Magnets and magnetic fields — OCR GCSE Physics
Test yourself on Magnets and magnetic fields with OCR GCSE practice questions.
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Magnets and magnetic fields explained
A permanent magnet has two poles, north and south, and the rule is simple: unlike poles attract and like poles repel.
Read the full explanation
Bring a north pole near a south pole and the magnets pull together; bring two north poles (or two south poles) near each other and they push apart. You can show this with field-line diagrams. Field lines leave a north pole and enter a south pole, and they never cross. For two attracting bar magnets placed end to end, the lines run from the north pole of one across the gap into the south pole of the other, showing the pull. For two repelling magnets with like poles facing, the lines curve away from each other and do not join across the gap, showing the push. The closer the lines, the stronger the field. A question may ask you to pick the correct statement or the correct diagram, so check both the pole labels and the direction of the arrows.
describe the difference between permanent and induced magnets
A permanent magnet keeps its own magnetic field all the time, whether or not it is near anything else; a bar magnet or a fridge magnet is an example. An induced magnet only becomes magnetic while it sits in the magnetic field of another magnet, and it loses its magnetism when that field is removed. A useful example is a steel paper clip: on its own it is not magnetic, but bring it near a permanent magnet and it is attracted and behaves like a magnet for as long as it stays in the field. Induced magnetism always produces attraction towards the inducing magnet, never repulsion. A question may ask you to choose the correct description or to decide which material is permanent and which is induced, so focus on whether the magnetism is there all the time or only while another magnet is nearby.
describe the characteristics of the magnetic field of a magnet, showing how strength and direction change from one point to another (to include: diagrams to show how the strength of the field varies around them and ways of investigating this)
A magnetic field is the region around a magnet where a magnetic force acts on another magnet or on a magnetic material. Field lines show direction: by convention they run from the north pole round to the south pole outside the magnet, and the arrow on a line gives the force direction on a north pole placed at that point. Field lines show strength through spacing: lines drawn close together mark a strong field, widely spaced lines mark a weak field. Around a bar magnet the lines crowd near the poles and spread out further away, so the field is strongest at the poles and weakest at the sides and at distance. Diagrams use arrows and line spacing to convey both direction and strength. Investigation methods include plotting with a small plotting compass moved around the magnet, and sprinkling iron filings to reveal the pattern.
explain how the behaviour of a magnetic (dipping) compass is related to evidence that the core of the Earth must be magnetic
A magnetic compass needle is a small magnet that turns freely. A plotting compass held horizontally aligns with the local field, so its needle points roughly north–south, which shows the Earth behaves as though it has a magnetic field. A dipping compass is balanced so the needle can tilt in the vertical plane; away from the equator the needle dips downward, and the size of the dip changes with latitude. A freely suspended magnet therefore does not stay horizontal: it tilts because the Earth's field has a vertical component as well as a horizontal one. The consistent alignment and dip of compass needles at many places on the Earth's surface are evidence that the Earth has its own magnetic field, which is explained by a magnetic core. The field behaves as if produced by a large magnet within the Earth, with the magnetic poles near the geographic poles.
describe how to show that a current can create a magnetic effect and describe the directions of the magnetic field around a conducting wire
A current-carrying wire is surrounded by a magnetic field, and this can be shown with a plotting compass or iron filings. Place a long straight wire through a card, connect it to a cell and switch, and pass a current. A small plotting compass placed near the wire deflects; reverse the current with the switch and the needle deflects the opposite way, showing the field is produced by the current. Iron filings sprinkled on the card form concentric circles centred on the wire. Around a straight wire the field lines are circles in a plane at right angles to the wire. Looking along the wire with the conventional current flowing away from you, the field lines run clockwise; with current towards you they run anticlockwise. Reversing the current reverses the field direction.
recall that the strength of the field depends on the current and the distance from the conductor
The magnetic field around a conductor is not the same everywhere. Its strength increases as the current through the conductor increases, so a larger current gives a stronger field and a greater deflection of a nearby plotting compass. Its strength decreases as the distance from the conductor increases, so the field is strongest close to the wire and weaker further away. The two factors act independently: doubling the current strengthens the field at a fixed distance, while moving further from the wire weakens it at a fixed current. A compass placed close to a high-current wire therefore deflects more than one placed far from a low-current wire. Field lines drawn closer together represent a stronger field.
