what determines how fast heat travels from a warmer object to a colder object

Department Learning Objectives

By the end of this section, you will be able to do the post-obit:

• Explain heat, rut capacity, and specific heat
• Distinguish between conduction, convection, and radiation
• Solve problems involving specific heat and oestrus transfer

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The learning objectives in this section will help your students master the following standards:

• (half dozen) Scientific discipline concepts. The pupil knows that changes occur inside a physical arrangement and applies the laws of conservation of energy and momentum. The student is expected to:
  • (F) contrast and give examples of unlike processes of thermal energy transfer, including conduction, convection, and radiation.

Section Fundamental Terms

conduction

convection

heat capacity

radiations

specific heat

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[BL] [OL] [AL] Review concepts of oestrus, temperature, and mass.

[AL] Bank check prior cognition of conduction and convection.

Estrus Transfer, Specific Oestrus, and Heat Capacity

We learned in the previous department that temperature is proportional to the boilerplate kinetic free energy of atoms and molecules in a substance, and that the average internal kinetic free energy of a substance is higher when the substance's temperature is higher.

If two objects at different temperatures are brought in contact with each other, energy is transferred from the hotter object (that is, the object with the greater temperature) to the colder (lower temperature) object, until both objects are at the same temperature. In that location is no net rut transfer once the temperatures are equal considering the amount of heat transferred from ane object to the other is the same as the amount of estrus returned. I of the major effects of rut transfer is temperature change: Heating increases the temperature while cooling decreases information technology. Experiments bear witness that the heat transferred to or from a substance depends on iii factors—the change in the substance's temperature, the mass of the substance, and certain physical backdrop related to the phase of the substance.

The equation for heat transfer Q is

where m is the mass of the substance and ΔT is the change in its temperature, in units of Celsius or Kelvin. The symbol c stands for specific estrus, and depends on the fabric and stage. The specific estrus is the corporeality of heat necessary to change the temperature of 1.00 kg of mass by 1.00 ºC. The specific rut c is a property of the substance; its SI unit is J/(kg $\cdot$ M) or J/(kg $\cdot$ $\text{°C}$ ). The temperature change ( $\Delta T$ ) is the same in units of kelvins and degrees Celsius (but not degrees Fahrenheit). Specific heat is closely related to the concept of oestrus capacity. Heat capacity is the corporeality of rut necessary to change the temperature of a substance by one.00 $\text{°C}$ . In equation form, oestrus capacity C is $C=\mathrm{thousand}c$ , where one thousand is mass and c is specific rut. Annotation that heat capacity is the same equally specific oestrus, but without whatsoever dependence on mass. Consequently, two objects fabricated upward of the aforementioned material but with dissimilar masses will have dissimilar heat capacities. This is because the rut capacity is a property of an object, but specific rut is a property of any object made of the same material.

Values of specific heat must be looked up in tables, because there is no unproblematic way to summate them. Table 11.2 gives the values of specific rut for a few substances as a handy reference. Nosotros see from this table that the specific oestrus of water is five times that of drinking glass, which means that it takes v times every bit much rut to raise the temperature of i kg of water than to raise the temperature of 1 kg of glass by the aforementioned number of degrees.

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[BL] [OL] [AL]Explain that this formula only works when in that location is no alter in phase of the substance. The transfer of thermal energy, heat, and phase change volition exist covered later in the affiliate.

Misconception Alert

The units of specific heat are J/(kg $\cdot \text{°C}$ ) and J/(kg $\cdot$ Thou). However, degrees Celsius and Kelvins are not always interchangeable. The formula for specific heat uses a difference in temperature and not absolute temperature. This is the reason that degrees Celsius may be used in identify of Kelvins.

