• Electric current is the rate of flow of charge through a conductor of cross-sectional area 𝑨.
• In electric circuits this charge is often carried by electrons moving through a wire.
• It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an
ionized gas (plasma).
Unit of Current
• The conventional symbol for current is 𝑰
• If ∆𝑸 is the amount of charge passing through an area in a time interval ∆𝒕, then the
average current 𝑰 𝒂𝒗𝒆 is defined as:
𝑰 𝒂𝒗𝒆 =
• The unit of current is coulomb per second.
• also called the ampere (𝑨).
Unit of Current
• Ampere is a large unit.
• Therefore, the current is usually expressed in:
𝑚𝑖𝑙𝑙𝑖𝑎𝑚𝑝𝑠 (1 𝑚𝐴 = 1 × 10−3
𝑚𝑖𝑐𝑟𝑜𝑎𝑚𝑝𝑠 (1 µ𝐴 = 1 × 10−6
𝑛𝑎𝑛𝑜𝑎𝑚𝑝𝑠 (1 𝑛𝐴 = 1 × 10−9
• In a conductive material, the moving charged particles that constitute the electric current are
called charge carriers.
• In metals, which make up the wires and other conductors in most electrical circuits, the
positively charged atomic nuclei of the atoms are held in a fixed position, and the negatively
charged electrons are the charge carriers, free to move about in the metal.
• In other materials, notably the semiconductors, the charge carriers can be positive or
negative, depending on the dopant used.
• Positive and negative charge carriers may even be present at the same time, as happens in
an electrolyte in an electrochemical cell.
• A flow of positive charges gives the same electric current, and has the same effect in a
circuit, as an equal flow of negative charges in the opposite direction.
• Since current can be the flow of either positive or negative charges, or both, a convention is
needed for the direction of current that is independent of the type of charge carriers.
• The direction of conventional current is arbitrarily defined as the same direction as positive
• The electrons, the charge carriers in an electrical circuit, flow in the opposite direction of the
conventional electric current.
Flow of Positive Charge
Flow of Electrons
What is enough current?
around 100 mA to 1 A of current is sufficient to
induce cardiac/respiratory arrest.
Device Current (A)
100-W lightbulb 1
Microwave 8 – 13
Laptop 2 – 3
Electric Fan 1
Television 1 – 3
Toaster 7 – 10
Fluorescent Light 1 – 2
radio/stereo 1 – 4
typical LED 20 X 10-3
Smart Phone 200 X 10-3
automobile starter 200