Question About Aquarium Heating FAST!!?

I got a 46 gallon tank and just bought a 300 watt Heater. Its submerged but I can see what looks like heat eminating from the sides of the heater. My heater died 3 days ago so my water might have been colder. My last heater was preset and I never saw this "heat" eminating from the heater. Is that normal? I have it set at 78 degrees.