Substances	Specific Oestrus (c)
Solids	J/(kg $\cdot \text{°C}$ )
Aluminum	900
Asbestos	800
Physical, granite (average)	840
Copper	387
Glass	840
Gilded	129
Human body (average)	3500
Ice (average)	2090
Iron, steel	452
Lead	128
Silver	235
Wood	1700
Liquids
Benzene	1740
Ethanol	2450
Glycerin	2410
Mercury	139
Water	4186
Gases (at ane atm constant pressure level)
Air (dry)	1015
Ammonia	2190
Carbon dioxide	833
Nitrogen	1040
Oxygen	913
Steam	2020

Table 11.2 Specific Heats of Various Substances.

Snap Lab

Temperature Change of Country and Water

What heats faster, land or water? You will answer this question by taking measurements to report differences in specific heat capacity.

• Open flame—Tie back all loose pilus and wear before igniting an open flame. Follow all of your teacher'south instructions on how to ignite the flame. Never leave an open flame unattended. Know the location of fire prophylactic equipment in the laboratory.

• Sand or soil
• Water
• Oven or heat lamp
• 2 small jars
• Two thermometers

Instructions

Procedure

1. Identify equal masses of dry out sand (or soil) and water at the same temperature into two small jars. (The average density of soil or sand is about 1.half-dozen times that of water, so you can become equal masses by using 50 per centum more water by volume.)
2. Rut both substances (using an oven or a heat lamp) for the aforementioned amount of fourth dimension.
3. Record the final temperatures of the two masses.
4. At present bring both jars to the same temperature by heating for a longer catamenia of time.
5. Remove the jars from the oestrus source and measure their temperature every 5 minutes for about 30 minutes.

Soil has an approximate specific heat of 800 J / kg °C. A farmer monitors both the soil temperature of his field and the temperature of a nearby pond every bit winter sets in. Will the field or the pond achieve 0 °C showtime and why?

1. The pond will reach 0 °C first because of water's greater specific heat.

2. The field will reach 0 °C first because of soil'south lower specific heat.

3. They volition accomplish 0° C at the same time because they are exposed to the same weather condition.

4. The h2o will take longer to heat besides as to absurd. This tells united states that the specific heat of h2o is greater than that of land.

Conduction, Convection, and Radiation

Whenever there is a temperature departure, heat transfer occurs. Rut transfer may happen rapidly, such equally through a cooking pan, or slowly, such as through the walls of an insulated libation.

There are three different estrus transfer methods: conduction, convection, and radiation. At times, all three may happen simultaneously. Run into Figure 11.3.

Figure 11.3 In a fireplace, heat transfer occurs by all three methods: conduction, convection, and radiation. Radiation is responsible for most of the heat transferred into the room. Heat transfer also occurs through conduction into the room, but at a much slower rate. Heat transfer by convection likewise occurs through common cold air inbound the room effectually windows and hot air leaving the room past ascent up the chimney.

Conduction is rut transfer through direct physical contact. Oestrus transferred between the electric burner of a stove and the bottom of a pan is transferred by conduction. Sometimes, we endeavour to control the conduction of heat to make ourselves more than comfortable. Since the charge per unit of heat transfer is different for unlike materials, we choose fabrics, such as a thick wool sweater, that boring downward the transfer of heat away from our bodies in wintertime.

As y'all walk barefoot beyond the living room carpeting, your anxiety experience relatively comfortable…until you lot stride onto the kitchen's tile floor. Since the carpet and tile flooring are both at the same temperature, why does one experience colder than the other? This is explained by unlike rates of rut transfer: The tile material removes estrus from your skin at a greater charge per unit than the rug, which makes it feel colder.

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[BL] [OL] [AL] Ask students what the current temperature in the classroom is. Inquire them if all the objects in the room are at the same temperature. One time this is established, ask them to place their hand on their desk or on a metal object. Does it feel colder? Why? If their desk is Formica laminate, then information technology will experience cool to their hand considering the laminate is a good conductor of rut and draws heat from their hand creating a sensation of "cold" due to heat leaving the torso.

Some materials simply conduct thermal energy faster than others. In general, metals (like copper, aluminum, gold, and argent) are good rut conductors, whereas materials like woods, plastic, and safety are poor heat conductors.

