Electronics > Electronics

Ohms Law and the resistor

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vipulan12:
hey guys,

I am kinda confused on how ohms law relates to a circuit with a resistor.

V=IR

-so if thats true if I were to increase the resistance or current i would ultimately increase the voltage but that doesn't apply to resistors

resistors provide resistance so by adding more resistance we should increase voltage but that's not the case
the resistor blocks current and voltage

so could someone explain to me why that is?

thanks

waltr:
Hold one of the three variable in the Ohm's law equation constant. Then change one of the other two variables and the third must change according the the equation.

Typical example is putting a variable resistor (potentiometer) across a constant voltage power supply. In this case the voltage (correctly called E or U in Ohm's Law, E = I*R) is constant and the current, I (measured in Amperes) is dependent on the resistor value. Turn the pot, changing the resistance, and the current changes.
This is calculated by rearranging the equation to I = E/R.

Now to your example: If you put a resistor across a constant current power supply. Then if you change the resistance, turn the pot, then the voltage will change as in E = I * R.

--- Quote --- the resistor blocks current and voltage
--- End quote ---
False.
resistors 'resist' the flow of current.
When a resistor is put across a constant voltage source, the resistors value does not matter (unless the source can not supply the required current). As in my first example the voltage does not change when the resistance value changes. Only the current through the resistor changes.

Two points to remember:
1- Voltage is the Potential across a resistor.
2- Current is the Flow of electrons through a resistor.

Your next task is to connect different value resistors to a voltage source, measure the voltage and resistance  then calculate the current. Also calculate the Power (P = E * I) and feel the heat dissipated by the resistor (proof that the equations do work).

Soeren:
Hi Waltr,

Thanks for touting the proper use of symbolic values! :)

I guess the mind slipped on the last one though ;)

--- Quote from: waltr on August 03, 2012, 04:34:21 PM ---Also calculate the Power (P = V * I) [...]

--- End quote ---
P=U*I (or P=E*I)

vipulan12 <- You are probably tinking in terms of a limited voltage supply. Think of a current supply with an infinite voltage. or think of a 9V battery with a 1 Ohm resistor shunting it - the voltage will be low (and the current high) - change the resistor to something huge - like the air between its terminals and the voltage will be high (but the current will be a few pA, depending on ambient humidity).

waltr:

--- Quote from: Soeren on August 04, 2012, 10:02:40 AM ---
I guess the mind slipped on the last one though ;)

--- Quote from: waltr on August 03, 2012, 04:34:21 PM ---Also calculate the Power (P = V * I) [...]

--- End quote ---
P=U*I (or P=E*I)

--- End quote ---

Ahh...thanks for pointing that out. I corrected my post above.