explain how solenoid arrangements can enhance the magnetic effect
A single straight wire carrying a current creates a weak circular magnetic field around it. Winding the wire into a coil, a solenoid, concentrates many turns so each turn's field adds to its neighbours' inside the coil, producing a much stronger, more uniform field along the axis. Adding an iron core magnetises the iron and strengthens the field further, making an electromagnet. Increasing the current or the number of turns per unit length also strengthens the effect. The field outside resembles a bar magnet's, with a north pole at one end and a south pole at the other. Learners should explain enhancement by reference to combining turns, adding a core and increasing current, not merely state that a solenoid is magnetic.
Your focus
- State the rule that unlike poles attract and like poles repel.
- Interpret a field-line diagram around two bar magnets to decide whether they attract or repel.
- Identify the direction of magnetic field lines leaving a north pole and entering a south pole.
Show all 21 objectives
- State that a permanent magnet has its own magnetic field at all times.
- Explain that an induced magnet is magnetic only while in another magnet's field and loses this when the field is removed.
- Recognise that induced magnetism always results in attraction towards the inducing magnet.
- Describe the direction of magnetic field lines around a magnet.
- Explain how line spacing shows where a field is strong or weak.
- Describe a method for investigating the field pattern of a magnet.
- Explain why a compass needle aligns with the Earth's magnetic field.
- Relate the dip of a compass needle to the vertical component of the Earth's field.
- Use compass behaviour as evidence that the Earth's core is magnetic.
- Describe a practical method that shows a current creates a magnetic field.
- Describe the circular shape and direction of the magnetic field around a straight conducting wire.
- Apply the right-hand grip rule to predict the field direction for a given current direction.
- Recall that magnetic field strength around a conductor increases with current.
- Recall that magnetic field strength decreases with distance from the conductor.
- Apply both factors to compare the field strength at two positions or currents.
- Explain how coiling a wire into a solenoid strengthens the magnetic effect.
- Explain how an iron core and increased current enhance the magnetic field.
- Describe the field pattern produced by a solenoid.
Magnets and magnetic fields exam tips
Marking Points
- States that unlike poles (north and south) attract each other
- States that like poles (north and north, or south and south) repel each other
- Identifies a field-line diagram in which lines run from a north pole across the gap into a south pole as showing attraction
- Identifies a field-line diagram in which lines curve away from like poles without joining across the gap as showing repulsion
- Recognises that magnetic field lines leave a north pole and enter a south pole and never cross
- States that a permanent magnet produces its own magnetic field all the time
- States that an induced magnet is only magnetic while it is in the magnetic field of another magnet
- States that an induced magnet loses its magnetism when the external field is removed
- Recognises that an induced magnet is always attracted towards the magnet that induces it
- Distinguishes a permanent magnet from an induced magnet by whether the magnetism persists without another magnet present
- States that field lines run from the north pole to the south pole outside the magnet.
- Explains that closely spaced field lines indicate a stronger field and widely spaced lines a weaker field.
- Identifies that the field of a bar magnet is strongest at its poles and weaker further away.
- Describes plotting the field with a small compass or iron filings to show direction and pattern.
- States that a compass needle is a small magnet that aligns with the local magnetic field.
- Explains that the needle dipping downward shows the Earth's field has a vertical component.
- Concludes that consistent alignment and dip of compass needles provide evidence that the Earth has a magnetic field and hence a magnetic core.
- Describes how the dip changes with position, for example being different at the equator and near the poles.
- States that a plotting compass near a current-carrying wire deflects, showing a magnetic effect.
- Describes sprinkling iron filings around a straight wire to reveal concentric circular field lines.
- Identifies that the field lines around a straight wire form circles in a plane perpendicular to the wire.
- Applies the right-hand grip rule to give the field direction for a stated current direction.
- Explains that reversing the current reverses the direction of the magnetic field.
- States that increasing the current increases the strength of the magnetic field.
- States that increasing the distance from the conductor decreases the strength of the magnetic field.
- Explains that the field is strongest close to the conductor and weaker further away.
- Identifies both current and distance as the two factors on which field strength depends.
