14.1 Heat

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• Define heat as transfer of energy.

In Work, Energy, and Energy Resources , we defined work as force times distance and learned that work done on an object changes its kinetic energy. We also saw in Temperature, Kinetic Theory, and the Gas Laws that temperature is proportional to the (average) kinetic energy of atoms and molecules. We say that a thermal system has a certain internal energy: its internal energy is higher if the temperature is higher. If two objects at different temperatures are brought in contact with each other, energy is transferred from the hotter to the colder object until equilibrium is reached and the bodies reach thermal equilibrium (i.e., they are at the same temperature). No work is done by either object, because no force acts through a distance. The transfer of energy is caused by the temperature difference, and ceases once the temperatures are equal. These observations lead to the following definition of heat    : Heat is the spontaneous transfer of energy due to a temperature difference.

As noted in Temperature, Kinetic Theory, and the Gas Laws , heat is often confused with temperature. For example, we may say the heat was unbearable, when we actually mean that the temperature was high. Heat is a form of energy, whereas temperature is not. The misconception arises because we are sensitive to the flow of heat, rather than the temperature.

Owing to the fact that heat is a form of energy, it has the SI unit of joule (J). The calorie (cal) is a common unit of energy, defined as the energy needed to change the temperature of 1.00 g of water by $1\text{.00ºC}$ —specifically, between $\text{14}\text{.}5ºC$ and $\text{15}\text{.}5ºC$ , since there is a slight temperature dependence. Perhaps the most common unit of heat is the kilocalorie    (kcal), which is the energy needed to change the temperature of 1.00 kg of water by $1\text{.}\text{00ºC}$ . Since mass is most often specified in kilograms, kilocalorie is commonly used. Food calories (given the notation Cal, and sometimes called “big calorie”) are actually kilocalories ( $1\phantom{\rule{0.25em}{0ex}}\text{kilocalorie}\phantom{\rule{0.25em}{0ex}}\text{=}\phantom{\rule{0.25em}{0ex}}\text{1000 calories}$ ), a fact not easily determined from package labeling.

Mechanical equivalent of heat

It is also possible to change the temperature of a substance by doing work. Work can transfer energy into or out of a system. This realization helped establish the fact that heat is a form of energy. James Prescott Joule (1818–1889) performed many experiments to establish the mechanical equivalent of heat    — the work needed to produce the same effects as heat transfer . In terms of the units used for these two terms, the best modern value for this equivalence is

$1\text{.}\text{000}\phantom{\rule{0.25em}{0ex}}\text{kcal}=\text{4186}\phantom{\rule{0.25em}{0ex}}\text{J}\text{.}$

We consider this equation as the conversion between two different units of energy.

The figure above shows one of Joule’s most famous experimental setups for demonstrating the mechanical equivalent of heat. It demonstrated that work and heat can produce the same effects, and helped establish the principle of conservation of energy. Gravitational potential energy (PE) (work done by the gravitational force) is converted into kinetic energy (KE), and then randomized by viscosity and turbulence into increased average kinetic energy of atoms and molecules in the system, producing a temperature increase. His contributions to the field of thermodynamics were so significant that the SI unit of energy was named after him.

Heat added or removed from a system changes its internal energy and thus its temperature. Such a temperature increase is observed while cooking. However, adding heat does not necessarily increase the temperature. An example is melting of ice; that is, when a substance changes from one phase to another. Work done on the system or by the system can also change the internal energy of the system. Joule demonstrated that the temperature of a system can be increased by stirring. If an ice cube is rubbed against a rough surface, work is done by the frictional force. A system has a well-defined internal energy, but we cannot say that it has a certain “heat content” or “work content”. We use the phrase “heat transfer” to emphasize its nature.

Two samples (A and B) of the same substance are kept in a lab. Someone adds 10 kilojoules (kJ) of heat to one sample, while 10 kJ of work is done on the other sample. How can you tell to which sample the heat was added?

