Gravitational, electric and magnetic fields, induction (HL). 4 subtopics, 15 key terms.
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Force per unit mass on a small test mass, in N kg⁻¹.
Newton's law of gravitation
Every pair of point masses attracts with a force proportional to the product of the masses and inversely proportional to the square of their separation.
Kepler's third law
For orbits around the same body, T² is proportional to r³.
Orbital speed
Speed of an object in a circular orbit, where gravity provides the centripetal force.
Spherical masses act as if all their mass were at the centre: measure \(r\) from the centre of the planet, not the surface.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Add the planet's radius to the height above the surface before using r in any gravitation equation.
D.2Electric and magnetic fields
Key terms
Electric charge
A property of matter, in C. Charge is conserved and quantised in units of e = 1.60 × 10⁻¹⁹ C.
Coulomb's law
The force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation.
Electric field strength (E)
Force per unit positive charge, in N C⁻¹ (or V m⁻¹).
Magnetic field
A region where a moving charge or magnetic material experiences a force; field lines run from north to south outside a magnet.
Like charges repel, unlike charges attract. Objects become charged by friction, contact or induction when electrons are transferred.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: Square the distance and convert μC to C: these are the two most common slips in Coulomb's law questions.
D.3Motion in electromagnetic fields
Key terms
Magnetic force on a charge
F = qvB sin θ, perpendicular to both v and B.
Motor effect
A current-carrying conductor in a magnetic field experiences a force F = BIL sin θ.
Tesla (T)
Unit of magnetic field strength: 1 T = 1 N A⁻¹ m⁻¹.
A magnetic force is always perpendicular to the velocity, so it does no work: speed stays constant and a charge moving perpendicular to a uniform field follows a circle.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: State that the magnetic force acts as the centripetal force before writing qvB = mv²/r.
D.4Induction HL
Key terms
Magnetic flux (Φ)
Φ = BA cos θ, in Wb.
Flux linkage
NΦ for a coil of N turns.
Faraday's law
The induced emf equals the rate of change of flux linkage.
Lenz's law
The induced current flows in a direction that opposes the change producing it.
An emf is induced whenever the flux linking a circuit changes: moving a magnet, moving a conductor, changing a current nearby or rotating a coil.
The full notes, worked example, interactive diagram and check questions for this subtopic are in the app.
Exam tip: For Lenz's law questions, say what change happens, what the induced current's field does about it, and therefore which way the current flows.