Quote from: larry_vw_1955 on Today at 12:10:06 AM

pennies these days are made of mainly zinc like 95 percent with a copper coating. zinc is not quite as good of a heat conductor as copper.

lets say room temperature is 85 deg F and your hand is 95 deg F. That's a temperature differential of 10 degrees. You have the coin sitting in the palm of your hand with one face exposed to 95 deg F and the other side exposed to room temperature. There is going to be some temperature differential when you actually toss the coin and the sides will not get into thermal equilibrium before the coin lands.

Okay, I'll bite. But in normal units. Let's assume there's a 1^oC (or 1K) temperature difference between both sides of the penny. And let's assume it's made of 100% zinc (Wiki: 97.5% Zn, 2.5% Cu).
The diameter is 19.05 mm, and it's 1.52 mm thick (again: Wiki. That means the surface area is 285 mm². The thermal conductivity is 112.2 W/mK.

It's been a while, so I had to look it up:

Quote from: Byjus.com

The thermal conductivity of a material is described by the following formula:

K = (QL)/(AΔT)

Where,

K is the thermal conductivity in W/m.K
Q is the amount of heat transferred through the material in Joules/second or Watts
L is the distance between the two isothermal planes
A is the area of the surface in square meters
ΔT is the difference in temperature in Kelvin

So:
K=112.2 W/mK
Q=<unknown> W
L=0.00152 m
A=0.000285 m²
ΔT=1 K

That means:
Q = K * A * ΔT / L = 112.2 * 0.000285 * 1 / 0.00152 = 21 W.

This confirms what I expected: you'll need to transfer massive amounts of energy through a penny to get a small temperature difference between both sides.
The specific heat of Zinc is 0.387 J/g K. A penny weights 2.5 g. That means adding 21 W for 1 second would be enough to raise the temperature of a penny by almost 21.7 K. It's safe to say you won't transfer 21W from your hand or a slightly warm surface into a penny, and it's safe to say the heat transfer inside the penny is large enough to keep both sides at almost exactly the same temperature.