Heat and work both change the internal energy of the substance. However, the properties of the sample only depend on the internal energy so that it is impossible to tell whether heat was added to sample A or B.

Summary

• Heat and work are the two distinct methods of energy transfer.
• Heat is energy transferred solely due to a temperature difference.
• Any energy unit can be used for heat transfer, and the most common are kilocalorie (kcal) and joule (J).
• Kilocalorie is defined to be the energy needed to change the temperature of 1.00 kg of water between $\text{14}\text{.}5ºC$ and $\text{15}\text{.}5ºC$ .
• The mechanical equivalent of this heat transfer is $1\text{.00 kcal}\phantom{\rule{0.25em}{0ex}}=\phantom{\rule{0.25em}{0ex}}\text{4186 J.}$

Conceptual questions

How is heat transfer related to temperature?

Describe a situation in which heat transfer occurs. What are the resulting forms of energy?

When heat transfers into a system, is the energy stored as heat? Explain briefly.

what is motion
a motion is the change in position of an object copmaring to a fixed point
ahmed
comparing*
ahmed
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ok
Chelsea
ok
babar
can u explain the term displacement in another form
displacement is the change in position of an object
Owusu
this is the a average distance covered
Lekunga
change in position with respect to reference point
babar
resultant of position vectors or difference of position vectors of a moving object
babar
Distance with a directional component.
Emmanuel
displacement is the difference between your final and initial positions independent of the path taken. distance is how far you actually moved.
Julia
is the acceleration ever in the same direction as a component of velocity
what is gravity
Singut
A 12V battery has a 1ohm resistor in series with a parallel system of 3ohms and 6ohms. How much current is going through the 3ohm resistor?
Total circuit current, I=12/3=4 amperes this means that 8v will be across the parallel resistors. Current through the 3 ohms is 8/3 amperes
Barnabas
How do you know there is 8v across the parallel resistors?
Ingrid
good question
Lekunga
Lekunga
3//6 combined is 2 ohms. Total cct R is 3 ohms, I=V/R=4amps. V across 3//6 is therefore = I x R = 8 volts.
Peter
thanks
Lekunga
a particle f suspended by two strings passing over smooth pulley isattached to two other particles q and r .. calculate the masses of p and q if mass of r is 2 kg
what is hall effect and what are its applications?
It's under sound wave
for speed detection.i.e speed of current
Ben
speed of the current is nothing but voltage
sharan
a uniform rod of length 3m is suspended at one end about an axis perpendicular to its length and it has mass of 2kg and an inertial of 6kgm^2. if the uniform rod was initially at horizontal. calculate its angular velocity when it is at an angle of 30° to the horizontal? please show your workings?
The rotational kinetic energy will equal the gpe lost by the beam as its centre of mass falls 3sin(30)=1.5m. You should get 10^(1/2) for omega but check it on paper.
Richard
the answer you gave is actually incorrect! can you check for any mistake
Olalekan
The centre of mass is at the centre of the beam so will have fallen 0.75m so that will reduce the overall angular speed.
Richard
okay.... does it mean the centre of mass itself has its own particular height? please explain to me sir... am confused!
Olalekan
During a race on an oval track, a car travels at an average speed of 200km/hr. Determine its average velocity at the end of its third lap
it's average velocity is zero
babar
zero
babar
great
Olalekan
does it matter that the track is elliptical instead of circular?
Julia
"lap" just means that you end up where you started from
Sean
so it could be elliptical, triangular, square: in any shape you like!
Sean
absolutely right shape doesn't matter
babar
what is mean fluid
which is capable of changing its shape or something which is viscous in nature
Supremo
fluid is a substance which deforms and flows when we apply small shear force
Manohar
Father of science is who?
Galileo
PzUn
why doesn't the initial position of the teacher or professor always start from zero ?
I always come across question like that how can I defined Newton's law of cooling.....?
kirchoff laws says what
who is the father of Electricity
BROWN
William Gilbert
yusuf
Alessandro Volta
Vasile