How a Simple Electric Circuit Works
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
A torch seems simple: press a switch and the bulb lights. Behind that small action is a complete electrical circuit, where charges move through a closed path and energy is transferred from a battery to the lamp.
The circuit must form a loop
Electric current can flow only when there is a complete conducting path. A battery, wires and a lamp can make that path, but only if every part connects in a closed loop. Open the switch and the path breaks, so the current stops.
The current is not used up as it travels around the circuit. Charge keeps moving through the loop. What changes is the energy carried by the charges. The battery gives them electrical energy, and the lamp transfers some of that energy into light and heat.
A helpful model is to separate charge from energy. Charges are already present throughout the wires, so closing the switch does not require one electron to travel all the way from the battery to the lamp before anything happens. The electric effect spreads through the circuit quickly, and charges begin moving around the loop together.
Voltage, current and resistance
Voltage describes the energy transferred per unit of charge. A battery provides a potential difference, which pushes charge around the circuit. Current measures how much charge passes a point each second.
Resistance describes how strongly a component opposes current. A thin wire, a lamp filament and a resistor can all limit the flow in different ways. For a fixed voltage, greater resistance usually means a smaller current. These three ideas work together, so changing one part of a circuit can affect the whole loop.
Scientists measure current with an ammeter placed in the path of the current, while a voltmeter is connected across a component to compare energy transfer. These instruments do not create the behaviour. They help reveal what the charges and energy are doing, much as a speedometer reveals motion without causing the car to move.
Series and parallel paths
In a series circuit, components share one path. The same current passes through each component, and adding more lamps usually makes them dimmer because the total resistance rises and the battery's voltage is shared.
In a parallel circuit, components sit on separate branches. Each branch has its own path back to the source, so one lamp can keep working if another branch is opened. Home wiring uses parallel connections for this reason. When reading a diagram, trace it calmly:
- •Find the battery or power source.
- •Follow every wire as a continuous path.
- •Check whether the switch closes the loop.
- •Notice whether components share one path or sit on branches.
Working with numbers and why short circuits are dangerous
Voltage, current and resistance are linked by a simple rule for many components: current equals voltage divided by resistance. If a 9 volt battery is connected across a 30 ohm resistor, the current is 9 divided by 30, or 0.3 amps. Double the resistance to 60 ohms and the current halves to 0.15 amps. The rule works well for ordinary resistors at steady temperatures. A filament lamp is a useful exception, because its resistance rises as the filament heats up, so its current does not increase in simple proportion to the voltage.
The same rule shows why a short circuit is dangerous. If a wire of very low resistance connects the two terminals of a source directly, bypassing the lamp, the current can become very large. Large currents heat wires quickly, which can melt insulation, damage batteries and start fires. A fuse contains a thin wire designed to melt and break the circuit when the current gets too high, while a circuit breaker switches off automatically and can be reset. Both mainly protect wiring, so homes also use residual current devices, which cut the power quickly to reduce the risk of electric shock.
Conventional current and the real direction of electrons
Circuit diagrams show current flowing out of the positive terminal of a battery, around the circuit and back to the negative terminal. This is called conventional current, and it was settled long before anyone knew what actually moves in a wire. In the eighteenth century, Benjamin Franklin and others described electricity as a fluid, and the labels positive and negative were chosen without knowing which way any particles travelled. The convention was already well established by the time the electron was discovered at the end of the nineteenth century.
In metal wires, the moving charges turn out to be electrons, which are negatively charged. They drift from the negative terminal towards the positive terminal, the opposite way to the arrows on a diagram. This sounds like a serious error, but it causes no problems in calculations. Positive charge moving one way has the same electrical effect as negative charge moving the other way, so the numbers for current, voltage and resistance all come out the same. In some conductors, such as salt solutions, positive and negative ions both move, travelling in opposite directions at once.
Power, and why a kettle needs a thick cable
Voltage and current together tell you how quickly energy is transferred, which is called power. Power in watts equals voltage multiplied by current. A small torch running on 3 volts with a current of 0.5 amps transfers energy at 1.5 watts. A kettle plugged into the 230 volt mains in Britain may draw about 13 amps, giving roughly 3,000 watts, or 3 kilowatts. That large current is why kettles need thick flexes and why their plugs carry a 13 amp fuse, the highest rating used in a standard British plug.
Energy is power multiplied by time, and electricity bills use a unit called the kilowatt-hour, the energy a 1 kilowatt appliance transfers in one hour. Boiling that 3 kilowatt kettle for four minutes, a fifteenth of an hour, uses 0.2 kilowatt-hours. A 10 watt LED bulb would need to stay on for twenty hours to use the same amount. Comparing appliances this way shows that devices which heat things, such as kettles, ovens, heaters and electric showers, tend to use far more energy than lights or phone chargers. Boiling only the water you need can save more than switching off a light.
Why an old battery still reads 1.5 volts
A battery is not a perfect source of voltage. The chemicals inside have resistance of their own, called internal resistance, and some of the battery's energy is spent pushing current through it. Take a cell that provides 1.5 volts, with an internal resistance of 0.5 ohms, connected to a lamp of 2.5 ohms. The total resistance is 3 ohms, so the current is 1.5 divided by 3, or 0.5 amps. Of the 1.5 volts, the lamp receives 0.5 amps times 2.5 ohms, which is 1.25 volts. The missing quarter of a volt is used inside the cell, gently warming it.
As a battery ages, its internal resistance usually rises. Suppose it reaches 5 ohms. The current is now 1.5 divided by 7.5, only 0.2 amps, and the lamp receives just 0.5 volts, so it glows feebly. Yet a voltmeter connected across the same battery may still read close to 1.5 volts. A voltmeter has a very high resistance and draws almost no current, so almost nothing is lost inside the cell while it measures. That is why battery testers apply a load before taking a reading. A battery can look healthy when idle and still fail once it has real work to do.
The takeaway
A working circuit is a closed loop that transfers energy from a source to components. Follow the path first, then think about voltage, current and resistance, and the symbols start to tell a clear story.