Simple phenomena of magnetism

Key points

Poles and forces

• Every magnet has two poles: a north pole (N pole) and a south pole (S pole). The magnetic forces are strongest at the poles.
• Like poles repel. An N pole pushes another N pole away. An S pole pushes another S pole away.
• Unlike poles attract. An N pole and an S pole pull towards each other.
• A magnetic material is a material that a magnet attracts. Iron, steel, nickel and cobalt are magnetic materials.
• A non-magnetic material is not attracted by a magnet. Examples: copper, aluminium, plastic, wood and glass.
• A piece of magnetic material can be magnetised (it is a magnet itself) or unmagnetised (it is not a magnet).
• A magnet attracts an unmagnetised magnetic material. Either pole of the magnet will attract it.
• A magnet never repels an unmagnetised material. Repulsion happens only between two magnets. So repulsion is the test for a magnet.

Induced magnetism

• Induced magnetism: a piece of unmagnetised magnetic material becomes a magnet when it is near a magnet or touching it.
• The end of the material nearest the magnet becomes the opposite pole. For example, the end nearest an N pole becomes an S pole.
• Unlike poles attract. This is why a magnet attracts an unmagnetised piece of iron.
• Example: a chain of paper clips can hang from one magnet. Each clip is magnetised by induction and attracts the next one.

Temporary and permanent magnets

• Soft iron is easy to magnetise. It also loses its magnetism easily when the magnet (or the current in a coil) is taken away. So soft iron makes temporary magnets.
• Steel is harder to magnetise. Once it is magnetised, it keeps its magnetism. So steel makes permanent magnets.
• Soft iron is used where the magnetism must switch off, for example the core of an electromagnet or a relay (see 4.5).

Magnetic fields

• A magnetic field is a region where a magnetic pole feels a force.
• We show a magnetic field with magnetic field lines.
• The direction of a magnetic field at a point is the direction of the force on an N pole placed at that point. A compass needle’s N end points in this direction.
• Around a bar magnet, the field lines:
  • come out of the N pole, curve round the magnet, and go into the S pole
  • have arrows that point from N to S outside the magnet
  • never cross or touch each other
  • are closest together near the poles.
• The spacing of the field lines shows the relative strength of the field. Lines closer together mean a stronger field. Lines further apart mean a weaker field. Extended
• So the field of a bar magnet is strongest at the poles, where the lines are closest. Extended
• A magnet has its own magnetic field. When two magnets are close, their fields interact. This interaction between the two fields causes the magnetic forces of attraction and repulsion. Extended

Plotting a magnetic field

• With a plotting compass (a small compass):
  1. Put the magnet on a sheet of paper and draw round it.
  2. Put the compass near the N pole. Mark a dot at the end the needle’s N end points to.
  3. Move the compass so the other end of the needle is over the dot. Mark a new dot.
  4. Repeat until you reach the S pole.
  5. Join the dots with a smooth curve. Add an arrow pointing the way the compass N end pointed.
  6. Start again from other points near the N pole to draw more lines.
• A compass also shows the direction of the field at any point: the field points the way the needle’s N end points.
• With iron filings: put a sheet of paper or card over the magnet. Sprinkle iron filings on it and tap it gently. The filings line up along the field lines.
• Iron filings show the pattern of the field quickly. They do not show its direction. You need a compass for the direction.

Uses of magnets

• A permanent magnet needs no electricity and stays magnetic. Uses: compasses, fridge door seals and magnetic catches, loudspeakers, and some motors and generators.
• An electromagnet is a coil of wire, usually around a soft-iron core. It is a magnet only while a current flows in the coil (see 4.5.3).
• An electromagnet is useful because it can be switched on and off, and its strength can be changed by changing the current.
• Uses of electromagnets: lifting and dropping scrap iron and steel in a scrapyard, relays, electric bells and trip switches.

Common mistakes

• Students write that a magnet attracting an object proves the object is a magnet. / The mark scheme wants: only repulsion proves it. A magnet also attracts unmagnetised iron or steel.
• Students write that all metals are magnetic. / The mark scheme wants: only some metals (iron, steel, nickel, cobalt) are magnetic. Copper and aluminium are not.
• Students draw field lines from S to N, or leave out the arrows. / The mark scheme wants arrows pointing from N to S outside the magnet.
• Students draw field lines that cross or that stop in mid-air. / The mark scheme wants smooth lines that start on the N pole, end on the S pole and never cross.
• Students write that permanent magnets are made of soft iron. / The mark scheme wants: steel for permanent magnets; soft iron for temporary magnets.
• Students write that the field is strongest in the middle of the magnet. / The mark scheme wants: strongest at the poles, where the field lines are closest together. Extended

Exam tips

• Draw the field of a bar magnet: several smooth lines from N to S, arrows on every line, no crossing lines, and lines closer together near the poles.
• Describe an experiment to plot a field: name the apparatus (plotting compass, paper, pencil), then give the steps in order. Say how you get the direction.
• State the difference between soft iron and steel with two parts: soft iron is easy to magnetise and easy to demagnetise; steel keeps its magnetism.
• Explain a choice of material by linking a property to the job. Example: “Soft iron is used in an electromagnet because it loses its magnetism when the current is switched off.”
• A typical 1-mark answer: “A magnetic field is a region where a magnetic pole feels a force.”
• A typical 2-mark answer to “How can you show that a steel bar is a magnet?”: “Bring one end near the N pole of a magnet, then near the S pole (1). If the bar repels one of them, it is a magnet (1).”
• A typical 2-mark answer to “Why does a magnet pick up an iron nail?”: “The nail becomes magnetised by induction (1). The end nearest the magnet becomes the opposite pole, so it is attracted (1).”
• A typical 1-mark answer: “Closer field lines show a stronger magnetic field.” Extended