Revise GCSE Physics


Question: If we pick up a metal fork and a plastic fork, the metal fork will feel colder than the plastic one . Why?

This is for top students only.

Many students would immediately think that the metal fork must be colder than the plastic fork. This is wrong! Both forks will be at room temperature!

So why does one feel colder?

First of all how do we feel temperature at all? In our skin there are temperature sensors. But these do not actually detect the temperature of objects that we touch. Instead our skins' temperature sensors only tell us about the temperature of the skin itself.

When we touch something colder than out skin, our skin transfers heat to that object and the temperature of the skin falls around the skin temperature sensor and the fall in temperature is reported to our brains.

Metal and plastic have very different thermal properties; plastic is a very poor thermal conductor and metal is a very good thermal conductor.

When we touch both forks, heat energy transfers from our skins to the fork simply because our skin is at a higher temperature than room temperature and heat always flows from hotter object to colder object.

In the metal heat flows quickly away from our hand along the metal and therefore the surface of the metal does not heat up quickly so our hand continues to lose energy until the whole fork warms up to skin temperature and our hand therefore feels colder because it loses a lot of energy and its temperature falls.

With the plastic fork, the surface of the plastic quickly heats up, because the heat energy flowing from our warmer skin cannot flow easily through the material.

As the surface of the plastic heats up the rate of heat flow from our skin falls and the actual rate of energy loss from our skin is much smaller than for the metal, because the temperature difference between our skin and the plastic fork decreases quickly.

This in turn means our skins' temperature falls by much less (remember the rest of our body will be trying to heat the skin as soon as the temperature starts to fall, so it may be that our body will replace our skins energy as fast as it is being lost and therefore after an initial small drop in temperature the temperature fall stops) and our skin does not feel cold, so the plastic fork does not feel cold to us.

So Plastic feels warmer than metal because it does not take heat from out skin as quickly.