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# PHY101 GDB Solution & Discussion Fall 2011

The topic for the discussion is “Oftenly it has observed that in winter a cold block of metal feels colder than a block of wood at the same temperature, in the same way in summer a hot block of metal feels hotter than a block of wood at the same temperature. Why?

Can you think ,is there any temperature at which the blocks feel equally hot or cold, if no give a proof if yes then what is that temperature?”

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### Replies to This Discussion

Can you think ,is there any temperature at which the blocks feel equally hot or cold, if no give a proof if yes then what is that temperature?” also explain it

Plz friends help us GDP today  is last date? specially Tariq Bhai

Just an Idea Don't Copy Paste...

Heat is a form of energy. Unless there is a draft or other outside interference, heat energy always flows from a hot region to a cold one, a process physicists call entropy. Thermal insulators can reduce this flow of energy by slowing it down, but they can never stop it completely. If they could, they would be breaking one of the fundamental laws of the universe, the second law of thermodynamics.

All materials have a thermal conductivity or k value. This value has units of Watts per Kelvin-meter (W/K-m). Degrees Kelvin are a scientific measure of temperature which uses the same scale as degrees Celsius. Thermal conductivity is a measure of how fast a given thickness of material will allow heat to flow. The higher the value, the faster heat will flow. One key point to note is that the flow of energy will be greater the wider the temperature difference.

Heat flows from hot to cold, so you would use a thermal insulator to help keep something colder than its surroundings. Thermos or vacuum flasks keep cold liquids cold and hot liquids hot. They use a double-walled construction to trap a partial vacuum between the walls. A partial vacuum has a low thermal conductivity so heat can only flow through it by radiation. By coating the inside of the flask with a shiny material, radiant heat loss is reduced, making it even more efficient as a cold insulator.

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Thermal Conductivity:
All materials have a thermal conductivity or k value.
This value has units of Watts per Kelvin-meter (W/K-m).
Degrees Kelvin are a scientific measure of temperature which uses the same scale as degrees Celsius.
Thermal conductivity is a measure of how fast a given thickness of material will allow heat to flow.
The higher the value, the faster heat will flow. One key point to note is that the flow of energy will be greater the wider the temperature difference.

K Values of Common Materials

Expanded polystyrene has a very low k value.

Most metals have k values of between 50 and 400 W/K-m.
That is why a block of metal at room temperature feels cold -- as you pick it up,
the heat flows out of your fingers into the metal.
Wood tends to have a value of about 0.15 W/K-m.
That makes wood quite a good thermal insulator and explains why a block of wood feels warm if you pick it up.
Loft insulation materials, such as expanded polystyrene or mineral wool, have k values of about 0.04 W/K-m.
Air has a value of 0.024 W/K-m.

all metal don't retain heat the reasons why has to do with the elements that make up the metal....
Metal is a good conductor of heat, whereas wood is a poor conductor of heat.
When you touch room-temperature metal, it draws heat from your skin faster than room-temperature wood does, so it feels cold.

As was stated above, this only holds for chairs that are at a lower temperature (say, room temperature) than your skin temperature.
If the chairs were at a higher temperature than your skin temperature (say, 100 degrees Fahrenheit),
the metal would transfer heat to your skin faster than the wood would transfer heat, so the metal chair would feel hotter than the wood chair.

Heat is conducted from hot objects to cold objects...

My guess is because metal is a better conductor of heat,
so the heat is pulled out of your hand faster when you touch cold metal as compared to cold wood.

Think of the temperature that you feel of a certain object as how much energy is being taken from your hand by the object (which is naturally colder than you).
Metal which is a good conductor will take a lot more energy than wood which is a poor conductor.

Because metal attracts cold.
And a wooden chair doesn't store the coldness of a chair something like that

I learned thsi ony my chem class, but i didn't learn it very well.

The principle behind this is called heat transfer.
Heat transfer says that heat (or thermal energy) always transfers from high energy objects(hot) to a low energy objects(cold).
If you touch a stove,
what you feel is a large amount of thermal energy being transferred into you through your contact with the stove.
The hotter the object, the more thermal energy will be transferred into you.
The skin can only handle so much thermal energy, so eventually you'll get burned.

On the opposite spectrum, if you touch an ice cube, you feel a cold sensation because thermal energy is transferred from your hand and into the ice cube.

So what does this all mean? There is no transfer of cold, just transfer of heat.
Heat gets transferred when the temperature of two objects are different and the two objects come into contact.
Heat always transfers from higher temp to a colder temp.

Your question is asking, how will I not feel hot or cold when touching a block of metal. That would only occur if the metal was the same temperature as you.

In reality though, the entire process is very complex. Thermal energy may not be uniformly distributed in the object. Our bodies are not a constant temperature everywhere, it fluctuates from area to area.
Our hands also have small sensors that get activated if heat gets transferred away and then there are other sensors that get activated if heat gets transferre

how evere, if the temperature of room equal to that of our body no transfer of heat will take place between our body and block (whatever be their thrmal connductivities)
ad so both will appeare hot or cold.

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