Heat transfer

Temperature is a measure of the ability of a substance, or more generally of any physical system, to transfer heat energy to another physical system. The higher the temperature of an object is, the greater the tendency of that object to transfer heat. The lower the temperature of an object is, the greater the tendency of that object to be on the receiving end of the heat transfer.

What is Thermal energy?

Thermal energy is the TOTAL potential & kinetic ENERGY of all the particles in an object.
Three factors determine an object’s Thermal energy:
Mass: More quantity particles  More Thermal Energy
Temperature: More motion - average kinetic energy  More Thermal Energy
Phase: Gases > Liquids > Solids

Heat is the transfer of thermal energy from a warmer object to a cooler object.
This transfer is due to a difference in the temperature.
Heat is energy in motion
Units to measure Heat: Joules, Calories

Heat travels by 3 different methods

Conduction: The mechanism in which heat is transferred from one object to another object through particle collisions is known as conduction. In conduction, there is no net transfer of physical stuff between the objects. Nothing material moves across the boundary. The changes in temperature are wholly explained as the result of the gains and losses of kinetic energy during collisions.

Heat Transfer by Convection: Heat typically does not flow through liquids and gases by means of conduction.
Liquids and gases are fluids; their particles are not fixed in place; they move about the bulk of the sample of matter. The model used for explaining heat transfer through the bulk of liquids and gases involves convection.
Convection is the process of heat transfer from one location to the next by the movement of fluids. The moving fluid carries energy with it. The fluid flows from a high temperature location to a low temperature location.

Heat Transfer by Radiation: A final method of heat transfer involves radiation. Radiation is the transfer of heat by means of electromagnetic waves. To radiate means to send out or spread from a central location. Whether it is light, sound, waves, rays, flower petals, wheel spokes or pain, if something radiates then it protrudes or spreads outward from an origin.
The transfer of heat by radiation involves the carrying of energy from an origin to the space surrounding it. The energy is carried by electromagnetic waves and does not involve the movement or the interaction of matter.
Thermal radiation can occur through matter or through a region of space that is void of matter (i.e., a vacuum). In fact, the heat received on Earth from the sun is the result of electromagnetic waves traveling through the void of space between the Earth and the sun.

Check Your Understanding

1. Consider Object A which has a temperature of 65°C and Object B which has a temperature of 15°C. The two objects are placed next to each other and the little bangers begin colliding. Will any of the collisions result in the transfer of energy from Object B to Object A? Explain.

Answer: Most certainly yes. The average kinetic energy of the particles in Object A is greater than the average kinetic energy of the particles in Object B. But there is a range of speeds and thus of kinetic energy in both objects. As such, there will be some highly energetic particles in Object B and some very non-energetic particles in Object A. When this combination of particles encounter a collision, there will a transfer of energy across the boundary from Object B (the colder object) to Object A (the hotter object). This is just one collision. Since majority of collisions result from the more energetic particles of Object A with less energetic particles of collision B, there will be a net kinetic energy transfer from Object A to Object B.

2. Suppose that Object A and Object B (from the previous problem) have reached a thermal equilibrium. Do the particles of the two objects still collide with each other? If so, do any of the collisions result in the transfer of energy between the two objects? Explain.

Answer: The collisions will still take place because the particles are still moving. Just because the temperatures are the same doesn't mean the collisions will stop. The fact that the temperature is identical means that the average kinetic energy of all the particles is the same for both objects. As such, there will be just as much energy transferred from Object B to Object A as there is energy transferred in the opposite direction. When the effect of these collisions is averaged, there is no net energy transfer. This explains why the temperature of the two objects remains the same. Thermal equilibrium persists.

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COLEGIO ANDES DE MAZATLÁN SECCIÓN SECUNDARIA
CIENCIAS 2 (FÍSICA) SEGUNDO GRADO

QUIZ 20. CALOR Y ENERGÍA

Alumn@: ___________________________________________ Calif.: _____

I. Selecciona la respuesta correcta:

1. Es una manifestación de la energía provocada por el movimiento molecular.
A) Trabajo B) Temperatura C) Calor D) Presión

2. ¿Qué nombre recibe la cantidad de energía térmica que un cuerpo pierde o gana en contacto con otro a diferentes temperaturas?
A) Trabajo B) Temperatura C) Calor D) Presión

3. El calor es:
A) Proporcional a la pérdida de energía B) Energía térmica en movimiento
C) Se mide con un termómetro D) Su unidad es el Pascal (Pa)

4. Cantidad de calor que es necesario aplicar a un gramo de cualesquiera sustancia para elevar su temperatura en un grado centígrado.
A) Caloría B) Joule C) Newton D) BTU

5. Propagación del calor presente en las corrientes marinas y los vientos debido a la diferencia de temperaturas.
A) Conducción B) Convección C) Radiación D) Refracción

6. Dispositivo que usamos cotidianamente para conservar “el calor” o “el frío”, ya que reduce al máximo la conducción, la convección y la radiación del calor.
A) Termo B) Olla presto C) Refrigerador D) Horno

7. Es todo dispositivo que transforma calor en trabajo, como los motores de combustión interna o externa, y las máquinas de refrigeración.
A) Reproductor CD B) Termostato C) Máquina térmica D) Batería

8. ¿Por qué algunos materiales se calientan más rápido que otros?






9. ¿Aumenta la temperatura de un cuerpo cuando se le transfiere energía mediante trabajo mecánico?






10. Un refrigerador disminuye la temperatura de los alimentos que se encuentran en su interior; ¿podemos decir que el refrigerador transfiere frío a los alimentos?, ¿por qué?