Electricity and Circuits

What makes a circuit?

An electric circuit is a closed loop through which electric charge can flow. It needs a source of energy (such as a battery), conducting paths (wires) and components that use the energy (such as bulbs). If the loop is broken anywhere — by an open switch or a broken filament — the current stops.

Voltage, current and resistance

• Voltage (V, in volts) is the energy given to each unit of charge — the “push” from the battery.
• Current (I, in amperes or amps) is the rate at which charge flows past a point.
• Resistance (R, in ohms, Ω) is how strongly a component opposes the current.

A useful picture is water in pipes: voltage is like pressure, current like the flow rate, and resistance like a narrow section of pipe. In metal wires the moving charges are electrons, which drift from the negative terminal to the positive one. By an old convention, conventional current is drawn in the opposite direction, from + to −.

Ohm’s law and power

For many conductors at constant temperature, current is proportional to voltage: V = I × R. This is Ohm’s law, named after Georg Ohm (1827). The power — the rate at which energy is converted, for example into light and heat in a bulb — is P = V × I. A bulb’s brightness depends on its power.

Series circuits

• There is only one path, so the same current flows through every component.
• The battery voltage is shared between the components; bigger resistances take a bigger share.
• Total resistance is the sum of the individual resistances, so adding bulbs makes them all dimmer.
• If one component fails, the whole circuit stops.

Parallel circuits

• Each component is on its own branch connected across the battery, so each gets the full voltage.
• The total current is the sum of the branch currents.
• Adding branches lowers the total resistance and increases the current from the battery.
• Each branch can be switched independently.

Formulas

Worked example

A 9 V battery is connected to two resistors, 100 Ω and 200 Ω.

• In series: total resistance R = 100 + 200 = 300 Ω, so the current is I = V / R = 9 / 300 = 0.03 A (30 mA) through both. The voltage divides in proportion to resistance: 0.03 × 100 = 3 V across the first and 0.03 × 200 = 6 V across the second, adding up to the battery’s 9 V.
• In parallel: each resistor has the full 9 V across it. The currents are 9 / 100 = 0.09 A and 9 / 200 = 0.045 A, so the battery supplies 0.135 A. The total resistance is 9 / 0.135 ≈ 66.7 Ω, less than either resistor alone.

Build both circuits in the simulator and check the meter readings against these numbers.

Key terms

Ammeter

Measures current; connected in series.

Voltmeter

Measures voltage; connected in parallel across a component.

Open and closed circuit

A broken loop with no current / a complete loop with current.

Short circuit

An accidental path with almost no resistance, allowing a dangerously large current.

Real-world applications

• Home wiring is parallel, so every appliance gets the full mains voltage and works independently.
• Fuses and circuit breakers cut the current if too much flows, preventing overheating and fires.
• Old holiday lights were wired in series — one failed bulb made the whole string go dark.
• Batteries in a torch are connected in series to add their voltages.

Common misconceptions

• “Current gets used up by the bulbs.” Current is the same before and after a bulb in series; it is energy that is converted.
• “Electricity flows from + to −.” In metals, electrons actually flow from − to +; conventional current is a historical convention.
• “More bulbs always means more resistance.” Only in series; in parallel, adding bulbs reduces total resistance.

Key takeaways

• V = I × R and P = V × I.
• Series: same current, shared voltage, resistances add.
• Parallel: same voltage, currents add, total resistance falls.
• Brightness depends on power, not just on current or voltage alone.

Model note: bulbs are treated as fixed resistors and the battery as ideal. Real filaments’ resistance rises as they heat up.