The difference between high power resistors and the low power ones

[x_stunner]

Hi everyone and thanks in advance for taking your time and answering me.
I am somehow stuck in a vicious circle and I would really appreaciate it if anyone could ansmer me fully and/or introduce a good book about this problem of mine:

I (I as me and i as current) know that V=Ri and every book says that a resistor is there as a brake in front of electrons to limit the current flow and I also know that if this happens then the "dissipated" power by the resistor equals P=Vi=Rii=VV/R , now by this basic knowledge and these formulas I can't explain the difference between a high power resistor and low power (you know the type the 1/4 watt colorful resistors) ones.
please help me understand why I can't connect a low power 5-Megaohm resistor to a 200-volt home power outlet while the aforementioned formulas say that the current will be 40 microamperes and the power will only be 8 milliwatts which is less than 1/4 of a watt ????

 

[albbg]

There are many types of resistors; typical are, carbon, metal film and wirewound.
Some of them have a maximum operating voltage that have to be considered even if the power dissipation is not exceded. This is due to the fact that over-voltage can cause irreversible composition modification (mainly the carbon types) that can also damage the component.

 

[keith1200rs]

You can connect a 5M ohms resistor across 200V provided its voltage rating is high enough. Resistor power is usually simply down to size - bigger ones can dissipate more power - but they also have a breakdown voltage so that has to be considered as well.

Keith

 

[x_stunner]

So this is what I've understood so far:
The withstandable voltage is more important than the amperage or power ??!!
and if a resistor voltage rating is observed there is no need to worry about the generated heat and the consequent burning.
and finally the formulas are true if the resistor doesn't burn.

 

[albbg]

No, both of them are important. Both maximum voltage and maximum amperage (that is: power rating) have not to be exceded.