# Fields

> IB Physics SL · IB SL Physics
> Source: https://www.owlsprep.com/study/ib-physics-sl-u4-overview/
> Weight: 17-19% of overall IB Physics SL exam

This unit introduces the core concept of fields, which describes how objects interact at a distance across gravity, electricity, and magnetism. Fields are foundational to understanding modern physics and many everyday technologies.

**Prerequisites:** Newtonian mechanics (forces, energy, circular motion); Basic understanding of electric charge and current

## Learning objectives

- Distinguish between gravitational, electric, and magnetic fields and describe their core properties
- Calculate field strength, potential, and force for gravitational, electric, and magnetic field systems
- Apply Faraday's and Lenz's laws to solve problems involving electromagnetic induction
- Connect field concepts to real-world technological applications of electricity and magnetism

## Unit at a Glance

This unit follows a logical progression from classical gravitational fields, through electric and magnetic fields, to the unifying concept of electromagnetic induction. You will notice that all three core field types share similar mathematical structures, which makes connecting concepts across topics simpler.

We start with the most familiar field (gravity) to build core concepts like field strength and potential, then extend these same frameworks to electric fields, before covering magnetic interactions and how changing fields induce electric current.

Below are the four core sub-topics in this unit:
- [Gravitational fields](https://www.owlsprep.com/study/ib-physics-sl-u4-gravitational-fields/) — Introduces core field concepts, gravitational field strength, potential, and orbital motion calculations.
- [Electric fields](https://www.owlsprep.com/study/ib-physics-sl-u4-electric-fields/) — Extends core field concepts to electricity, covering Coulomb's law, field strength, and electric potential.
- [Magnetic fields](https://www.owlsprep.com/study/ib-physics-sl-u4-magnetic-fields/) — Explores magnetic force on moving charges and current-carrying wires in magnetic fields.
- [Electromagnetic induction](https://www.owlsprep.com/study/ib-physics-sl-u4-electromagnetic-induction/) — Covers Faraday's law, Lenz's law, and calculation of induced emf and current.

## Common pitfalls

- **Wrong:** Confusing gravitational potential (always negative) with gravitational potential energy.
  - Why it fails: Gravitational potential is energy per unit mass, with zero reference set at infinity, leading to negative values for all bound systems.
  - Correct: Remember potential = potential energy divided by test mass/charge, with zero potential defined at infinity for both gravity and electric fields.
- **Wrong:** Mixing up the direction of magnetic force on positive vs negative charges.
  - Why it fails: Standard hand rules for magnetic force assume a positive moving charge, so direction flips for negative charges.
  - Correct: Reverse the force direction if the moving charge is negative, or use cross product rules consistently for sign.
- **Wrong:** Forgetting Lenz's law when finding the direction of induced current.
  - Why it fails: Faraday's law only gives the magnitude of induced emf, direction comes from energy conservation via Lenz's law.
  - Correct: Always apply Lenz's law first to determine the direction of induced current before calculating magnitude.

## Cheatsheet

| Concept | Formula |
| --- | --- |
| Gravitational field strength (point mass) | $g = \frac{GM}{r^2}$ |
| Coulomb's force between point charges | $F = k\frac{q_1 q_2}{r^2}$ |
| Electric field strength (point charge) | $E = \frac{kq}{r^2}$ |
| Magnetic force on moving charge | $F = qvB\sin\theta$ |
| Magnetic force on current-carrying wire | $F = BIL\sin\theta$ |
| Magnetic flux | $\Phi = BA\cos\theta$ |
| Faraday's law (induced emf magnitude) | $\varepsilon = N\left\|\frac{\Delta \Phi}{\Delta t}\right\|$ |

## What's next

Begin by exploring gravitational fields, the first sub-topic of this unit, where you will learn core field concepts that extend to electric and magnetic fields. After completing all four sub-topics of this unit on Fields, continue your learning with the first sub-topic of the next unit on current electricity.

- [Gravitational fields (First sub-topic of this unit)](https://www.owlsprep.com/study/ib-physics-sl-u4-gravitational-fields/)
- [Electric fields](https://www.owlsprep.com/study/ib-physics-sl-u4-electric-fields/)
- [Magnetic fields](https://www.owlsprep.com/study/ib-physics-sl-u4-magnetic-fields/)

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