0625 · 4.1Simple phenomena of magnetism

Poles and magnetic materials

0:00A magnet can pull an iron nail across a gap without touching it. Another magnet can be pushed away across that same gap. These effects follow a small set of connected ideas about poles, materials and magnetic fields. By the end of this video, you will understand these connections and use them to explain magnetic behaviour. Start with the idea of a force. A force is a push or a pull, and it can change an object’s motion.

0:23Magnetic forces act without contact, so the objects do not need to touch. Attraction pulls objects towards each other, while repulsion pushes them apart. Every bar magnet has a north pole and a south pole. Its magnetic effects are strongest near these poles, which lie at opposite ends. Like poles repel, whether both are north or both are south. Unlike poles attract, so a north pole attracts a south pole.

0:48Both magnets experience a force during attraction or repulsion. Turning one magnet around changes which poles face each other, so it alters the interaction. This makes repulsion especially useful when identifying an unknown magnet. Attraction alone is less decisive, because unmagnetised iron and steel are also pulled towards a magnet. Magnetic materials can be magnetised, and they are attracted to a magnet even when unmagnetised. Important examples are iron, steel, nickel and cobalt.

1:17Non-magnetic materials, such as wood, plastic and copper, are not attracted to a magnet in this way. Being a metal does not automatically make a material magnetic. An unmagnetised iron nail is therefore different from a non-magnetic plastic object. The nail can become magnetised, meaning that it develops magnetic poles and behaves as a magnet. The plastic cannot become a magnet in this way. This distinction explains why a magnet can attract an ordinary iron nail that starts out unmagnetised.

Induced magnetism and magnetic materials

1:45When a material becomes magnetised by being placed in a magnetic field, this is called induced magnetism. Bring the north pole of a magnet near an unmagnetised iron nail. The nearer end of the nail becomes a south pole. Its farther end turns into a north pole. The nail is now an induced magnet. The nearer poles are unlike, so they attract. If you turn the original magnet around, the induced poles in the nail reverse.

2:10Its nearest end again becomes the opposite pole to the approaching magnet. This explains why either pole of a magnet attracts an unmagnetised magnetic material. Induced magnetism does not require the objects to touch. The magnetic field can magnetise the nail across a gap. The induced nail can then attract another iron object, because it now produces its own magnetic field. Whether it remains magnetised afterwards depends on the material used.

2:35Soft iron is easily magnetised, and it also loses its magnetism readily. It usually loses most of its magnetism when the magnetising field is removed, making it suitable for temporary magnets. Steel is harder to magnetise, but it retains its magnetism much more effectively. This makes steel suitable for permanent magnets. Temporary and permanent describe how well the material keeps its magnetism, not simply how strong its magnetic force is.

3:01A temporary magnet can produce a strong attraction while it is magnetised. A permanent magnet keeps its magnetism after the external magnetising field is removed. Here, the word soft describes magnetic behaviour, not how the metal feels to touch.

Magnetic fields and field lines

3:16A magnetic field is a region in which a magnetic pole experiences a force. This definition connects the invisible field to an effect we can detect. Place a small magnet nearby, and its poles experience forces. Magnetic field lines help us represent the field’s pattern and direction on a diagram. Outside a bar magnet, magnetic field lines run from the north pole to the south pole. They curve around the magnet, spreading out away from the poles.

3:42To draw the pattern, start with a labelled bar magnet and add several smooth curves around it. Put arrowheads on the curves pointing from north towards south outside the magnet. The direction of a magnetic field at a point is the direction of the force on the north pole of a magnet at that point. Think of placing a tiny compass at that position. Its north pole points along the local field direction.

4:03The direction follows the curve at that point, rather than always pointing directly towards the magnet. Field lines never cross, because the magnetic field has one direction at any particular point. Crossing lines would incorrectly show two different directions at the same position. The lines are not physical strings surrounding the magnet. They are a drawing convention that represents an invisible field. For the Supplement, or Extended, content, line spacing represents the relative strength of a magnetic field.

4:33Closely spaced lines indicate a stronger field, while wider gaps show a weaker one. Around a bar magnet, the lines are most concentrated near the poles. This matches the stronger magnetic effects observed there. The other Extended idea is that magnetic forces are due to interactions between magnetic fields. Each magnet produces a field, and the fields interact when the magnets are brought together. Depending on their orientation, the magnets experience attraction or repulsion.

