Electric current and circuit

Electrons are present in a wire even when the lamp is off. What changes when the switch closes? The battery need not deliver new electrons; it organizes the motion of charges already present.

Without a directing field, mobile electrons in metal move randomly: on average, equal amounts of charge cross an imagined section in opposite directions. An electric field adds a directed component to this motion. Electric current is this ordered motion of charged particles. Thermal motion continues. Mobile charges are therefore necessary, but their presence alone does not produce current.

Why does current not end as soon as a small redistribution occurs? The source uses another form of energy to separate charges and maintain the conditions for an electric field in the circuit. A battery uses chemical energy; a generator uses mechanical energy. Wires provide carrier paths, while a lamp or other load converts the transferred energy. The source supplies energy, not a continuous supply of newly created electrons replacing ones supposedly lost in the lamp.

Steady current in a simple battery circuit needs a complete path through source, wires and load. Break a wire: carriers first redistribute slightly near the gap, and the resulting electric action stops further directed motion. Current ends after a brief transient. A single wire from one terminal to the lamp is therefore insufficient. This concerns steady current in a simple circuit, not the impossibility of every brief charge movement.

Current direction is defined as the direction positive carriers would move. Outside a battery, this is from positive to negative. Metal electrons are negative, so the field drives them oppositely: their directed motion is from negative to positive. Inside the source, its action completes the path. A current arrow need not match the motion arrow of the actual carriers.

How can we compare weak and strong currents without counting along the entire wire? Imagine cutting across it and count the net charge q transferred through this section in time t. Charge q is measured in coulombs and t in seconds. This is not the total charge of every electron in the wire: positive charges in the material largely balance them. A wire that is neutral overall can still transfer charge through a section and carry current.

To remove dependence on observation duration, divide transferred charge by time: average current I = q/t for t > 0. The unit is the ampere: 1 А = 1 Кл/с. Twice the charge in the same time gives twice I; the same charge in twice the time gives half I. No net transfer means zero average current. For constant current, increasing time increases q proportionally, leaving the ratio unchanged.

Directed charge motion can be detected through its effects: a wire heats, a magnetic field appears nearby, or chemical changes occur in suitable media. Lamps and other devices show energy conversion, not disappearing charge. Never test for current using your body: its effects on tissue are real too. School experiments use a suitable low-voltage source and a correctly connected circuit.

A charge of 12 C crosses a section in 4 s. Find the average current.

Find the average charge transfer per second. Use I = q/t, not the product of charge and time.

I = 12/4 = 3 Кл/с = 3 А. Check: over 4 s at this average current, the charge transferred is 3·4 = 12 Кл.

An average of 3 A does not prove identical current in every second; that requires knowing the current is constant. Electrons are not used up in the lamp; energy is converted.

Average current: 3 A. Over the stated interval, an average charge of 3 C passes per second.

The field adds directed motion to the carriers' existing random motion.

The source maintains energy transfer, and a closed simple circuit provides a path for steady current.

I = q/t measures charge transferred per time; conventional current in metal is opposite to electron motion.

Next, measure current with an ammeter and explain what voltage measures.