Example: Start with the actual specification of the resistor: maximum power at specified case temperature. Not all "1/4-W" resistors have the same specification.
Then perform the calculation of heat conduction to either flowing water or through oil to a very large heat sink.
After that, remember that the heat sink will, of course, heat up so you need to calculate the temperature that the heat sink achieves given the heat flow into it.
These questions are quantitative, and hand-waving will not go very far.
If your hopeless at maths, I guess u can experiment instead, and ull get the truth the same.
Then you don’t understand the role of the math. Experimenting is very instructive, but it lacks the predictive power of calculations. So they serve different purposes.
What they aren’t is substitutes for each other: math only lets you calculate for the factors you know and model into your equations, so the simplified models often used only get us close to how real-world circuits and components behave. Experimentation, on the other hand, gives you glimpses into real-world behavior, but often doesn’t explain why something is the way it is.
As always, your lack of structured electronics training — and above all, your belief that experimentation is an adequate (or superior!) substitute for it — is something you need to rectify.