Current and potential difference

"Which wire carries more current: one where 3 coulombs of charge pass a point every second, or one where 5 coulombs pass?" The second. Electric current is how much charge flows past a point each second:

I = Q / t

Charge (Q) is measured in coulombs (C), time in seconds, and current in amperes (A): 1 A is 1 coulomb per second.

In a metal wire, the charges that move are electrons. Each carries a tiny charge of 1.6 × 10⁻¹⁹ C, so it takes 6.25 × 10¹⁸ electrons (about 6 × 10¹⁸) to make one coulomb: 1 ÷ (1.6 × 10⁻¹⁹) = 6.25 × 10¹⁸.

By an old convention, current is said to flow from the positive terminal to the negative one. Electrons actually move the other way. The convention was fixed before electrons were discovered, and it is still used.

There are no minutes in physics formulas: convert to seconds first. A bulb draws 0.5 A for 10 minutes: t = 600 s, so Q = It = 0.5 × 600 = 300 C.

"What makes the charges move?" In a crowded bus, you move forward because someone pushes from behind. In a wire, a cell or battery provides the push: a potential difference between its two ends. No potential difference, no current.

Potential difference (also called voltage) is the work done to move each coulomb of charge:

V = W / Q

1 volt means 1 joule of work for every coulomb. So 220 V at home means 220 joules of push for every coulomb of charge.

Measuring: