Electronic – Creating tiny heater for ant farm

Problem description

I have a small formicarium for ant keeping, it is basically a tiny glass tank with inside dimensions of 20cm x 10cm x 2cm, filled with sand or soil. Ants I am keeping, like to dwell in the nature under rocks, which are heated by sun rays, and they are using its heat for faster pupae (final stage of ant cocoon) hatching. They are moving pupae to such vicinity of the stone so the correct temperature is achieved. I would like to replicate this in indoor conditions for my ants.

Solution suggestion

I intend to use old laptop power source, characteristics from its sheet:

• 90W AC Adapter
• Output DC 19,5V 4,62A

And basically just put resistor to its output connector big enough to provide power of about 10W. Then either put this resistor to e.g. some concrete mixture to create artificial stone or (if it will be too big) attach it to the outside glass to create one spot temperature increase. Close to this resistor there will be thermostat sensor which will cut of power to the laptop power source as soon as the temperature increase would suffice. It will be operated indoor and desired temperature increase will be 3-5 degrees of Celsius for the 40 cubical centimeters of soil/sand.

My Questions

1. If I put five 2W 10R resistors just after each other I am effectively creating 10W 50R resistor, correct?
2. If having 50R resistor, my math is telling me that with the 19,5V of the power source, it will be drawing 0,39A and will provide power of 7,6W. Is my math correct? If not please provide description of how to calculate this.
3. I totally do not know how would I calculate if this power would suffice to provide desired temperature increase for that amount of soil/sand and artificial concrete stone. I intend to experiment with different resistor size, but if someone would be able to provide me with the info on this or point me to some articles on this topic it could be nice.
4. Safety – it will be in wet environment most of the time. Do you have any tips on how to insulate this water proof, electrical proof but with low thermal insulation? It needed to be cheap and possible to apply in the small size since it will be quite small

My level of knowledge

I have high school education in electronics but I am not working in that field for 20 years. So my knowledge is fuzzy at least 🙂

What I (believe) cannot use and why

1. Artifical heating stones from pet shops – they are too big for my use case. Maximum dimension 2cm
2. Heating cables – they are too long and thus most of the heat would dissipate without use and I am not sure what to do with the rest of the heating cable length
3. Light bulb – its too big and is heating entire formicarium and not only part of it
4. Heating mats which work by spreading infrared light – they are too big and thus would heat larger area