Current as rate of flow of charge
Charge crossing a wire in a given time.
Two wires, the same charge
Wire X and wire Y both let 6 C of charge cross a point. Wire X takes 2 s to do it and wire Y takes 3 s.
Which wire carries the larger electric current?
What this lesson covers
The idea
Electric current is the rate of flow of electric charge: if a net charge Q flows across any cross-section of a conductor in time t, the current is I = Q/t.
Two wires, the same charge
Wire X and wire Y both let 6 C of charge cross a point. Wire X takes 2 s to do it and wire Y takes 3 s.
Which wire carries the larger electric current?
- Wire X, because it gets the charge across in less time
- Wire Y, because it takes longer
- Both carry the same current because the charge is the same
Charge and time
The current is shown in A (amperes, explained soon). Change the charge that crosses and the time it takes.
A rate
The current went up when more charge crossed in the same time, and went down when the same charge took longer. So current tells us how fast charge flows.
Electric current is the rate of flow of electric charge. If a net charge Q flows across any cross-section of a conductor in time t, the current is I = Q/t.
Notes
Electric current is the rate of flow of charge: if a net charge Q crosses a cross-section in time t, then I = Q/t.
Check yourself
A charge of 12 C flows across a cross-section of a wire in 4 s. What is the current, in A?
Answer: 3 A
I = Q/t = 12 C divided by 4 s = 3 A.
A charge of 10 C flows across a cross-section of a wire in 5 s. What is the current, in A?
Answer: 2 A
I = Q/t = 10 C divided by 5 s = 2 A.
Wire X lets 6 C cross in 2 s. Wire Y lets 6 C cross in 3 s. Which carries the larger current?
Which statement about electric current is correct?
- Wire Y. Wire Y takes 3 s for the same charge, so its current is 6 C divided by 3 s = 2 A, which is less than the 3 A of wire X.
- They carry equal currents, since both move 6 C. Current depends on the time as well. The same charge in a shorter time is a larger rate of flow.
- Wire X — correct. Yes!
- It is the charge crossing a cross-section in each unit of time — correct. Yes!
- It is the total charge stored in the wire. Current is a rate. It tells how much charge crosses in each unit of time, not how much is stored.
- It is the time a charge takes to cross the wire. Time is only part of it. The current is the charge divided by that time.