# If V=IR Why are voltage and current interchangeable through a constant resistance [duplicate]

I hear people say things like "I only put 5 amps through the circuit but I put a bunch of volts". I don't understand how this is possible if V=IR. Lets say you have a circuit with 5 ohms of resistance so V=I(5). The amount of voltage and current I am allowed to put through it has to be proportional.

Can anyone can give a good intuitive answer (don't go too in depth with math) that is understandable?

• Transformers don't "just" exchange volts and amps. – Ignacio Vazquez-Abrams Jan 15 '16 at 5:32
• Everything obeys Maxwell's equations, but these require calculus and vector math and are pretty complicated. So we use a simpler model called lumped constant where we assume resistors, voltage sources, etc. connected by ideal wires. For DC or steady-state circuits we also assume the voltage and currents are constant. This is where we use Ohm's law and KVL and KCL. But transformers don't work at DC, they require alternating current. AC analysis is more complicated; energy is still conserved but peak voltage and peak current may happen at different times. – MarkU Jan 15 '16 at 5:37
• There are all kinds of circuits that are not resistors. Ohm's Law only applies to resistors and resistances. – Daniel Jan 15 '16 at 5:50
• So are voltage and amperage always proportional in a DC circuit with resistance? – Rhezner Jan 15 '16 at 5:53
• Voltage and current are always in the ratio of the resistance. If the resistance changes, then the ratio changes. There are devices with variable resistance - any wire for instance has a slight temperature coefficient of resistance - though thermistors have a much much greater variation. – Neil_UK Jan 15 '16 at 6:23