SAT Physics Thermal Properties - Heat And Heat Transfer

SAT Physics Thermal Properties - Heat And Heat Transfer

HEAT AND HEAT TRANSFER
Heat is often misinterpreted by beginning physics students since the word heat is used incorrectly in everyday life. When students touch a warm object, they think it contains a large quantity of heat energy. This is not necessarily correct. The temperature felt by touching a warm object is a result of the thermal energy of the vibrating particles in that object. Try not to confuse the word heat with the word hot. Hot refers to the temperature of an object alone. Temperature is a measure of the average kinetic energy of the particles in that object. Heat refers not only to temperature but also to the number of particles (mass) that are involved. Consider a hot cup of coffee and the ocean. The coffee is hotter in temperature. However, the ocean contains more heat since it contains significantly more particles than the coffee. Then what is heat? Heat is similar to work. Work is a mechanical change in energy that can be seen by the eye (macroscopic). When an object is pushed by a force through a distance, its kinetic energy changes. Work equals the change in macroscopic kinetic energy. Heat, Q, is a change in thermal energy that cannot be seen by the eye (microscopic). When a flame is applied to an object, the object’s thermal energy changes. Heat equals the change in thermal energy. Although work is the quantity of mechanical energy transferred from one system to another, heat is the quantity of thermal energy transferred from one system to another system.

Heat Transfer
Heat transfer is the process of transferring thermal energy from one system to another. In order for heat transfer to take place between two systems, the systems must be at different temperatures. The natural direction of heat transfer is from the high-temperature system to the low-temperature system. The particles in the high temperature system are vibrating or moving faster. When they come into contact with the slower particles in the low-temperature system, collisions transfer energy (heat) from the high-temperature system to the low temperature system. The particles in the high-temperature system lose energy, slow down, and become cooler. The particles in the low-temperature system gain energy, speed up, and become hotter. Energy transfer continues until both systems reach the same temperature. Thermal equilibrium describes the condition when two objects have the same temperature, and no net heat transfer will take place between them on their own. Consider the coffee and ocean example. Pouring the coffee into the ocean transfers heat from the coffee, which is hotter, to the ocean, which is cooler. Both will ultimately achieve the same temperature, or thermal equilibrium. However, the direction of heat transfer always goes from the object with the higher temperature to that with the lower temperature.
    Heat transfer occurs by three methods. In conduction, heat is transferred when two objects at different temperatures physically touch each other. Convection is heat transfer by fluids (liquids and gases). Radiation is heat transfer due to the absorption of light energy.

Conduction
Transfer of heat by conduction requires objects to touch each other physically. This normally involves a hotter solid object touching a colder solid object. This is similar to the process of conduction in electricity, where charges transfer energy from one conductor to another conductor when they touch each other. In fact, substances that are good conductors of electricity, such as metals, are also good conductors of heat.

Rate of Heat Transfer and Thermal Conductivity
The rate of heat transfer, Q/Δt, through an object is dependent on the length, L, the heat must travel through the object, the cross-sectional area of the object, the cross-sectional area of the object, the temperature difference between the ends of length L, and the thermal conductivity, k, of the object. The following formula solves for the rate of heat transfer:
3-1
Thermal conductivity is a physical property of an object. It indicates how well heat is conducted through an object. Every substance has a unique thermal conductivity. The higher the thermal conductivity of the substance, the faster heat transfers through it. Copper has a thermal conductivity of 400 watts/meter • kelvin, while stainless steel has a thermal conductivity of 14 watts/meter • kelvin. This means that copper pots and pans transfer heat through them at a much faster rate, allowing for faster meal preparation and quicker adjustments in temperature when cooking.
Insulators can be used to reduce the rate of heat transfer. This is similar to the process of insulation in electricity. In fact, substances that are good insulators of electricity, such as nonmetals, are also good insulators of heat. Insulation is used in homes to prevent summer heat from entering and winter heat from escaping. Insulating materials need to prevent the rapid transfer of heat, and these substances will have very low thermal conductivities.
    Thermal conductivity also explains why objects at the same temperature can feel like they have different temperatures when touched. Metals have high thermal conductivities, and wood has a very low thermal conductivity. Even if a piece of metal and a piece of wood are both at room temperature, the metal will feel cooler to the touch. The human body is warmer than room temperature. When these objects are touched, heat transfers from the fingers into both the metal and the wood. However, heat transfers into the metal at a faster rate. The rapid loss of body heat as it moves into the piece of metal makes the fingers feel colder.

Convection
Heat transfer through convection involves fluids. Fluids can flow. This means both liquid and gases are fluids. When a hot fluid mixes with a cold fluid, thermal energy is transferred as the faster-moving particles collide with the slower-moving particles. Heating a home is an example of convection. Hot air flows through ducts in the home, eventually pouring into a room, much like a river pours into a lake. A moving fluid with a different temperature than its surroundings is known as a convection current.

Radiation
Heat can also be transferred by electromagnetic radiation. Light waves carry energy that can be absorbed when the waves strike objects. The absorption of light will increase the thermal energy of an object, causing its temperature to increase.

