Analogue electronics: voltage, current and resistance, Ohm's law, electrical power, and combining resistors in series and in parallel.

What this key area is asking

The SQA wants you to work with analogue quantities - voltage, current and resistance - using Ohm's law, calculate electrical power, and combine resistors in series and in parallel.

Analogue signals

Ohm's law

Rearranging gives $I = \dfrac{V}{R}$ and $R = \dfrac{V}{I}$. Resistance is the opposition to current: a higher resistance gives a smaller current for the same voltage. For a resistor at constant temperature, a graph of voltage against current is a straight line through the origin.

Electrical power

Power is the rate at which a component transforms electrical energy. If you know the current and voltage, use $P = IV$. If you know the current and resistance, use $P = I^2R$. If you know the voltage and resistance, use $P = V^2/R$. All three are in the data booklet.

Resistors in series and parallel

In series the same current flows through every resistor, so their oppositions add. In parallel the current splits between branches, giving it more paths, so the total resistance falls. A quick sanity check: a series total must be bigger than each resistor; a parallel total must be smaller than each resistor. A useful special case is two equal resistors in parallel - the total is always exactly half of one of them, because the two equal branches share the current equally.

You should also be able to read the voltage and current rules that go with these combinations. In a series circuit the current is the same at every point and the supply voltage is shared between the components. In a parallel circuit the voltage across each branch is the same (the full supply) and the current splits between the branches. These rules are the mirror image of each other, so it pays to learn them as a pair alongside the resistance rules.

Why analogue electronics matters

Analogue electronics is the foundation for the rest of the area. The voltage divider, input transducers, transistor switching and the operational amplifier all rely on Ohm's law and on the series and parallel rules. Getting these calculations automatic frees you to focus on the circuit design.

Try this

Q1. A $6.0 \text{ V}$ supply drives $0.20 \text{ A}$ through a resistor. Calculate the resistance. [2 marks]

• Cue. $R = V/I = 6.0/0.20 = 30 \text{ }\Omega$.

Q2. Resistors of $100 \text{ }\Omega$ and $150 \text{ }\Omega$ are in series. Find the total resistance. [1 mark]

• Cue. $R_T = 100 + 150 = 250 \text{ }\Omega$.

Q3. A lamp transfers $24 \text{ W}$ when $2.0 \text{ A}$ flows through it. Calculate the voltage across it. [2 marks]

• Cue. $V = P/I = 24/2.0 = 12 \text{ V}$.