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6.1.2Magnetism and Electromagnetism

Magnetic Fields

5 min read 5 parts Lesson 3 of 16 1 quiz

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Section 1 of 5Field Lines

6.1.2 Magnetic Fields

Field lines are a model that shows what the magnetic field is doing at each point in space.

Magnetic field
  • A region in which a magnet or a magnetic material experiences a force, such as the space around a bar magnet.
Direction
  • Shown by arrows on the field lines, which point from the north pole towards the south pole outside a magnet.
Strength
  • Shown by spacinglines close together mean a strong field, and lines far apart mean a weak field.
Drawing rules
  • Field lines never cross or touch, because the field has only one direction at any point.

Section 2 of 5Patterns Around Magnets

The pattern depends on whether the poles are alone, facing like poles or facing unlike poles.

Patterns Around Magnets
Single bar magnet
  • The field is strongest at the poles, where the lines are closest, and weakens with distance from the magnet.
Like poles facing
  • The lines bend away from the gap between the two north poles, and a neutral point lies midway where the fields cancel.
Unlike poles facing
  • Lines run from the north pole of one magnet to the south pole of the other, showing that the magnets attract.
Uniform field
  • Parallel, evenly spaced lines appear in the gap between two opposite poles placed a few centimetres apart.

Section 3 of 5Mapping a Field with a Compass

A plotting compass shows the field direction at each point, because its needle aligns with the local field.

1
Place a bar magnet on paper and mark a dot beside one end of the magnet.
2
Place a small plotting compass next to the dot so that one end of its needle points away from the dot, then mark a new dot at the other end of the needle.
3
Move the compass so its needle points away from the newest dot, and repeat until the chain of dots reaches the far end of the magnet.
4
Join the dots with a smooth curve, then repeat to build a complete set of field lines with arrows from north to south.

Section 4 of 5Worked Example

Question Explain where the magnetic field of a bar magnet is strongest.

  1. Identify the region where the field lines are most closely packed.
  2. Recall that closely spaced lines show a strong field.
  3. Note that the lines are closest together at the poles of the magnet.

Answer The field is strongest at the poles, where the field lines are closest together.

Section 5 of 5Exam Checklist & Tips

6.1.2 – Key Points for the Exam

  • Magnetic field A region in which a magnet or magnetic material experiences a force.
  • Field line A line showing the direction of the field, with arrows pointing from north to south outside a magnet.
  • Uniform field A field with the same strength and direction at every point, shown by parallel, evenly spaced lines.
  • Neutral point A point where the magnetic fields from two magnets cancel, so the net field is zero.
  • Plotting compass A small magnetised needle used to find the direction of a magnetic field at a point.

Common Mistakes

  • Drawing field lines that cross or touch each other, which is never allowed.
  • Drawing arrows from south to north outside a magnet; outside the magnet the field runs from north to south.
  • Stating that the field is strongest in the middle of a bar magnet rather than at its poles.
  • Describing a uniform field as curved lines; a uniform field has parallel, evenly spaced lines.
Transformer Golden Equations Physics Standard

Transformer Turns Ratio & Efficiency

Ratio: Vp / Vs = Np / Ns = Is / Ip

  • Step-up reduces grid thermal loss (P = I2R).
  • Laminated core prevents eddy currents.