Current is how fast charge flows. Charge stored is how much charge is sitting on an object. The first is measured in amperes and describes movement. The second is measured in coulombs and describes an amount.
This lesson brings together charging through electron transfer and current from charge and time, because one describes stored charge and the other describes charge on the move.
Why are they easy to mix up?
Both involve charge, and everyday language blurs them. Phone batteries are sold in “mAh”, which looks like a current but is really a charge. A charged balloon “has electricity” though no current flows in it at all.
A useful picture is a water tank and a pipe. The water held in the tank is like stored charge. The litres per second through the pipe are like current.
How do the ideas connect?
When a charged object discharges through a wire, its stored charge leaves as a current. The average current is I = Q ÷ t, where Q is the charge that left and t is the time it took.
Also, a steady current moving through a wire does not store anything in it. Charge flows in at one end and out at the other, so the total charge on the wire stays zero.
Worked example
A charged metal dome holds 4.0 × 10⁻⁶ C (4.0 microcoulombs). It discharges through a wire in 0.020 s. Find the average current. (Invented example data.)
Step 1, name each quantity: 4.0 × 10⁻⁶ C is the stored charge Q. The time is t = 0.020 s.
Step 2, relationship: average current I = Q ÷ t.
Step 3, calculate: I = 4.0 × 10⁻⁶ ÷ 0.020 = 2.0 × 10⁻⁴ A.
Step 4, express sensibly: 2.0 × 10⁻⁴ A = 0.20 mA.
Step 5, check: 0.20 mA is 0.00020 A, and 0.00020 × 0.020 = 4.0 × 10⁻⁶ C, which matches the stored charge.
The current is small, yet it would take a current of 0.20 mA flowing for a full hour to move 0.72 C (0.00020 × 3600). Stored charge on everyday objects is tiny, but a current lasting a long time moves a lot of charge.
The mistake to watch for
Students often say that a bigger current means more charge stored.
Mistaken answer: Sphere X has a larger current, so it stored more charge than sphere Y.
Current tells you how fast charge is moving, not how much was stored. A small charge released very quickly gives a larger current than a big charge released slowly.
The correction is to ask “is this a rate or an amount?” before choosing a unit. If it is a rate, the unit is A. If it is an amount, the unit is C.
Check yourself
Try these, then open each answer.
1. A phone battery is rated at 3000 mAh. Treat this as the charge delivered by a steady current. How many coulombs is that?
Show answer
3000 mAh = 3.0 Ah = 3.0 A for 1 hour. Q = 3.0 × 3600 = 10 800 C.
2. A student says, “A cell stores current.” Rewrite the sentence correctly.
Show answer
A cell stores chemical energy and can make charge flow. Current is the rate of flow of charge, and it is not stored.
3. A sphere holding 9.0 × 10⁻⁶ C discharges in 0.030 s. Find the average current in mA.
Show answer
I = 9.0 × 10⁻⁶ ÷ 0.030 = 3.0 × 10⁻⁴ A = 0.30 mA.
Where this leads next
This finishes the lessons in the module. Test every skill with the charge and current practice set, and return to the charge and current module for the study order.
Quantities with similar names are where careful questioning helps most. That is the kind of conversation our online one-to-one Physics tuition is built around.