Does serving food on a hot plate really keep it warm longer? I live in Ireland where serving food on hot plates is considered “good cooking practice” to ensure the food remains warm – I come from France where I have rarely seen it done.
I am wondering if this practice really is useful.  I assume it depends on the difference of temperature between the food and the plate, as heat won’t transfer as fast to the plate.  But wouldn’t the extra time the food remains above an “acceptable” temperature threshold just be marginal?
 A: Yes, if Newton's law of cooling applies . Use this calculator to prove it to yourself. Assume the specific heat capacity of the plate and the food are approximately the same. Assume the heat transfer between the food and plate is much faster than to the air, and represent the initial temperature of the dish-food compound as the mass-weighted average of the food and dish temperatures. Over time, temperatures everywhere should converge anyway. 
Do an experiment to test your theory.
A: There are two points to remember:


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*The options are not "hot plate or no plate", they are "hot plate or cold plate". Bringing food into contact with a room temperature plate will cool it rapidly.

*Many plate designs have only a small area in contact with the table, and a much larger area in contact with the food. 

A: The plates may affect the food to a certain degree, but look at the whole thing. 


*

*There's cold air surrounding the food. 

*The plate isn't maintained at a high temperature (assuming). 


Food kept for a long time wouldn't be kept hot for very long, unless the plate is hot enough to burn, which I doubt. I have seen hot plates being served, but keeping the food hot isn't the reason. It's mostly because the plates are heated to sanitize them, and served immediately after that - thus the plates remain warm. 
Or, as @Kevin says, there's a psychological effect. Could you elaborate on that? :)
