# Homework Help: Concept of thermal resistance

1. Jun 7, 2015

### Sahil Dev

Hi,

I shall be grateful if someone can help with this fundamental understanding,

Thermal resistance (in case of conduction mode of heat tarnsfer) can be defined as;

Rt = L / kA

where k is the thermal conductivity

Thus.

thermal resistance if inversely proportional to the thermal conductivity..

Low thermal conductivity means high thermal resistance

My question is;

1) What is good? To have high thermal resistance or low thermal resistance?
2) High thermal resistance would result less heat transferred per unit area. Right?
3) So, am I right in saying that whether you need high thermal resistance or low thermal resistance is problem dependent?

Sorry for being too basic.

2. Jun 7, 2015

### Staff: Mentor

Your questions are fine, and yes, it depends on the application.

For example, we want low thermal resistance for devices like "heat sinks", that are used to pull heat out of semiconductor devices in high-power electronics applications (like power amplifiers).

On the other hand, you wan high thermal resistance if you are trying to insulate something from a heat source (like the thermal insulation foam strips that are used in home attics to insulate the home from the heat absorbed by the roof on hot days, and to hold in the heat on cold days).

Does that make sense? Can think of some other applications for low- and high-thermal-resistance?

3. Jun 7, 2015

### Sahil Dev

Yes, it makes sense fundamentally. Your second example is simple to understand;

Can you give a simpler example than first where you want low thermal resistance (cause I'm not from Electronics field)

4. Jun 7, 2015

### Staff: Mentor

How about the radiator in your car's engine? What do you want from a radiator's materials and construction? What is done with car radiators to ensure that they have a low thermal resistance between the coolant liquid inside them and the airflow outside of them? There are several factors that go into good car radiator design...

5. Jun 7, 2015

### billy_joule

Or another example; A cooking pot, you want the heat from your stovetop element to pass through the pot material easily to heat your food.

6. Jun 8, 2015

### Sahil Dev

billy_joule, you said:

See attached screenshot.

1) Does it mean (in your example), the thermal conductivity of the material of the vessel should be high (hence thermal resistance low) in order that heat be transferred from the hot plate to the vessel?
2) IS this example a heat transfer mode by convection?

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7. Jun 8, 2015

### billy_joule

1) Yes, that's right. An oven mitt, on the other hand, is something you want to have poor thermal conductivity for obvious reasons.
2) Are you referring to the attached image? It's too small to see what's going on. Convection only occurs in fluids and is due to buoyancy.

8. Jun 8, 2015

### sophiecentaur

The accepted use of the word "Resistance" is, perhaps, not very helpful because it suggests more than just a simple similarity in the Maths of electrical and thermal situations. People frequently ask "What is Electrical Resistance?", as if it has some extra meaning. The only proper answer to that question is that it's the Ratio between two quantities (V and I). Likewise, thermal Resistance is really no more than the ratio between Temperature difference and rate of Heat flow. (The Mechanical use of the word Resistance can throw up more confusion.) In all three cases, the Units are different.