DC electrical circuits: charge, current, voltage and resistance, Ohm's law, series and parallel resistors, Kirchhoff's current and voltage laws, the potential divider, and power.

What this dot point is asking

Every circuit you analyse in WJEC Electronics rests on the same handful of DC rules. The specification expects you to define current, voltage and resistance, apply Ohm's law, combine resistors in series and parallel, use Kirchhoff's two laws to find unknown currents and voltages, set an output with a potential divider, and work out power. The potential divider in particular is the single most reused circuit on the whole course, because it is how every sensor turns a resistance change into a voltage.

The answer

Current, voltage and resistance

Ohm's law states that the current through an ohmic conductor is proportional to the voltage across it at constant temperature, so $V = IR$ with $R$ constant. Many electronic components (diodes, transistors, lamps) are non-ohmic, so Ohm's law is a tool for resistors and metal conductors, not a universal rule.

Combining resistors

In series the same current flows through every resistor and the voltages add. In parallel the same voltage appears across every resistor and the currents add. The parallel total is always smaller than the smallest individual resistor.

Kirchhoff's laws

These two laws let you find every current and voltage in a network, even one too tangled to reduce by simple series and parallel rules.

The potential divider

Two resistors in series across a supply split the supply voltage in the ratio of their resistances. The output is taken across $R_2$. This is the workhorse of input sub-systems: replace $R_2$ with a thermistor or LDR and the output voltage tracks temperature or light.

Power

Power is the rate of energy transfer, in watts. The three forms are interchangeable: pick the one that uses the quantities you already know. Power matters for choosing component ratings, since a resistor that dissipates more than its rating overheats.

Examples in context

Example 1. Setting a comparator reference

A fixed potential divider of two equal resistors gives exactly half the supply, a stable reference voltage for a comparator. Because the divider current is small, the reference barely changes when the comparator input draws a tiny current, which is why dividers are the standard way to set thresholds.

Example 2. Limiting current to an LED

An LED dropping $2.0\,\text{V}$ from a $5.0\,\text{V}$ supply needs a series resistor to take the remaining $3.0\,\text{V}$. Ohm's law sizes it: for $15\,\text{mA}$, $R = \frac{3.0}{0.015} = 200\,\Omega$. Power in the resistor, $P = I^2 R = 0.045\,\text{W}$, confirms a standard quarter-watt resistor is fine.

Example 3. Sharing current in parallel loads

Two equal resistive heaters in parallel across a supply each see the full supply voltage, so each draws the same current and the total current doubles. Kirchhoff's first law accounts for it: the supply current equals the sum of the branch currents.

Try this

Q1. A $3.0\,\text{k}\Omega$ and a $6.0\,\text{k}\Omega$ resistor are in parallel. Find the combined resistance. [2 marks]

• Cue. $\frac{1}{R} = \frac{1}{3.0} + \frac{1}{6.0} = \frac{3}{6.0}\,\text{k}\Omega^{-1}$, so $R = 2.0\,\text{k}\Omega$.

Q2. A resistor carries $20\,\text{mA}$ with $5.0\,\text{V}$ across it. Find its resistance and the power it dissipates. [3 marks]

• Cue. $R = \frac{V}{I} = \frac{5.0}{0.020} = 250\,\Omega$; $P = VI = 5.0 \times 0.020 = 0.10\,\text{W}$.