# Twice as hot?

1. Dec 15, 2005

### pivoxa15

Does it make sense to say that one object is twice as hot as another object?

For example, a 40 degree object is twice as hot as a 20 degree object.

2. Dec 15, 2005

### Homer Simpson

It would be OK to say that one object's temperature is twice that of another.

To me the word hot brings to mind heat. You can not say that one object has twice as much heat as another. 'Heat' is just a term for the transfer of energy from an object with a higher temperature to an object with a lower one.

In social circles, you can however say that, for instance, Pam Anderson is at least 10 times hotter than Kathy Bates.

3. Dec 15, 2005

### Theoretician

Surely, it is only technically acceptable to say it if the temperatures are measured on an absolute scale (such as the Kelvin scale)?

4. Dec 15, 2005

### ZapperZ

Staff Emeritus
The problem with this question is that you need to put it in the context in which it is being asked. Typically, such a thing is used in casual conversation. In such a case, there's nothing wrong with saying that.

However, in physics, the term "hot" doesn't have any definite meaning. You can talk about "temperature" and "heat" because those quantities are clearly defined. One can say an object has twice the temperature of the other, or an object has twice the heat of the other, since each of these quantities has a clear mathematical description on how it is measured. So when there's well-defined quantity, then talking about something having twice the value will make sense.

Zz.

5. Dec 15, 2005

### Danger

Hey, don't be dissin' my Kath... cripes! What am I saying?!

6. Dec 15, 2005

### Homer Simpson

Best Heat explanation I've seen below. Plus a little something for Danger.

http://www.stardestroyer.net/Empire/Science/Heat-Explanations.html

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7. Dec 16, 2005

### ZapperZ

Staff Emeritus
Can you please compare what you accept as the "explanation" for heat with what one can find in a standard Thermodynamics text? Why do you accept something you read off some website more than a Thermo text?

Zz.

8. Dec 16, 2005

### pivoxa15

If one object was 40 degrees celcius and another 80 degree celcius than the second is twice the temperture of the first.

If we convert these two objects to kelvin, the first one is 313.15K and the second 353.15 so in this repect the second object is not twice the temperture of the first.

9. Dec 16, 2005

### Bystander

No. Celsius and Fahrenheit temperature scales are segments of the Kelvin and Rankine scales, respectively. Neither is useful by itself for correlating physical properties with temperature.

10. Dec 16, 2005

### ZapperZ

Staff Emeritus
That's why I said that one has to talk about this in the proper CONTEXT! I could also use your example and show it is a "contradiction" in many cases, such as saying an object at one location has twice the potential energy as it did in another location, since where I call zero potential energy is arbitrary.

Zz.

11. Dec 16, 2005

### vanesch

Staff Emeritus
:rofl:
Ok, let's try this one: my keyboard is at 50 cm and my screen is at 100 cm. So my screen is twice as far as my keyboard. However, when measured from the wall, my keyboard is at 150cm, and my screen is at 200cm. So now my screen is NOT twice as far as my keyboard. Also a contradiction :tongue:

12. Dec 16, 2005

### Homer Simpson

I've got the dust off.

Just blew the dust off my thermo text (Fundamentals of Thermodynamics: Sonntag, Borgnakke, Van Wylen) 5th ed.

Def'n of heat, pg 82:

I think the best thing to get from all this is that saying a bowl of water is 'hotter' than another bowl of water means that the one bowl has more internal kinetic energy of its molecules. Those molecules could have been excited from mechanical work input, heat transfer (conduction, convection,radiation), neutron absorbtion...

As far as the twice as hot thing... just go around saying "oh my, there is a lot of kinetic energy in the air today" or something like that. Kelvin and Rankine scales do go from absolute 0 and up, and these scales can be used to give direct effeciencies of heat engines, for example, so it's likely much more technically correct.

Outside of a thermodynamic and scientific context, it's all semantics. It's like that T-shirt that says: "YOUR RETARDED" When someone corrects you on the grammer, just mutter "you're retarded".

13. Dec 16, 2005

### DaveC426913

What's even dumber is 'twice as cold'.

'It's twice as cold today as it was yesterday!'

14. Dec 16, 2005

### Homer Simpson

Yesterday is what -1 C here and today it is plus 2 C. So it is -2 times as hot.

15. Dec 16, 2005

### ZapperZ

Staff Emeritus
Ah, wonderful. Now look at that definition, and then see if there is such a thing as "energy". If we go by that definition, then every other forms of energy can also be defined that way, i.e. a moving object really doesn't have a "kinetic energy", but rather it has something that can be transfered to another.

We can look at the Thermo's First Law and clearly see that, without work being done on the system, the amount of "heat" being given equals to the amount of change in the internal energy of the system. In turn, this comes right out of the statistical accounting of the speed distribution of the system. If the internal energy is "energy", and the addition of heat increases its energy, then heat is a form of energy on par with KE, PE, Work Done, Internal Energy, etc. To say that heat isn't energy, but just "energy transfer" is like saying that I have money in my bank account, but when I'm transfering money from one account to another, during the transfer, it isn't money.

Zz.

16. Dec 16, 2005

### Homer Simpson

I don't have a great grasp on the concept, but the way I see it is this:

A block is moving. It KE is 1/2 mv^2. How it got that KE is unknown, and does not matter. (could have been collision with other block, could have been gravity)

A bunch of molecules are moving in an object. There KE is also 1/2 mv^2 as a sum. How these molecules got this KE is unknown and does not matter. (could have been in contact with another object whose molecules had greater KE, could have had work applied)

Different energies can be tranferred. 'Heat' is the term for the transfer of molecules Kinetic Energy from one object of a higher temperature to another with lower T.

Comparing it to electricity terms. The object with a higher internal kinetic energy equates to object with higher voltage. Bring this object in contact with another object with lower voltage. There is a transfer: current. To me the best comparison is that Heat is the Current. The voltages represent internal KE. An object can not pocess current, and nor is current a form of energy. There is not necesarily any work done in the process. I realize this is a fairly weak comparison.

EDIT: OK, scrap the current thing, the more I think of it the more I dont like it.

Last edited: Dec 16, 2005
17. Dec 16, 2005

### D H

Staff Emeritus
The definition Homer quoted from his thermo book is a good one: "a body never contains heat". The concept of a body containing heat, the caloric model of heat is an old idea. The caloric model of heat was gradually discarded over the course of the 19th century because it doesn't work.

The idea of heat content is appealing: just define the total heat content of some object as

$$Q = \int_{\mathrm{absolute\ zero}}^{\mathrm{current\ state}} dQ$$

The concept doesn't work because the amount of heat needed to change the thermodynamic state of an object from state A to state B depends on the path taken between the two states. Because the integral is path dependent, there can be no definitive answer to the question "what is the heat content of object A?"

This means that one cannot say that body A is twice as hot as body B. The bodies can be compared in terms of temperature, enthalpy, free energy, and a host of other thermodynamic state variables, but not heat.

Last edited: Dec 16, 2005
18. Dec 16, 2005

### Morbid Steve

Context, context, context!

19. Dec 24, 2005

### skywolf

couldnt you just say that the average speed of a particle is twice as fast as the speed of another particle, as long as they are the same particle?

20. Dec 26, 2005

### pivoxa15

As we have seen from the examples earlier, it is dangerous to claim twice or any other multiples of the original quantity when comparing the magnitude of a measurement. Look at the celcius and kelvin example.

But if you keep all the units constant throughout or if you like, keep the context constant than you are able to get away with multiples of magnitudes.