Let me clarify my question, if two conductors of different materials placed in contact with each other and one material is heated, the heat energy is transferred to the other material by conduction, as a result the temperature of sceond material rises.
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Now let us take our query, as heater coil gets heated the contact wires must also get heated due to the heat conduction from the heater coil. But this is not happening in the real case. (In essence the temperature of the contact wire almost remains unchanged) What is the reason?
Please consider this question in a very fundamental manner. For example if we connect a small piece of nicrome coil to a lead acid battery via a copper wire without any insulation the same thing is happening.
re Review
2006-10-11 17:19:25 · 5 answers · asked by Anonymous in Science & Mathematics ➔ Physics
the materials don't conduct heat the same way. Materials transfer heat energy differently.
One material gets hotter from the electric due to the resistance to the flow of electrons.
2006-10-11 17:30:36 · answer #1 · answered by Anonymous · 0⤊ 0⤋
The effect is due primarily to the differences in thermal conductivity between the heater and the connection wire, as well as any differences in diameter between the two.
It sounds like you're looking for some level of detail, so let's consider your nichrome coil to copper wire example. In particular, let's look at what happens with heat flow right near the connection point, say the last mm of nichrome and the first mm of copper. Assume a certain conductive heat flow through both wires which is the same. Heat flow Q/t = k * A * (delta T) / L, where k is the thermal conductivity, A is area of the wire, L is the length (1 mm each) and delta T is the temperature difference across each respective 1mm length.
This tells us:
(delta T)/L = (Q/t) * 1/(k*A)
(delta T_n)/(delta T_c)=(k_c*A_c)/(k_n*A_n)
k of copper is about 35 times that of nichrome. The diameter of the copper is also typically larger (say just over 3x), then we see that in this example, the temperature drop across the nichrome will be about 350 times that across the copper. In other words, even though the wire will heat up slightly, the connection point stays essentially at the temperature of the wire, not the hot coil.
2006-10-11 23:03:51 · answer #2 · answered by or_try_this 3 · 0⤊ 0⤋
In the example of electric heater leads, the leads are copper, the best thermal conductor and the heater element is some resistive filament/ceramic combination. The element is designed to produce heat and radiate it away. The conductors are designed to deliver the current required, with minimal heating AND are sized accordingly. The mass of the copper is able to radiate the small amount of heat energy generated by the current and absorbed from the element very efficiently. As the element is of higher electrical and thermal resistance than the leads, very little heat is transferred to them.
2006-10-11 17:36:42 · answer #3 · answered by shiro_se 2 · 0⤊ 0⤋
It's because most of the heat energy leaves the heating coil through radiation and convection. Very little of it leaves through conduction to the wires.
After all, there's a lot of surface area of the coil exposed to the air, but very little surface area making a connection to the wires.
2006-10-11 17:38:00 · answer #4 · answered by extton 5 · 0⤊ 0⤋
That is because the connection wires have less resistance to the flow of electric current that the coil in the toaster.
2006-10-11 17:41:56 · answer #5 · answered by ag_iitkgp 7 · 0⤊ 0⤋