoms or molecules in the system.
•The zeroth law of thermodynamics says that no heat is transferred between two objects in thermal equilibrium; therefore, they are the same temperature.
•We can calculate the heat released or absorbed using the specific heat capacity C , the mass of the substance m , and the change in temperature ΔT in the equation:
q=m×C×ΔT
•Heat, q , is thermal energy transferred from a hotter system to a cooler system that are in contact.
•Temperature is a measure of the average kinetic energy of the atoms or molecules in the system.
•The zeroth law of thermodynamics says that no heat is transferred between two objects in thermal equilibrium; therefore, they are the same temperature.
•We can calculate the heat released or absorbed using the specific heat capacity C , the mass of the substance m , and the change in temperature ΔT in the equation:
What contains more heat, a cup of coffee or a glass of iced tea? In chemistry class, that would be a trick question (sorry!). In thermodynamics, heat has a very specific meaning that is different from how we might use the word in everyday speech. Scientists define heat as thermal energy transferred between two systems at different temperatures that come in contact. Heat is written with the symbol q or Q, and it has units of Joules (J ).
Heat is sometimes called a process quantity, because it is defined in the context of a process by which energy can be transferred. We don't talk about a cup of coffee containing heat, but we can talk about the heat transferred from the cup of hot coffee to your hand. Heat is also an extensive property, so the change in temperature resulting from heat transferred to a system depends on how many molecules are in the system.
Heat and temperature are two different but closely related concepts. Note that they have different units: temperature typically has units of degrees Celsius (∘C ) or Kelvin (K ), and heat has units of energy, Joules (J ). Temperature is a measure of the average kinetic energy of the atoms or molecules in the system. The water molecules in a cup of hot coffee have a higher average kinetic energy than the water molecules in a cup of iced tea, which also means they are moving at a higher velocity. Temperature is also an intensive property, which means that the temperature doesn't change no matter how much of a substance you have (as long as it is all at the same temperature!). This is why chemists can use the melting point to help identify a pure substance− the temperature at which it melts is a property of the substance with no dependence on the mass of a sample.
On an atomic level, the molecules in each object are constantly in motion and colliding with each other. Every time molecules collide, kinetic energy can be transferred. When the two systems are in contact, heat will be transferred through molecular collisions from the hotter system to the cooler system. The thermal energy will flow in that direction until the two objects are at the same temperature. When the two systems in contact are at the same temperature, we say they are in thermal equilibrium.
The zeroth law of thermodynamics defines thermal equilibrium within an isolated system. The zeroth law says when two objects at thermal equilibrium are in contact, there is no net heat transfer between the objects; therefore, they are the same temperature. Another way to state the zeroth law is to say that if two objects are both separately in thermal equilibrium with a third object, then they are in thermal equilibrium with each other.
The zeroth law allows us to measure the temperature of objects. Any time we use a thermometer, we are using the zeroth law of thermodynamics. Let's say we are measuring the temperature of a water bath. In order to make sure the reading is accurate, we usually want to wait for the temperature reading to stay constant. We are waiting for the thermometer and the water to reach thermal equilibrium! At thermal equilibrium, the temperature of the thermometer bulb and the water bath will be the same, and there should be no net heat transfer from one object to the other (assuming no other loss of heat to the surroundings).
How can we measure heat? Here are some things we know about heat so far:
•When a system absorbs or loses heat, the average kinetic energy of the molecules will change. Thus, heat transfer results in a change in the system's temperature as long as the system is not undergoing a phase change.
•The change in temperature resulting from heat transferred to or from a system depends on how many molecules are in the system.
We can use a thermometer to measure the change in a system's temperature. How can we use the change in temperature to calculate the heat transferred?
In order to figure out how the heat transferred to a system will change the temperature of the system, we need to know at least 2 things:
•The number of molecules in the system
We can use the heat capacity to determine the heat released or absorbed by a material using the following formula:
q=m×C×ΔT
where m is the mass of the substance (in grams), C is the specific heat capacity, and ΔT is the change in temperature during the heat transfer. Note that both mass and specific heat capacity can only have positive values, so the sign of q will depend on the sign of ΔT . We can calculate ΔT using the following equation:
ΔT=Tfinal−Tinitial
Let's say that we have 250mL of hot tea which we would like to cool down before we try to drink it. The tea is currently at 370K , and we'd like to cool it down to 350K . How much thermal energy has to be transferred from the tea to the surroundings to cool the tea?
We are going to assume that the tea is mostly water, so we can use the density and heat capacity of water in our calculations. The specific heat capacity of water is 4.18Jg⋅K , and the density of water is 1.00gmL . We can calculate the energy transferred in the process of cooling the tea using the following steps:
We can calculate the mass of the tea/water using the volume and density of water:
m=250mL×1.00gmL=250g
We can calculate the change in temperature, ΔT , from the initial and final temperatures:
ΔT=Tfinal−Tinitial=350K−370K=−20K
Mar 26, 2024 · The key concept is that heat is a form of energy corresponding to a definite amount of mechanical work. What are the laws of thermodynamics? Thermodynamics is the science of the relationship between heat, work, temperature, and energy. (more) See all videos for this article.
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In thermodynamics, heat is the thermal energy transferred between systems due to a temperature difference. [1] . In colloquial use, heat sometimes refers to thermal energy itself. Thermal energy is the kinetic energy of vibrating and colliding atoms in a substance.
Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation.
where P is the pressure of a gas, V is the volume it occupies, N is the number of particles (atoms or molecules) in the gas, and T is its absolute temperature.The constant k is called the Boltzmann constant and has the value k = 1.38 × 10 −23 J/K, k = 1.38 × 10 −23 J/K, For the purposes of this chapter, we will not go into calculations using the ideal gas law.