Effigy 11.four shows particles (either atoms or molecules) in 2 bodies at dissimilar temperatures. The (average) kinetic energy of a particle in the hot torso is higher than in the colder trunk. If 2 particles collide, energy transfers from the particle with greater kinetic free energy to the particle with less kinetic free energy. When two bodies are in contact, many particle collisions occur, resulting in a internet flux of heat from the higher-temperature body to the lower-temperature trunk. The estrus flux depends on the temperature departure $\Delta T=T_{\text{hot}}-T_{\text{cold}}$ . Therefore, yous will become a more severe burn from humid water than from hot tap water.

Figure 11.4 The particles in ii bodies at dissimilar temperatures have different average kinetic energies. Collisions occurring at the contact surface tend to transfer energy from loftier-temperature regions to low-temperature regions. In this illustration, a particle in the lower-temperature region (right side) has low kinetic energy before collision, just its kinetic free energy increases afterward colliding with the contact surface. In contrast, a particle in the higher-temperature region (left side) has more kinetic energy before standoff, but its energy decreases after colliding with the contact surface.

Convection is heat transfer by the movement of a fluid. This blazon of heat transfer happens, for instance, in a pot boiling on the stove, or in thunderstorms, where hot air rises up to the base of the clouds.

Tips For Success

In everyday language, the term fluid is commonly taken to mean liquid. For example, when you are ill and the physician tells y'all to "button fluids," that only ways to drink more beverages—not to breath more than air. Nevertheless, in physics, fluid means a liquid or a gas. Fluids move differently than solid material, and even take their own branch of physics, known as fluid dynamics, that studies how they move.

Every bit the temperature of fluids increment, they expand and become less dense. For example, Effigy 11.4 could represent the wall of a balloon with different temperature gases inside the balloon than outside in the environment. The hotter and thus faster moving gas particles inside the airship strike the surface with more force than the libation air outside, causing the balloon to aggrandize. This decrease in density relative to its environment creates buoyancy (the tendency to rise). Convection is driven by buoyancy—hot air rises considering it is less dense than the surrounding air.

Sometimes, nosotros control the temperature of our homes or ourselves past controlling air movement. Sealing leaks around doors with weather condition stripping keeps out the cold current of air in winter. The house in Figure xi.5 and the pot of water on the stove in Figure eleven.half dozen are both examples of convection and buoyancy past human pattern. Sea currents and big-calibration atmospheric circulation transfer energy from one office of the world to another, and are examples of natural convection.

Figure 11.5 Air heated by the so-chosen gravity furnace expands and rises, forming a convective loop that transfers free energy to other parts of the room. As the air is cooled at the ceiling and outside walls, information technology contracts, eventually condign denser than room air and sinking to the floor. A properly designed heating arrangement similar this ane, which uses natural convection, can be quite efficient in uniformly heating a home.

Effigy 11.6 Convection plays an important part in rut transfer within this pot of water. Once conducted to the within fluid, heat transfer to other parts of the pot is mostly by convection. The hotter water expands, decreases in density, and rises to transfer rut to other regions of the water, while colder water sinks to the bottom. This process repeats as long as there is h2o in the pot.

Radiation is a form of heat transfer that occurs when electromagnetic radiation is emitted or absorbed. Electromagnetic radiation includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays, all of which have unlike wavelengths and amounts of energy (shorter wavelengths accept higher frequency and more energy).

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[BL] [OL] Electromagnetic waves are also often referred to every bit EM waves. We perceive EM waves of different frequencies differently. Simply as we are able to see certain frequencies every bit visible calorie-free, we perceive certain others as heat.

You lot can feel the heat transfer from a fire and from the sun. Similarly, yous can sometimes tell that the oven is hot without touching its door or looking inside—it may just warm you as you walk by. Another example is thermal radiation from the man trunk; people are constantly emitting infrared radiation, which is not visible to the homo eye, merely is felt as estrus.

