Conventional current is taken to flow from the positive terminal of a cell, round the circuit, to the negative terminal. The electrons in a metal move the other way, from negative to positive. Questions use both ideas, sometimes in the same sentence.
This follows calculating current from charge and time: the equation gives the size of a current, and this lesson gives its direction.
Why are there two directions?
Early scientists chose a direction for current before they knew what carried it. They picked positive to negative. Later it turned out that in metals the charge carriers are electrons, which are negative, so they move the opposite way.
The convention stayed. All circuit diagrams, arrows, rules and exam answers use conventional current. Electron motion is used only when a question asks about the particles themselves.
What actually moves in a wire?
In a metal wire, free electrons are already spread along the whole length. The positive ions stay fixed in their places. When a cell is connected, the electrons all drift toward the positive terminal together.
That is why a lamp lights as soon as the switch is closed. The electrons do not have to travel all the way from the cell first.
Worked example
A current of 0.25 A flows clockwise (by the conventional direction) around a simple loop containing a cell, a switch and a lamp. The positive terminal of the cell is on the left. (Invented example data.) State the direction of the electrons, and the charge that passes the lamp in 4.0 s.
Step 1, conventional current: it leaves the positive terminal, so it travels clockwise from the left of the cell through the lamp.
Step 2, electrons: they move the opposite way, anticlockwise, from the negative terminal toward the positive terminal.
Step 3, charge: Q = I × t = 0.25 × 4.0 = 1.0 C.
Step 4, check: 1.0 C ÷ 4.0 s = 0.25 A, which matches the current.
Answer: electrons move anticlockwise, and 1.0 C passes the lamp in 4.0 s.
The mistake to watch for
Students often draw the arrows on a diagram in the electron direction, or claim the current gets smaller after the lamp.
Mistaken answer: Current flows out of the negative terminal, and some is used up in the lamp, so the ammeter after the lamp reads less.
The arrow shows electron motion, not conventional current, and the lamp takes energy, not current.
The correction has two parts. Draw current arrows from positive to negative. Then remember that in a series circuit the same current flows at every point, so both ammeters show the same reading.
Check yourself
Try these, then open each answer.
1. Conventional current in a wire flows from left to right. In which direction do the electrons move?
Show answer
Right to left, opposite to conventional current.
2. A student says the lamp lights instantly because electrons race from the cell to the lamp. Explain what is really happening.
Show answer
Free electrons are already all along the wire, including inside the lamp filament. When the switch closes, they all start drifting together, so the lamp responds almost immediately.
3. An ammeter before a lamp reads 0.40 A. What does an ammeter placed after the lamp read in the same series circuit, and why?
Show answer
0.40 A. Current is not used up. The same charge per second passes every point in a series loop.
Where this leads next
To put directions on a diagram correctly you need a reliable set of drawings, covered in using a circuit symbol consistently. The charge and current module shows how this fits with the rest of the topic.
Direction questions are easy to explain in a conversation and easy to get wrong alone. Our online one-to-one Physics tuition gives time to talk a circuit through.