3.1.9Magnetism and Electromagnetism
Magnetic Field Patterns
5 min read 4 parts Lesson 10 of 15 Unit quiz
Section 1 of 4Straight Wire
Shape
- The field around a straight wire is a set of concentric circles centred on the wire, with no poles.
Spacing
- The circles get further apart as the distance from the wire increases, showing that the field is weaker further away.
Right-hand thumb rule
- Point the thumb along the current and curl the fingers; they show the direction of the field around the wire.
Reversing current
- Reversing the current reverses the field direction, and no current means no field.
Increasing current
- A larger current strengthens the field, so the circles move closer together.
Section 2 of 4Coil and Solenoid
The solenoid is the most useful pattern to remember.
Coil and Solenoid
Flat circular coil
- The field lines loop through the centre of the coil, where the field is strongest.
Solenoid
- A long coil makes the fields of each turn add up inside it, producing a strong, almost uniform field along its centre.
Outside
- Outside the solenoid the field spreads out and is weaker, with a pattern like that of a bar magnet.
Strengthening
- Increase the current, add more turns, or place an iron core in the coil.
Section 3 of 4Finding the Poles
1
Look at one end of the solenoid and note the direction of the current going round the end.
2
If the current flows clockwise when viewed from that end, that end is a south pole.
3
If the current flows anticlockwise when viewed from that end, that end is a north pole.
4
The field lines inside the solenoid run from the south pole end to the north pole end, and outside they run from north to south.
5
Reversing the current swaps the poles, and with no current there is no field at all.
| Arrangement | Field shape | Poles? |
|---|---|---|
| Straight wire | Concentric circles | No |
| Flat circular coil | Loops through the centre | Faces act like poles |
| Solenoid | Like a bar magnet; uniform inside | Yes, one at each end |
Section 4 of 4Exam Checklist & Tips
3.1.9 – Key Points for the Exam
- Magnetic field The region around a magnet or current where a magnetic material experiences a force.
- Field line A line showing the direction a free north pole would move at each point.
- Concentric Sharing the same centre, like the rings around a straight wire.
- Uniform field A field with the same strength and direction everywhere, shown by evenly spaced parallel lines.
- Right-hand rule A hand method for finding the direction of a magnetic field around a current.
Common Mistakes
- Drawing field lines that cross or touch; they never do.
- Forgetting arrows on field lines, so that the direction is missing.
- Mixing up clockwise and anticlockwise when finding the pole of a solenoid.
- Saying a straight wire has a north and a south pole; its field has no poles.
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.