Electric current

• • Electric current is the ordered flow of electric charges through a conducting material.
  • In a metal the charge carriers are free electrons; in an electrolyte they are positive and negative ions; in a gas they can be both ions and electrons.
• • Without a potential difference the free electrons of a metal move randomly in all directions, so there is no net flow of charge across any section of the wire.
  • When a source is connected, an electric field is set up along the conductor and the electrons acquire a small, steady drift in the direction opposite to the field, superimposed on their random motion.
  • This ordered drift is the current.

• Current intensity (I) is the quantity of charge that crosses a section of the conductor each second:

  I = Q / t

  Q is the charge in coulomb (C), t is the time in second (s) and I is in ampere (A). So 1 A = 1 C/s: one ampere is the current when one coulomb of charge passes a section every second.

• Charge passed in a given time: Q = I × t. Always convert minutes or hours into seconds first (1 min = 60 s, 1 h = 3600 s). A very common mistake is to leave the time in minutes.

• The number of electrons that carry a charge Q is n = Q / e, where e = 1.6 × 10⁻¹⁹ C is the charge of one electron.

• The current is measured by an ammeter, which is always connected in series in the branch whose current is wanted (it must have a very small resistance so that it does not change the current it measures).

Worked example 1.1. A current of 4.5 A flows through a lamp for 2 minutes. Find the charge that passes and the number of electrons involved. Step 1: t = 2 × 60 = 120 s. Step 2: Q = I t = 4.5 × 120 = 540 C. Step 3: n = Q / e = 540 / (1.6 × 10⁻¹⁹) = 3.375 × 10²¹ electrons.