Current and circuits

IB Physics SL· IB Physics SL Unit 2: The particulate nature of matter, Topic 6· 12 min read

1. Definition of Electric Current★★☆☆☆⏱ 10 min

Electric current describes how much electric charge moves past a fixed point in a conducting wire every second. In metallic conductors, the charge carriers are free electrons that move randomly through the metal lattice, and a net flow of these electrons creates a measurable current.

📘 Definition

Electric Current

$I = \frac{Δ Q}{Δ t}$

The rate of net electric charge passing through a cross-sectional area of a conductor

Example:

A current of 2 A means 2 coulombs of charge flow past a point every 1 second

📐 Worked Example

Calculate the total charge that flows through a circuit bulb if a constant current of 0.3 A runs for 2 minutes.

1
First convert the time from minutes to SI units (seconds):
$2 \text{ min} = 2 \times 60 = 120 \text{ s}$
2
Rearrange the current formula to solve for total charge:
$\Delta Q = I \times \Delta t$
3
Substitute the given values:
$\Delta Q = 0.3 \times 120 = 36 \text{ C}$

Exam tip:

Always convert time to seconds before plugging into the I=Q/t formula, exam questions often give time in minutes or hours to trick you.

2. Conventional Current vs Electron Flow★★☆☆☆⏱ 8 min

Early models of electricity assumed positive charges were the mobile charge carriers, leading to the definition of conventional current flowing from the positive terminal of a power supply to the negative terminal. In reality, in metallic wires, only negatively charged electrons move, so their flow is in the exact opposite direction to conventional current.

📐 Worked Example

A copper wire is connected between the positive and negative terminals of a 1.5 V cell. State the direction of conventional current and electron flow in this wire.

1
Conventional current follows the historical positive charge model: flows from the positive terminal to the negative terminal through the wire
2
Electron flow follows the actual movement of negative charge carriers: flows from the negative terminal to the positive terminal through the wire

✓ Quick check

Test your understanding of flow directions:

Which direction does conventional current move through a wire connected to a battery?
- From negative to positive terminal
- From positive to negative terminal
- No fixed direction
- Same as electron flow
Reveal answer
From positive to negative terminal —
This is the standard definition of conventional current used in all circuit calculations.

3. Standard Circuit Symbols★☆☆☆☆⏱ 7 min

All IB Physics SL circuit diagrams use a fixed set of standard symbols to represent components, so you do not need to draw realistic pictures of batteries or bulbs. You are required to recognize and draw the required circuit symbols for the SL assessment.

Component Name	Circuit Symbol Description
Cell	Two parallel lines, one longer (positive) one shorter (negative)
Battery	Multiple connected cell symbols
Lamp / Bulb	A circle with a cross inside
Resistor	A rectangular box
Switch (open)	A break in the line with a tilted lever
Ammeter	A circle with the letter A inside
Voltmeter	A circle with the letter V inside
Wire junction	A filled dot where two wires connect

📐 Worked Example

Draw a simple circuit containing one cell, one open switch, one lamp, and one ammeter in series, and label all components.

1
Start with the cell, mark the positive terminal with a + sign
2
Connect one terminal of the cell to the open switch symbol
3
Connect the other side of the switch to the lamp symbol
4
Connect the lamp to the ammeter symbol, then run the final wire back to the negative terminal of the cell
5
Confirm all components are connected in a single unbroken loop

4. Basic Series Circuit Rules★★★☆☆⏱ 10 min

A series circuit is a single closed loop with no branches, so the same current flows through every component in the circuit. There are no alternate paths for charge carriers to take, so no charge is lost or gained at any point in the loop.

🔬 Derivation

Goal:

Prove that current is constant everywhere in a series circuit

Starting from:

Conservation of charge

1
Charge cannot be created or destroyed in the circuit
2
There are no branches to split the flow of charge
3
The number of electrons entering any point in the wire per second must equal the number leaving that point per second

Result:

The value of current I is identical at all positions in a series circuit

📐 Worked Example

A series circuit has a cell, two resistors, and an ammeter that reads 1.2 A. What is the current through the second resistor?

1
Recall that current is constant at all points in a series circuit
2
The ammeter measures the total current in the single loop
3
The current through the second resistor is equal to the measured ammeter value, 1.2 A

5. Common Pitfalls

Wrong move:

Using time in minutes or hours directly in the I=Q/t formula

Why:

The SI unit for time is seconds, so your final current value will be off by a large factor and you lose method marks

Correct move:

Always convert all time values to seconds before substituting into any current or charge equation

Wrong move:

Stating that conventional current flows from negative to positive

Why:

This is the direction of electron flow, not the universal convention used for all circuit calculations

Correct move:

Default to conventional positive-to-negative flow for all circuit analysis unless explicitly asked for electron direction

Wrong move:

Drawing a non-standard circuit symbol for an ammeter

Why:

IB markers are instructed to deduct marks for unrecognizable or non-standard component symbols

Correct move:

Use the standard circle with a capital A inside for all ammeter diagrams

Wrong move:

Claiming current gets 'used up' after flowing through a bulb in a series circuit

Why:

Charge is conserved, no charge carriers are destroyed when energy is transferred to the bulb

Correct move:

The current is identical before and after the bulb; only electrical potential energy is converted to light and heat

Wrong move:

Confusing the symbol for a cell and a battery

Why:

A cell is a single power source, a battery is multiple connected cells, and they have slightly different standard symbols

Correct move:

Draw two parallel lines (one long, one short) for a single cell, and repeat the pattern for a multi-cell battery

6. Quick Reference Cheatsheet

Quantity	Formula	SI Unit
Electric Current	$I = Δ Q /Δ t$	Ampere (A)
Charge	$Δ Q = I \times Δ t$	Coulomb (C)
Time	$Δ t = Δ Q / I$	Second (s)
Series Circuit Rule	Current is identical at all points	N/A

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2025 · Paper 1
Current calculation multiple choice
2024 · Paper 2
Circuit symbol identification
2023 · Paper 1
Conventional vs electron flow

What's Next

You have now mastered the foundational definitions and rules for current and basic circuits in IB Physics SL. This knowledge is a mandatory prerequisite for upcoming topics including potential difference, Ohm's law, parallel circuit analysis, and electrical power calculations. These concepts are heavily weighted in both Paper 1 and Paper 2, so make sure you complete the associated practice questions to solidify your understanding before moving on. Mastering these basics will also prevent common cascading errors when you tackle more complex multi-component circuit problems later in the unit.