5:00Field lines show the resulting pattern, but they are not objects that collide or pull like ropes.

Plotting fields and identifying magnets

5:05A plotting compass lets us map the field experimentally. Place a bar magnet on a table, cover it with paper, and mark its outline and poles. Position the compass near one pole and mark the positions of both ends of its needle. The needle aligns with the magnetic field at that position. Move the compass so that one end of its needle lies over the previous mark made by the other end.

5:26Mark the new position of the needle’s other end, then repeat the process. Join the resulting dots with a smooth curve to trace one field line. Use the compass’s north pole to determine the arrow direction. Repeat from several different starting positions to build up the pattern around the magnet. Each compass reading gives the direction at one particular location. Following a sequence of readings reveals how that direction changes through space.

5:51Keep other magnets and magnetic objects away, because their fields could affect the compass. Iron filings provide another way to reveal the pattern. Place paper over a bar magnet, sprinkle the filings thinly, and tap it gently. The filings become temporarily magnetised and arrange themselves along the local field. The result shows curved patterns with a greater concentration near the poles. Iron filings show the shape of the field, but they do not directly reveal its direction.

6:17The pattern alone does not tell you which way arrowheads should point. Use a compass to determine direction, or use the known positions of the magnet’s poles. For a practical investigation, the compass provides a direct directional measurement. Consider a worked example involving an unknown steel bar. Its left end repels the north pole of a known magnet. The task is to identify that end and decide whether the bar is magnetised.

6:41Begin with repulsion, because this observation gives stronger evidence than attraction alone. Like poles repel, so the steel bar’s left end must be a north pole. Its opposite end must therefore be a south pole. The bar must already be a magnet, because unmagnetised steel would gain induced poles and be attracted instead. Repulsion confirms that the unknown object is already acting as a magnet. Suppose the bar had been attracted instead.

7:07Its nearest end could have been the south pole of an existing magnet. Alternatively, the bar could have been unmagnetised steel that became magnetised by induction. Attraction alone would not distinguish these possibilities, so it could not prove that the bar was already magnetised.

Magnetic applications and exam recap

7:22Permanent magnets are useful when a magnetic effect is needed continuously. Examples include cupboard catches and magnetic compasses. An electromagnet produces a magnetic field when electric current flows through its coil. Its main practical advantage is that its magnetic effect can be switched on and off. A scrapyard crane uses an electromagnet to pick up magnetic materials and release them when the current is switched off. A soft iron core is suitable because it is easily magnetised and readily loses its magnetism afterwards.

7:52Electromagnets are also used in relays and electric bells. How current produces these fields belongs in the video on electromagnetic effects. In examinations, you may be asked to describe induced magnetism, compare soft iron with steel, or draw a magnetic field. You may also need to explain a practical method for plotting field lines. Pay attention to the command word: describe what happens, or explain the reason behind it. For drawings, include both the field pattern and its direction.

8:20Try this exam-style situation: a north pole approaches an unmagnetised soft iron bar, then is removed. Describe the induced poles and attraction, and state what happens afterwards. First identify the end nearest the original magnet, then apply the rule for induced poles. Finally, use the properties of soft iron rather than those of steel. The end nearest the magnet becomes a south pole. The farther end becomes a north pole.

8:45The bar is attracted because unlike poles face each other. The soft iron loses most of its induced magnetism when the original magnet is removed. The main mistakes come from confusing different ideas. A magnetic material is not necessarily already magnetised, and attraction does not prove that an object is a permanent magnet. Iron filings reveal the field pattern, not its direction. Widely spaced field lines indicate a weaker field, not a stronger one.

9:11To recap, like poles repel and unlike poles attract. Magnets attract unmagnetised magnetic materials by inducing opposite poles at their nearer ends. Soft iron is suitable for temporary magnets, while steel retains magnetism for permanent magnets. These material properties help us choose magnets for different applications. A magnetic field is a region in which a magnetic pole experiences a force. Outside a bar magnet, field lines point from north to south, and closer spacing represents greater strength.

9:42A compass reveals direction, while iron filings reveal the pattern. Magnetic forces arise from interactions between fields, connecting all these observations into one explanation.