HEATING AND COOLING
Specific Heat
If a substance is heated, it will absorb energy and its temperature will increase. If a substance is cooled, it will lose energy and its temperature will decrease. Every substance absorbs or loses energy at a set rate, which can be quantified. Specific heat, c, is the amount of heat needed to raise the temperature of 1 kilogram of a substance by 1 kelvin. For example, the specific heat of liquid water is 4,190 joules per kilogram • kelvin. In order to raise the temperature of 1 kilogram of water by 1 kelvin, 4,190 joules of heat must be transferred to the water. The following equation is used to solve for the heat needed to change the temperature of a substance with a mass of m and a temperature difference of ΔT.
Q = mc ΔT
The specific heat, c, is a physical property that is unique for every substance. In addition, each state of matter has a unique specific heat. For example, The specific heat of solid water, cs, is 2,090 joules per kilogram • kelvin. However the specific heat of liquid water, cL, is 4,190 joules per kilogram • kelvin.
    Liquid water has the highest specific heat of substances commonly found on Earth. As a result, a tremendous amount of energy is needed to increase the temperature of water. Similarly, water can retain a tremendous amount of energy. This allows for the temperature of water to change quite slowly in either direction. It also allows water to be a heat sink, whereby it can store large quantities of energy in places such as rivers, lakes, and particularly oceans.

Specific Heat
The specific heat of aluminum is 900 joules/kilogram • kelvin. How much heat is required to raise 2.0 kilograms of aluminum 10°C?
 
WHAT'S THE TRICK?

For quantities other than 1 kilogram and 1 kelvin, use the equation:
3-2
Note that a change of 10°C is the same as a change of 10 K.
Q = 18,000 J

Phase Changes
Matter on Earth is commonly found in three phases: solid, liquid, or gas. A phase change is when a substance physically changes from one phase to another. Phase changes occur when substances reach critical temperatures. Solids change into liquids at their melting point, and liquids change into gases at their boiling point. The melting and boiling points are two important physical properties of a substance. Different substances will have different melting and boiling points. However, every substance has a set melting and boiling point under specific environmental conditions.

Heat of Transformation (Latent Heat)
When a substance is heated, its temperature will increase until it reaches the critical temperature (melting point or boiling point) at which a phase change can occur. At this critical temperature, all the thermal energy added to the substance is used to conduct the phase change. The heat of transformation, also known as latent heat, is the energy added during the phase change. This energy is used to weaken the intermolecular forces that hold molecules together as solids and as liquids. Since all the energy added is involved in the transformation, the temperature of the substance does not change during the phase change. When all of the substance has completed the phase change, the temperature of the substance can then resume its rise.
    The heat of transformation or latent heat, L, is a physical property of a substance. Each substance has a unique value for this quantity. The heat of fusion or latent heat of fusion, Lf is the heat energy needed to convert 1 kilogram of a substance from its solid form to its liquid form. The heat of vaporiztion or latent heat of vaporization, Lv, is the heat energy needed to convert 1 kilogram of a substance from its liquid form to its gaseous form. The heat of vaporization is always significantly larger than the heat of fusion. More energy is required for the phase change from liquid to gas. The values given for latent heat will be the amount of heat needed for exactly 1 kilogram of a substance. To solve for the heat needed in phase changes involving a substance with mass m, use the following formula:
Q = mL
 
Heat of Transformation
Water has a heat of fusion of 3.33 x 10s joules per kilogram and a heat of vaporization of 22.6 x 10s joules per kilogram.How much heat energy is needed to melt 2.0 kilograms of ice at 0°C?
 
WHAT'S THE TRICK?

Melting involves the heat of fusion. The given heat of vaporization is a distracter.
Q = mL
Q = (2.0 kg)(3.33 x 105 J/kg) = 6.66 x 105 J

Heating and Cooling Curve
Heating a substance from a solid to a liquid can be summarized in a graph known as the heating and cooling curve. In Figure 18.2, a substance starting as a solid is heated by adding heat at a constant rate. The temperature of the substance is graphed versus time.
3-3 
Figure 18.2.
Heating and cooling curve
The temperature of the solid rises as heat is added, Q= mcs ΔT. When the substance reaches its melting point, the temperature becomes constant while the heat of fusion, Q = mLF, is added to convert the solid into a liquid. When the entire substance has become a liquid, the heat added, Q = mciΔT, raises the temperature until the substance reaches its boiling point. At the boiling point, the temperature is again constant while the heat of vaporization, Q = mLv,converts the substance into a gas. Once the phase change is completed, the gas changes temperature with the addition of more heat, Q= mcgΔT. When boiling water on a stove, the last line of the heating curve would not be possible as the steam attains the highest temperature at the boiling point. However, it is possible to capture the steam in a pipe and add more thermal energy by applying a flame to increase the water’s temperature beyond the boiling point.
    Note how the sloped sections differ. This is because the specific heat differs for a solid, ,a liquid, cv During the phase changes, the temperature does not rise and the graph remains horizontal. The energy added is used to conduct the phase change. The temperature will not rise until the entire substance has completed the change. In addition, the horizontal sections have different lengths. More energy is required to change a substance into a gas than to change it into a liquid. Since heat is added at a continuous rate, more time is needed to change the substance into a gas than to change it into a liquid.
    The graph is identical for a substance that is cooled but is reversed in terms of time. When cooled, a substance could start as a gas and cool until it condenses into a liquid and then freezes into a solid. The mathematics for heating and cooling are the same. It is just a question of whether heat is added or removed and whether temperature rises or decreases. Either way, the formulas and calculated values are the same.


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