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valentinak56 [21]
3 years ago
10

Describe the three mechanisms of heat flow (conduction, convection, and radiation), and the factors that influence their rates.

Chemistry
1 answer:
trapecia [35]3 years ago
7 0

Answer:

Just as interesting as the effects of heat transfer on a system are the methods by which it occurs. Whenever there is a temperature difference, heat transfer occurs. It may occur rapidly, as through a cooking pan, or slowly, as through the walls of a picnic ice chest. So many processes involve heat transfer that it is hard to imagine a situation where no heat transfer occurs. Yet every heat transfer takes place by only three methods:

Conduction is heat transfer through stationary matter by physical contact. (The matter is stationary on a macroscopic scale—we know that thermal motion of the atoms and molecules occurs at any temperature above absolute zero.) Heat transferred from the burner of a stove through the bottom of a pan to food in the pan is transferred by conduction.

Convection is the heat transfer by the macroscopic movement of a fluid. This type of transfer takes place in a forced-air furnace and in weather systems, for example.

Heat transfer by radiation occurs when microwaves, infrared radiation, visible light, or another form of electromagnetic radiation is emitted or absorbed. An obvious example is the warming of Earth by the Sun. A less obvious example is thermal radiation from the human body.

:)

Further more info...

Conduction:

As  you walk barefoot across the living room carpet in a cold house and then step onto the kitchen tile floor, your feet feel colder on the tile. This result is intriguing, since the carpet and tile floor are both at the same temperature. The different sensation is explained by the different rates of heat transfer: The heat loss is faster for skin in contact with the tiles than with the carpet, so the sensation of cold is more intense.

Convection :

In convection, thermal energy is carried by the large-scale flow of matter. It can be divided into two types. In forced convection, the flow is driven by fans, pumps, and the like. A simple example is a fan that blows air past you in hot surroundings and cools you by replacing the air heated by your body with cooler air. A more complicated example is the cooling system of a typical car, in which a pump moves coolant through the radiator and engine to cool the engine and a fan blows air to cool the radiator.

Radiation :

You can feel the heat transfer from the Sun. The space between Earth and the Sun is largely empty, so the Sun warms us without any possibility of heat transfer by convection or conduction. Similarly, you can sometimes tell that the oven is hot without touching its door or looking inside—it may just warm you as you walk by. In these examples, heat is transferred by radiation. That is, the hot body emits electromagnetic waves that are absorbed by the skin. No medium is required for electromagnetic waves to propagate. Different names are used for electromagnetic waves of different wavelengths: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.

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How long does it take electrons to get from the car battery to the starting motor? Assume the current is 137 A and the electrons
WARRIOR [948]

Answer:

t = 55.79 min

Explanation:

First, the problem is asking for calculate the time that it takes electrons from the battery to the motor.

The general formula to calculate time is:

<em>t = d/V (1)</em>

Where:

d: distance or length

V: speed

Now, we don't have data of speed, but we can know an expression of current density in function of the distance which is the following:

<em>J = n*q*V (2)</em>

Where:

q: charge of the particle (1.6x10^-19 C)

n: number of charge carriers per unit of volume

Current density (J) is actually current per Area so:

<em>J = I/A (3)</em>

Replacing (3) in (2) we have:

I/A = nqV

Solving for V:

<em>V = I/Anq (4)</em>

Finally, if we replace this expression in (1) we have:

<em>t = nqAd / I (5)</em>

Now, the value of n, it's not given but it can be calculated because we have mass density, molar mass and avogadro's number, so this value of "n" can be calculated using the following expression:

<em>n = D * Av / MM (6)</em>

Where:

D: mass density (kg/m³)

Av: avogadro number (6.02x10^23 atom/mol)

MM: molar mass (kg/mol)

Putting the data that we know to calculate n we have:

n = 8960 * 6.02x10^23 / 0.0635

n = 8.49x10^28 atom/m³

Now with the value of n, we can finally calculate the time:

<em>t = nqAd / I </em>

A is the area and it should be in m²: 44.6 mm² / 1x10^6 m = 4.46x10^-5 m²

d is the length in meter: 75.7 cm / 100 cm/m = 0.757 m

so replacing these data in (5):

t = 8.49x10^28 * 1.6x10^-19 * 4.46x10^-5 * 0.757 / 137

t = 3,347.63 s

But the answer is in minute so:

t = 3,347.63 / 60

<em>t = 55.79 min</em>

so the electrons takes 56 min aprox. to go from the car battery to the starting motor.

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