Physics unit guide
Fields
Gravitational, electric and magnetic fields, potential, field lines and motion within fields.
Unit 4 is where physics stops being about objects and starts being about space, and it is the unit students most often describe as the hardest. Much of the difficulty is that gravitational and electric fields are mathematically almost identical while differing in one crucial respect: gravity only attracts. Revising the two together, as one set of relationships with one sign difference, is far more efficient than treating them as separate topics.
Topics in this unit
What gets examined
- Calculating field strength and potential for point masses and point charges, and knowing which is a vector and which is a scalar.
- Sketching field lines and equipotential surfaces, and the fact that they always meet at right angles.
- Work done moving a charge or mass between two points, using potential difference rather than force.
- The motion of a charged particle in a uniform electric field, and in a magnetic field where the force is always perpendicular to velocity.
- Using F = BIL sinθ and F = qvB sinθ, including getting the direction right.
- Escape speed and orbital energy, and why total orbital energy is negative.
How to revise it
Learn gravity and electrostatics as one topic
The inverse-square forms are the same shape. Build one table with the gravitational and electric version of every quantity side by side, and the unit halves in size.
Keep vector and scalar straight
Field strength is a vector and needs direction; potential is a scalar and adds arithmetically. Confusing the two is the single most common conceptual error here.
Use potential for energy questions, not force
Work done is charge times potential difference. Integrating force is almost never what the question wants, and reaching for it wastes time you need elsewhere.
Practise the hand rules until they are automatic
Direction marks in magnetism are all-or-nothing and cannot be recovered by working. A few minutes a day beats an hour the night before.
Where marks get lost
- Adding potentials as vectors, or field strengths as scalars.
- Forgetting that gravitational potential is always negative.
- Using the distance from the surface rather than from the centre in an inverse-square calculation.
- Assuming a magnetic force does work on a charged particle - it never does, since it is perpendicular to motion.
- Dropping sinθ when the current and field are not perpendicular.
Practise this unit
Reading about a unit only goes so far. Work through real IB-style questions on it, with mark schemes.