MYP Physics

L.1 Electrical Circuits

Understanding current, potential difference, electrical resistance, and building series and parallel circuits.

Questions to explore

? Why do the lights in your home not all go out when one bulb fails?
? What really flows through a wire when we say 'electricity'?

💡 Key ideas, explained simply

⚡ Current, voltage and resistance

Current ( $I$ ) is the flow of charge, in amps. Voltage ( $V$ , potential difference) is the push that drives it, in volts. Resistance ( $R$ ) is how much the component slows the flow, in ohms.

They link through Ohm's law: $V = IR$ . More voltage → more current; more resistance → less current.

🔀 Series vs parallel

In series there is one loop: the current is the same everywhere and the voltage shares out between components. Add a bulb and all of them dim.

In parallel each component has its own branch: they all get the full voltage and work independently. That is why home wiring is parallel — one light can fail without the rest going dark.

Series circuits give components one single path (same current everywhere). Parallel circuits give each component its own branch (same voltage across each).

Series vs parallel circuits

Quantity	Series	Parallel
Current	Same at every point	Splits between branches
Voltage	Shares across components	Full voltage across each
Add a component	All get dimmer	Others unaffected
Total resistance	Adds up (increases)	Decreases below smallest

📖 Key terms

Current $I$: Rate of flow of charge, in amperes (A).
Potential difference $V$: The energy given to charge, i.e. voltage, in volts (V).
Resistance $R$: Opposition to current, in ohms (Ω); $R = \frac{V}{I}$ .
Ohm's law: $V = IR$ for an ohmic conductor at constant temperature.
Series: Components on one single loop — same current everywhere.
Parallel: Components on separate branches — same voltage across each.

1 Key Formulas

Ohm's Law

V = IR

Electrical Power

P = VI

Resistors in Series

R_{total} = R_1 + R_2 + \dots

Resistors in Parallel

\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \dots

✏️ Worked example

Ohm's law

A $12\text{ V}$ battery drives current through a $4\,\Omega$ resistor. Find the current.

1

Rearrange Ohm's law

$V = IR \Rightarrow I = \dfrac{V}{R}$
2

Substitute

$I = \dfrac{12}{4}$

✓

I = 3 A. Halve the resistance and the current would double.

🔬

Interactive Lab

Circuit Design Lab

Build series and parallel circuits, add components and watch current and voltage change.

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🎄 Physics around you

Cheap fairy lights are wired in series, so when one bulb blows the whole string dies. Better ones use parallel branches — a single dead bulb leaves the rest shining.

🎯 Nail it in the exam

Circuit Analysis – Series vs Parallel

This is a core skill tested in Criteria A and C. You must know:

Series circuits:

- Current: same everywhere ( $I = I_1 = I_2 = ...$ ).

- Voltage: splits across components ( $V_{total} = V_1 + V_2 + ...$ ).

- Resistance: $R_{total} = R_1 + R_2$ .

- Adding more resistors increases total resistance and decreases current.

Parallel circuits:

- Current: splits at junctions ( $I_{total} = I_1 + I_2 + ...$ ).

- Voltage: same across each branch ( $V_{total} = V_1 = V_2$ ).

- Resistance: $1/R_{total} = 1/R_1 + 1/R_2$ , total resistance is always less than the smallest individual resistance.

- Adding more resistors in parallel decreases total resistance and increases total current from the battery.

Many questions combine Ohm’s law with circuit rules – always identify series/parallel first.

Pro Exam Strategy

Draw the circuit and label currents and voltages before calculating.
For parallel resistors, total resistance is smaller than the smallest resistor – use this as a quick check.
If a question asks ‘Explain why bulbs in parallel are brighter than in series’, use the voltage and current rules to justify.

I–V Characteristics and Non‑Ohmic Conductors

You must be able to recognise and sketch I–V graphs (current on y‑axis, voltage on x‑axis).

- Ohmic conductor (resistor): straight line through origin; constant resistance.

- Filament lamp: S‑shaped curve; resistance increases with current because the filament heats up.

- Diode: very low current in reverse bias (almost zero), steep increase in forward bias above threshold voltage.

Typical ‘explain’ question: “Why does the resistance of a filament lamp increase as the current increases?” – Because the filament gets hotter, causing the metal ions to vibrate more, making it harder for electrons to flow.

Pro Exam Strategy

Resistance = V/I, not the gradient – only for a straight‑line (ohmic) graph gradient equals resistance.
Thermistors and LDRs may appear: resistance of thermistor decreases with temperature; LDR resistance decreases with light intensity.
When describing a graph, quote data points.

🧠 Check your understanding

Tap an answer to see if you're right — and why.

Q1. If a potential difference of 12 V is applied across a 4 Ohm resistor, what is the current?

$3\, A$
$8\, A$
$16\, A$
$48\, A$

Q2. In a series circuit, how behaves the current at different points?

It divides among the components
It is the same at all points
It increases with each resistor
It drops to zero after the first component

📝 Exam-style questions

Try each one, then reveal the model answer.