Magnetism and Magnetic Fields | OCR GCSE Physics A, Higher tier (J249)

Magnetism and Magnetic Fields

135 questions
8 subtopics
Paper 3
Paper 3

Magnetism and Magnetic Fields is examined in Physics A Paper 3.

It covers permanent and induced magnets, and magnetic poles, magnetic fields, and the dipping compass, the magnetic effect of a current, and solenoids, the motor effect and Fleming's left-hand rule, the force on a conductor, and how a motor turns, induced potential, alternators and dynamos, transformers and the turns ratio and the microphone.

Sample questions from Magnetism and Magnetic Fields

Answer each one closed book first, then open the answer.

Permanent and induced magnets, and magnetic poles

Describe the field lines in the gap when a north pole and a south pole are placed facing each other.

Show the answer
They run straight across the gap from the north pole to the south pole, linking the two magnets together.
Magnetic fields, and the dipping compass

How does a plotting compass show the direction of a magnetic field at a point?

Show the answer
Its needle turns until the north end points along the field line at that point.
The magnetic effect of a current, and solenoids

What two things decide the strength of the magnetic field at a point near a straight current-carrying wire?

Show the answer
The size of the current and the distance from the wire.
The motor effect and Fleming's left-hand rule

Give two ways of increasing the force on a current-carrying wire in a magnetic field.

Show the answer
Increase the current in the wire, or use a stronger magnetic field.
The force on a conductor, and how a motor turns

A wire of length 0.60 m lies at right angles to a field of magnetic flux density 0.15 T and feels a force of 1.8 N. Calculate the current in the wire.

Show the answer
The current is 20 A.
Induced potential, alternators and dynamos

A magnet is pulled out of a coil instead of being pushed in. What happens to the induced potential difference?

Show the answer
It is induced in the opposite direction.
Transformers and the turns ratio

Why is the core of a transformer made of iron?

Show the answer
Iron is magnetised very easily, so it carries the changing magnetic field from the primary coil across to the secondary coil.
The microphone

Explain how a dynamic microphone produces an electrical signal from a sound wave.

Show the answer
The pressure variations in the sound make the diaphragm and its coil vibrate near the magnet, so the magnetic field through the coil keeps changing and induces a varying potential difference across it.

The 8 subtopics

One subtopic is one session. Work down the list.

Subtopic	What it covers	Questions
Permanent and induced magnets, and magnetic poles	Attraction and repulsion between poles as a non-contact force, the field lines between like and unlike poles and the neutral point, how the force weakens with distance, permanent and induced magnets, why induced magnetism only attracts, the magnetic metals, and why steel suits permanent magnets and iron suits electromagnets.	17
Magnetic fields, and the dipping compass	What a magnetic field is, where it is strongest and how line spacing shows strength, field direction, plotting a field with a compass or iron filings, the dipping compass and what its dip shows about the field and core of the Earth, and why the magnetic pole in the far north behaves as a south pole.	17
The magnetic effect of a current, and solenoids	Showing that a current has a magnetic effect, the circular field around a straight wire and the right-hand grip rule, how current and distance change the field strength, the field inside and outside a solenoid, ways of making a solenoid stronger, and electromagnets.	21
The motor effect and Fleming's left-hand rule	The force on a current-carrying wire in a magnetic field and the equal force on the magnet, how the angle to the field, the current and the field strength change the force, what reversing the current or field does, and Fleming's left-hand rule with worked directions.	16
The force on a conductor, and how a motor turns	Force = magnetic flux density × current × length, the tesla and the units needed, when the equation applies, calculations of force, flux density, current and length, why a coil turns in a field, reversing the current every half turn, and making a motor faster or reversing it.	18
Induced potential, alternators and dynamos	Electromagnetic induction and ways to induce a potential difference, when a current flows and how its field opposes the change, making the induced potential difference larger, and how an alternator with slip rings and a dynamo with a split-ring commutator produce alternating and direct current.	16
Transformers and the turns ratio	The primary and secondary coils and the iron core, how a changing field transfers energy and why a transformer needs alternating current, step-up and step-down transformers, the turns ratio equation with calculations, stepping up for the grid, and power in and out of an ideal transformer.	21
The microphone	What a microphone does, which part the sound moves, the coil attached to the diaphragm, how the induced signal is produced, and how louder or higher-pitched sounds change it.	9

Magnetism and Magnetic Fields is 135 of the 1,584 questions in the guide.Get the guide, £7

How the guide is worked

Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.

Step 1 · Closed book
Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.
Step 2 · Open book
Go back to the top. Read each printed answer and write it out in full, including the ones you had right.
Step 3 · Closed book again
Same questions, same order, from memory. The gap between pass one and pass three is the session result.

Read the full method, the return schedule and the research behind it.