# Good conductor of electricity and bad conductor of heat

I wonder, how a graphite can be a good conductor of electricity but at the same time, be a bad conductor of heat.?! As we know, a body conducting electrons are bound to produce heat by resistance, which will, in turn increase the entropy and heat. So therefore thermal conductivity is directly proportional to electrical conductivity.

Also I guess it is not necessary that a good conductor of heat be also a good conductor of electricity because vibrational energy can also be the cause of it like in the case of diamond.

• Define 'bad'. According to engineeringtoolbox.com/thermal-conductivity-d_429.html, graphite is a better conductor of heat than iron or steel, in the same order of magnitude as aluminum. – Ján Lalinský Nov 12 '18 at 21:20
• Data isn't very solid, because on google.co.in/url?sa=t&source=web&rct=j&url=https://… – RS2000 Nov 12 '18 at 21:26
• Also some other substances also show this property, being good conductor of electricity and not a good conductor of heat why is it so? Also, graphite being not a good conductor, still is true. – RS2000 Nov 12 '18 at 21:30
• And steel is not a good conductor of heat, it's used in airplane engines. Bad here doesn't refer to as bad as air and just nothing, it refers to the fact that how can diamond be a far better conductor. – RS2000 Nov 12 '18 at 21:34
• Then you should ask about diamond - graphite has ordinary conductivity comparable to common metals. Diamond is the outlier. – Ján Lalinský Nov 12 '18 at 22:04

I wonder, how a graphite can be a good conductor of electricity but at the same time, be a bad conductor of heat.?!

You have to be more precise. Can you give values? Graphite is a very anisotropic material. Its thermal conductivity is insanely high in the x-y plane (about 4 times that of copper). But in the z direction, the thermal conductivity is very low, about 2 orders of magnitude less than copper. Something similar happens with its electrical conductivity.

therefore thermal conductivity is directly proportional to electrical conductivity.

Not quite. The so-called Wiedemann-Franz law (which works reasonable well for metals) stipulates that the ratio between the thermal conductivity and electrical conductivity is proportional to temperature (and not merely a constant!). It also assumes that only electrons contribute to heat transfer, not phonons. But keep in mind that this law does not apply "as is" for semiconductors or semimetals.

Also I guess it is not necessary that a good conductor of heat be also a good conductor of electricity because vibrational energy can also be the cause of it like in the case of diamond.

You guessed right. This would hold reasonable well for metals, but not for heavily doped semiconductors (good thermoelectric materials), or insulators. The more the phonons have a non negligible contribution to the heat transfer, the less the statement linking $$\kappa$$ to $$\sigma$$ should hold.

• Thanks for your input. I just wanted to ask that why alloys like steel have thermal conductivity pretty less than it's corresponding metals like iron? – RS2000 Nov 19 '18 at 14:37
• For the case of steel, I expect the electrons to scatter with the lattice disturbances due to the carbon atoms. This impacts (negatively) on both the electrical and thermal conductivities. – thermomagnetic condensed boson Nov 19 '18 at 15:46

No, Graphite is a good conductor of heat also.

To relate Electrical Conductivity to Thermal conductivity, we have Wiedemann-Franz law(applicable at low and high temperatures). According to this law they are both directly related at a given temperature.

If we flow a current through graphite ,it's temperature will become high and it will be heated at high temperature in less time

• No it's not. Though on Google, few of the answer says it is, but it's actually not(I read it and also my teacher told) – RS2000 Nov 12 '18 at 18:48
• And if u r still not satisfied, I have other examples too to demonstrate that. – RS2000 Nov 12 '18 at 18:51
• When we connect graphite to terminals of battery it becomes very much warm in less time – user212727 Nov 12 '18 at 20:59