Unit Overview
Fields
IB Physics SLΒ· 5 min read π 17-19% of overall IB Physics SL exam
1. 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
Introduces core field concepts, gravitational field strength, potential, and orbital motion calculations.
β β β± 15 min
Electric fields
Extends core field concepts to electricity, covering Coulomb's law, field strength, and electric potential.
β β β β± 15 min
Magnetic fields
Explores magnetic force on moving charges and current-carrying wires in magnetic fields.
β β β β± 12 min
Electromagnetic induction
Covers Faraday's law, Lenz's law, and calculation of induced emf and current.
β β β β β± 18 min
2. Common Pitfalls
Wrong move:
Confusing gravitational potential (always negative) with gravitational potential energy.
Why:
Gravitational potential is energy per unit mass, with zero reference set at infinity, leading to negative values for all bound systems.
Correct move:
Remember potential = potential energy divided by test mass/charge, with zero potential defined at infinity for both gravity and electric fields.
Wrong move:
Mixing up the direction of magnetic force on positive vs negative charges.
Why:
Standard hand rules for magnetic force assume a positive moving charge, so direction flips for negative charges.
Correct move:
Reverse the force direction if the moving charge is negative, or use cross product rules consistently for sign.
Wrong move:
Forgetting Lenz's law when finding the direction of induced current.
Why:
Faraday's law only gives the magnitude of induced emf, direction comes from energy conservation via Lenz's law.
Correct move:
Always apply Lenz's law first to determine the direction of induced current before calculating magnitude.
3. Quick Reference Cheatsheet
Concept | Formula |
|---|---|
Gravitational field strength (point mass) | |
Coulomb's force between point charges | |
Electric field strength (point charge) | |
Magnetic force on moving charge | |
Magnetic force on current-carrying wire | |
Magnetic flux | |
Faraday's law (induced emf magnitude) |
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.