Electric heaters
Hi, an electric heater may be one of the simplest circuits used in everyday life. Its working follow the fundamentl Ohm low where I=V/R, where as I gueess you already know, I is current, V is voltage and R is the resistance of the heater.
NOw, about your specific question, there are as many varieties of heater as you can imagine. It depends where you are pluging your heater, I mean to the outlets of your electricity distribution in your house, or like somebody mentioned, it maybe a battery powered heater, or if the voltage in your country is 110v or 220v.
In few words your question is too general. you didnt mention if you want to built a heater or if you want to modify an existing circuits. Therefore I am giving you a general answer.
Usually you have V fixed and I can be calculated from the Ohm law, therefore, your variable is R, and that is what heater controls do, switch the value of R, lets assume your heater has 3 resistor R1, R2, and R3 which have the same value r ohms. then you can have 3 different degrees of heat from your heater, using a connetor switch R can be R1 or R1+R2 or R1+R2+R3. By the way power is equal to P = V*I
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Sal