Radiations is the only method of heat transfer where no medium is required, meaning that the estrus doesn't need to come up into direct contact with or be transported by any matter. The infinite betwixt Globe and the sun is largely empty, without whatsoever possibility of oestrus transfer by convection or conduction. Instead, estrus is transferred past radiations, and Globe is warmed as it absorbs electromagnetic radiation emitted past the sun.

Figure 11.7 Most of the heat transfer from this fire to the observers is through infrared radiation. The visible light transfers relatively little thermal energy. Since skin is very sensitive to infrared radiations, you lot tin can sense the presence of a burn down without looking at information technology directly. (Daniel X. O'Neil)

All objects absorb and emit electromagnetic radiation (see Figure 11.7). The charge per unit of heat transfer by radiations depends mainly on the color of the object. Black is the most constructive absorber and radiator, and white is the to the lowest degree constructive. People living in hot climates by and large avoid wearing black clothing, for instance. Similarly, black cobblestone in a parking lot volition be hotter than adjacent patches of grass on a summertime day, because black absorbs better than green. The reverse is also true—black radiates amend than greenish. On a articulate summer night, the black cobblestone will be colder than the green patch of grass, considering black radiates free energy faster than green. In contrast, white is a poor cushion and likewise a poor radiator. A white object reflects nearly all radiation, like a mirror.

Teacher Back up

Teacher Back up

Ask students to give examples of conduction, convection, and radiations.

Virtual Physics

Energy Forms and Changes

In this blitheness, yous volition explore rut transfer with dissimilar materials. Experiment with heating and cooling the atomic number 26, brick, and h2o. This is done by dragging and dropping the object onto the pedestal and and then holding the lever either to Heat or Cool. Drag a thermometer beside each object to mensurate its temperature—y'all can watch how quickly it heats or cools in real time.

Now let's attempt transferring heat between objects. Heat the brick and then place it in the cool water. Now oestrus the brick once again, only and so place it on pinnacle of the atomic number 26. What do you notice?

Selecting the fast forwards choice lets you lot speed upwardly the heat transfers, to salve fourth dimension.

Compare how quickly the dissimilar materials are heated or cooled. Based on these results, what material do y'all think has the greatest specific heat? Why? Which has the smallest specific heat? Can y'all think of a real-globe situation where you would desire to apply an object with large specific estrus?

1. Water will take the longest, and iron will have the shortest fourth dimension to heat, too as to cool. Objects with greater specific heat would be desirable for insulation. For instance, woolen clothes with large specific heat would prevent rut loss from the torso.

2. Water volition take the shortest, and iron will take the longest fourth dimension to heat, besides as to absurd. Objects with greater specific estrus would be desirable for insulation. For example, woolen clothes with large specific oestrus would forbid heat loss from the body.

3. Brick volition take shortest and iron will take longest time to heat upward as well as to cool down. Objects with greater specific oestrus would be desirable for insulation. For case, woolen apparel with large specific rut would preclude oestrus loss from the trunk.

4. Water will take shortest and brick will accept longest time to heat upwardly as well as to cool downwardly. Objects with greater specific heat would be desirable for insulation. For instance, woolen clothes with large specific oestrus would preclude heat loss from the body.

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Have students consider the differences in the interactive exercise results if different materials were used. For example, enquire them whether the temperature alter would be greater or smaller if the brick were replaced with a cake of fe with the same mass as the brick. Ask students to consider identical masses of the metals aluminum, golden, and copper. Later on they have stated whether the temperature change is greater or less for each metal, have them refer to Tabular array 11.2 and bank check whether their predictions were correct.

Solving Estrus Transfer Issues

Worked Example

Calculating the Required Heat: Heating H2o in an Aluminum Pan

A 0.500 kg aluminum pan on a stove is used to heat 0.250 L of water from 20.0 $\text{°C}$ to 80.0 $\text{°C}$ . (a) How much heat is required? What percentage of the oestrus is used to raise the temperature of (b) the pan and (c) the h2o?

Strategy

The pan and the water are always at the same temperature. When you put the pan on the stove, the temperature of the water and the pan is increased by the same amount. We utilise the equation for heat transfer for the given temperature change and masses of h2o and aluminum. The specific heat values for water and aluminum are given in the previous table.

