# Current and circuits

> IB Physics SL · IB Physics SL
> Source: https://www.owlsprep.com/study/ib-physics-sl-u2-current-and-circuits/

We cover current definition, conventional vs electron flow, the I=Q/t relationship, standard circuit symbols, and basic series circuit behavior for IB Physics SL assessments.

**Prerequisites:** [Basic properties of electric charge](https://www.owlsprep.com/study/ib-physics-sl-u2-current-and-circuits/); [SI units and prefixes for physics measurements](https://www.owlsprep.com/study/ib-physics-sl-u1-si-units-measurement/)

## Learning objectives

- Define electric current as the rate of charge flow in a conductor
- Distinguish between conventional current and electron flow direction
- Apply the I=Q/t relationship to solve quantitative problems
- Identify standard circuit symbols and interpret simple series circuit diagrams

## Definition of Electric Current

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.

**Electric Current** — The rate of net electric charge passing through a cross-sectional area of a conductor

*Notation:* $I = \frac{\Delta Q}{\Delta t}$

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

> **warning**
>
> Note that current is a scalar quantity, even though we refer to 'direction' of flow, it does not follow vector addition rules.

**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.

## Conventional Current vs Electron Flow

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.

**Exam command terms**

IB exam questions often use these terms to test your understanding:

- **Conventional current direction** — Always stated as positive to negative around the full circuit, regardless of actual charge carrier type

- **Electron flow direction** — Only relevant for metallic conductors, flows from negative to positive terminal

**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

**Check your understanding**

Test your understanding of flow directions:

1. 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

   *Why:* This is the standard definition of conventional current used in all circuit calculations.

## Standard Circuit Symbols

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

> **tip**
>
> You will lose marks in Paper 2 if you draw non-standard symbols, so practice drawing these 8 symbols until they are automatic.

## Basic Series Circuit Rules

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:** 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

*Conclusion:* 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

## Common pitfalls

- **Wrong:** Using time in minutes or hours directly in the I=Q/t formula
  - Why it fails: 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: Always convert all time values to seconds before substituting into any current or charge equation
- **Wrong:** Stating that conventional current flows from negative to positive
  - Why it fails: This is the direction of electron flow, not the universal convention used for all circuit calculations
  - Correct: Default to conventional positive-to-negative flow for all circuit analysis unless explicitly asked for electron direction
- **Wrong:** Drawing a non-standard circuit symbol for an ammeter
  - Why it fails: IB markers are instructed to deduct marks for unrecognizable or non-standard component symbols
  - Correct: Use the standard circle with a capital A inside for all ammeter diagrams
- **Wrong:** Claiming current gets 'used up' after flowing through a bulb in a series circuit
  - Why it fails: Charge is conserved, no charge carriers are destroyed when energy is transferred to the bulb
  - Correct: The current is identical before and after the bulb; only electrical potential energy is converted to light and heat
- **Wrong:** Confusing the symbol for a cell and a battery
  - Why it fails: A cell is a single power source, a battery is multiple connected cells, and they have slightly different standard symbols
  - Correct: Draw two parallel lines (one long, one short) for a single cell, and repeat the pattern for a multi-cell battery

## Cheatsheet

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

## 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.

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