Electric current and circuits explained: voltage, resistance and Ohm's law
What electric current, voltage and resistance mean, how Ohm's law links them, and the rules that separate series circuits from parallel circuits.
An electric circuit needs a source of potential difference to push charge around a closed loop. Current is the rate of flow of charge, voltage is the energy transferred per unit charge, resistance opposes the flow, and Ohm's law links them as V = IR. In a series circuit the current is the same everywhere; in a parallel circuit the voltage is the same across each branch.
Current, voltage and resistance
Electric current is the flow of electric charge around a circuit, measured in amperes (A). It is the amount of charge passing a point per second, so charge equals current multiplied by time. A current can only flow in a closed circuit that includes a source of potential difference, such as a cell or battery.
Potential difference, also called voltage, is the difference in energy between two points in a circuit, measured in volts (V). One volt is the potential difference when one coulomb of charge transfers one joule of energy, so potential difference equals energy divided by charge (V = E/Q). Resistance, measured in ohms, is how much a component opposes the flow of current; all components have some resistance, and a resistor is a component designed to provide it.
Ohm's law and a worked example
Ohm's law states that the potential difference across a component equals the current through it multiplied by its resistance: V = I x R. For a fixed resistance, doubling the potential difference doubles the current, and for a fixed voltage, increasing the resistance reduces the current.
A standard worked example: if a current of 2 A flows through a resistance of 40 ohms, the potential difference is 2 x 40 = 80 V. The same relation can be rearranged to find current (I = V/R) or resistance (R = V/I). Remember the measuring instruments too: an ammeter is connected in series with a component to measure current, while a voltmeter is connected in parallel across a component to measure potential difference.
Series vs parallel circuits
In a series circuit there is a single loop, so the same current flows through every component, and the voltages across the components add up to the supply voltage. Total resistance is the sum of the individual resistances, and if one component breaks, the whole circuit stops working. This is why old-style fairy lights went out together when one bulb failed.
In a parallel circuit each component is connected across the same two points, so the voltage across each branch is the same as the supply voltage, while the current splits between the branches and the branch currents add up to the total current. Adding more branches lowers the overall resistance because the current has more paths. If one branch fails, the others keep working, which is why household wiring uses parallel circuits so that each appliance receives the full supply voltage.
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Frequently asked questions
What is electric current?
The rate of flow of electric charge in a circuit, measured in amperes (A). It equals the charge passing a point divided by the time taken.
What is Ohm's law?
V = IR: potential difference (V, volts) equals current (I, amps) multiplied by resistance (R, ohms). It can be rearranged as I = V/R or R = V/I.
What is the potential difference if 2 A flows through 40 ohms?
80 V, because V = I x R = 2 x 40 = 80.
What stays the same in a series circuit?
The current is the same through every component, while the supply voltage is shared between them and total resistance is the sum of the resistances.
What stays the same in a parallel circuit?
The voltage is the same across every branch, while the current splits between branches and adds up to the total current. If one branch fails, the others keep working.