Word

In this example, most of the total estrus transferred is used to estrus the water, even though the pan has twice as much mass. This is because the specific estrus of water is over iv times greater than the specific heat of aluminum. Therefore, it takes a bit more than twice equally much heat to achieve the given temperature change for the water than for the aluminum pan.

H2o can absorb a tremendous amount of energy with very picayune resulting temperature modify. This property of water allows for life on Globe because it stabilizes temperatures. Other planets are less habitable because wild temperature swings make for a harsh environment. Yous may have noticed that climates closer to big bodies of water, such every bit oceans, are milder than climates landlocked in the heart of a large continent. This is due to the climate-moderating issue of h2o's big rut chapters—water stores large amounts of heat during hot weather and releases heat gradually when it's cold outside.

Worked Example

Calculating Temperature Increment: Truck Brakes Overheat on Downhill Runs

When a truck headed downhill brakes, the brakes must do work to catechumen the gravitational potential energy of the truck to internal free energy of the brakes. This conversion prevents the gravitational potential energy from being converted into kinetic free energy of the truck, and keeps the truck from speeding up and losing command. The increased internal energy of the brakes raises their temperature. When the hill is peculiarly steep, the temperature increase may happen too rapidly and cause the brakes to overheat.

Calculate the temperature increase of 100 kg of brake textile with an average specific estrus of 800 J/kg $\cdot \text{°C}$ from a 10,000 kg truck descending 75.0 1000 (in vertical deportation) at a constant speed.

Strategy

We first calculate the gravitational potential free energy (Mgh) of the truck, and then find the temperature increase produced in the brakes.

Discussion

This temperature is shut to the humid betoken of water. If the truck had been traveling for some time, so only before the descent, the brake temperature would likely exist higher than the ambient temperature. The temperature increase in the descent would likely raise the temperature of the restriction textile higher up the humid point of water, which would be hard on the brakes. This is why truck drivers sometimes use a dissimilar technique for chosen "engine braking" to avert burning their brakes during steep descents. Engine braking is using the slowing forces of an engine in low gear rather than brakes to slow down.

Practice Problems

five .

How much heat does it take to raise the temperature of 10.0 kg of h2o by 1.0 °C ?

1. 84 J
2. 42 J
3. 84 kJ
4. 42 kJ

6 .

Summate the change in temperature of 1.0 kg of water that is initially at room temperature if three.0 kJ of rut is added.

1. 358 °C
2. 716 °C
3. 0.36 °C
4. 0.72 °C

Bank check Your Understanding

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Use these questions to appraise student accomplishment of the section'southward learning objectives. If students are struggling with a specific objective, these questions will help identify which and straight students to the relevant content.

7 .

What causes rut transfer?

1. The mass deviation between two objects causes heat transfer.

2. The density difference between two objects causes heat transfer.

3. The temperature deviation between 2 systems causes heat transfer.

4. The pressure departure betwixt ii objects causes heat transfer.

8 .

When two bodies of different temperatures are in contact, what is the overall direction of heat transfer?

1. The overall management of heat transfer is from the higher-temperature object to the lower-temperature object.

2. The overall direction of rut transfer is from the lower-temperature object to the higher-temperature object.

3. The direction of oestrus transfer is kickoff from the lower-temperature object to the higher-temperature object, so dorsum again to the lower-temperature object, and then-forth, until the objects are in thermal equilibrium.

4. The direction of heat transfer is first from the higher-temperature object to the lower-temperature object, then dorsum again to the college-temperature object, and and so-forth, until the objects are in thermal equilibrium.

nine .

What are the different methods of heat transfer?

1. conduction, radiation, and reflection

2. conduction, reflection, and convection

3. convection, radiation, and reflection

4. conduction, radiations, and convection

10 .

Truthful or simulated—Conduction and convection cannot happen simultaneously

1. True
2. Fake

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Source: https://openstax.org/books/physics/pages/11-2-heat-specific-heat-and-heat-transfer

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