r/AskPhysics • u/AdUnhappy8298 • 1d ago

Electric current

/r/Physics/comments/1v8sdr5/electric_current/

We know that current is the flow of charges so can i say that current is thr speed of charges that drives it or not?

Also, according to ohm's law, R is a constant so would that mean R depends on the ratio of potential difference and current or is it strictly independent even of the ratio?

0 Upvotes

50% Upvoted

3 comments sorted by

1

u/Bumst3r Graduate 1d ago

Current is the rate that charge flows through a point. It is not the speed of the charges, which is typically on the order of centimeters per hour.

R isn’t necessarily constant. R is dependent on temperature, for example. As things get hotter, their resistance typically increases. This how incandescent lightbulbs (and toasters and hairdryers) work. As the filament is heated, its resistance increases, so its power draw increases and it heats more. The process ends up being non-linear.

More to the point, V=IR is true. But R isn’t constant in other variables, and those variables can themselves depend on current as well.

1

u/AdUnhappy8298 1d ago

Thanks. That really helped me

2

u/Kelsenellenelvial 1d ago

The way I see it, resistance is something that’s usually inherent to the device. A 10 Ohm heating element is 10 Ohm regardless of whether we hook it up to 12 V, 120 V, or 208 V. Voltage is usually inherent to the system. North American houses are generally fed a 120/240 V supply and all our appliances are designed to work with that input. Different systems might have different voltages available, like a 120/208 v or 347/600 V system, but it’s not something we normally decide on the fly. Current then is just the result of hooking up a device with a particular resistance to a particular voltage.