Charge
On the board sit a battery, a bulb and a switch, joined by wires. The switch is open, so there is a gap in the path. Nothing flows and the bulb stays dark.
Zoom into a metal wire. It is packed with electrons: tiny particles that each carry a negative electric charge. Charge is measured in coulombs (C). One coulomb is the charge of about 6.24 billion billion electrons.
Voltage
First, a picture. A pump pushes water round a loop of pipe. The pump's push, its pressure, is like voltage, the electric push. The amount of water flowing past a point each second is like current, the electric flow.
Now drop the pipes. The battery's + end sits 1.5 volts (V) above its − end. Voltage is energy per charge: 1 volt gives each coulomb 1 joule (J), the unit of energy. With the switch open, still nothing moves.
Current
Close the switch. Now the path is a closed loop: out of the battery's + end, through the bulb and back into its − end. Charge can flow all the way round, so a current flows and the bulb lights.
Current is how much charge passes a point each second, measured in amperes (A). 1 ampere means 1 coulomb per second. Watch the meter: 2 coulombs pass in 4 seconds, so the current here is 0.5 A.
By convention, current flows out of the battery's + end, round the loop and back into its − end. The arrows and the dashes show this direction. The rule was fixed long before electrons were discovered in 1897.
Electrons are negative, so they drift the other way: from − to +, against the arrows. They creep slowly, much less than a millimetre per second here. Yet the bulb lights at once, because charges all round the loop start moving together.
Energy
Each coulomb leaves the battery with 1.5 joules and spends them in the bulb as light and heat. With 0.5 coulomb passing each second, that is 0.75 joules per second: a power of 0.75 watts (W).
Open the switch and the loop breaks. Current stops everywhere in the loop at once, the meter reads 0 A and the bulb goes dark. The battery still pushes with 1.5 V, but with a gap there is no current.
In detail
;; step 1/10 · parts Parts battery the push wires the path bulb uses the energy (light and heat) switch open = a gap in the path, closed = no gap
The board circuit is a teaching model: the 0.5 A bulb current and the 0.5 mm² wire are round numbers chosen to show the idea. The 1.5 V is the nominal voltage of one AA alkaline cell. Units: the coulomb (C) is the unit of charge, the joule (J) of energy, the ampere (A) of current (1 A = 1 C/s), the volt (V) of energy per charge (1 V = 1 J/C) and the watt (W) of power (1 W = 1 J/s), as defined in the SI. Drift speed: v = I / (n e A) with n = 8.34 × 10²⁸ free electrons per m³ for copper (OpenStax). For 0.5 A in 0.5 mm² of copper that is about 0.075 mm/s.