- Identifies that winding the wire into a coil concentrates many turns so their individual fields combine and reinforce inside the coil.
- Explains that inserting an iron core becomes magnetised and strengthens the magnetic field, forming an electromagnet.
- States that increasing the current or the number of turns increases the strength of the magnetic effect.
- Describes the field pattern of a solenoid as similar to a bar magnet, with a north pole at one end and a south pole at the other.
Examiner Tips
- 💡Read the pole labels on each diagram before choosing, because the same picture can show attraction or repulsion depending on which poles face each other.
- 💡Check the arrow direction: lines leave a north pole and enter a south pole.
- 💡Eliminate options that say like poles attract or unlike poles repel, since these reverse the rule.
- 💡Ask yourself whether the object is magnetic on its own; if it needs another magnet nearby, it is induced.
- 💡Remember that induced magnetism only ever causes attraction, so reject any option describing repulsion of an induced magnet.
- 💡Use a concrete example such as a steel paper clip near a bar magnet to check each option.
- 💡Read the question for whether it asks about direction, strength or both, and answer each part.
- 💡Use the spacing of lines, not their length, as the evidence for field strength.
- 💡When describing an investigation, name the apparatus (plotting compass or iron filings) and what it reveals.
- 💡Link each observation of the compass (alignment, dip) to the conclusion about the Earth's field.
- 💡Use the word 'evidence' explicitly when explaining why the core is inferred to be magnetic.
- 💡Distinguish the horizontal alignment from the vertical dip when both are relevant.
- 💡Name the apparatus explicitly, such as a plotting compass or iron filings, rather than saying only that a field is detected.
- 💡State the plane of the circles: perpendicular to the wire, not along it.
- 💡Use the right-hand grip rule and state the current direction before giving the field direction.
- 💡Link each factor to its effect in one clear sentence: more current means stronger field, greater distance means weaker field.
- 💡Use the spacing of field lines to justify a statement about strength.
- 💡Check that both variables, current and distance, appear in your answer when the question asks what field strength depends on.
- 💡Read the question for the word explain, then give a reason for each enhancement rather than listing features.
- 💡Link each enhancement to how the individual magnetic fields combine or reinforce.
- 💡Check whether the option or answer mentions the iron core, current and number of turns, since all can enhance the effect.
Common Mistakes
- Claims that two north poles attract each other; correct this by stating that like poles repel
- Reads a diagram with lines joining across the gap as repulsion; correct this by noting that joining lines from north to south show attraction
- Assumes field lines cross where two fields meet; correct this by stating that field lines never cross
- Describes an induced magnet as keeping its magnetism after the other magnet is removed; correct this by stating that induced magnetism lasts only while the external field is present
- Claims that an induced magnet can be repelled by the inducing magnet; correct this by stating that induced magnetism always gives attraction
- Treats a permanent magnet as needing another magnet nearby to be magnetic; correct this by stating that a permanent magnet has its own field at all times
- Draws field lines running from south to north outside the magnet; reverse them to run from north to south outside.
- Treats evenly spaced lines as showing a stronger field; use closer spacing to show greater strength.
- Places the strongest field at the middle of a bar magnet; locate it at the poles instead.
- Claims a compass needle points to the geographic North Pole exactly; state that it aligns with the Earth's magnetic field, which is close to but not identical with geographic north.
- Assumes the needle always stays horizontal; explain that a dipping compass tilts because of the vertical component of the Earth's field.
- Treats the dip as random; link the consistent dip pattern to evidence for a magnetic core.
- Draws the field lines as straight lines parallel to the wire; draw them as concentric circles around the wire.
- Claims the compass needle points along the wire; note that the needle aligns with the circular field, so it turns around the wire.
- States that reversing the current leaves the field direction unchanged; state that reversing the current reverses the field direction.
- Claims that a larger current weakens the field; state that a larger current strengthens the field.
- States that field strength is the same at all distances from the wire; state that it decreases as distance increases.
- Names only one factor, such as current, and omits distance; name both current and distance.
- Claims a solenoid has no field outside the coil; correct this by noting the external field resembles a bar magnet's.
- Asserts that adding a core weakens the field; correct this by explaining the iron core becomes magnetised and strengthens it.
- Believes more turns reduce the effect; correct this by stating that more turns per unit length increase the field strength.