Section 1 of 5Magnetic Fields and Field Lines
A magnetic field is the region around a magnet where another magnet or magnetic material experiences a force.
Magnetic materials
- Iron, steel, cobalt and nickel are attracted to magnets, while materials such as copper, plastic and wood are not.
Field line direction
- Field lines are drawn with arrows pointing from the north pole to the south pole of the magnet.
Field line spacing
- Lines that are close together show a strong field, and lines that are far apart show a weak field.
Drawing rules
- Field lines never cross or touch, and they are continuous curves that leave and enter the magnet at the poles.
Section 2 of 5Magnets and Poles
Magnets and Poles
Bar magnet
- The field is strongest at the poles, where the lines are closest together, and it weakens with distance from the magnet.
Like poles
- Two north poles or two south poles repel each other, and the field lines between them bend away from each other.
Unlike poles
- A north pole and a south pole attract each other, and the field lines run straight across the gap between them.
Uniform field
- Parallel, equally spaced lines between opposite poles of two flat magnets show a field of the same strength everywhere.
| Poles brought together | Result |
|---|---|
| North and north | Repel |
| South and south | Repel |
| North and south | Attract |
Section 3 of 5Plotting and Earth's Field
1
Place a plotting compass near the north pole of a bar magnet and mark a dot at each end of the compass needle.
2
Move the compass so that its tail sits on the dot at the head of the needle, then mark a new dot at the other end.
3
Repeat these steps and join the dots to give one field line, which shows the direction a free north pole would move.
4
A compass needle is a small magnet; it lines up with the Earth's magnetic field and points towards geographic north.
5
This happens because the Earth behaves like a giant bar magnet whose magnetic south pole is near the geographic north pole.
Section 4 of 5Fields Caused by Electric Currents
Straight wire
- A current in a straight wire produces circular field lines around it, and the field is stronger closer to the wire.
Reversing current
- Reversing the direction of the current reverses the direction of the field lines around the wire.
Solenoid
- A coil of wire has a field like a bar magnet, with a strong, almost uniform field inside the coil.
Electromagnet
- A solenoid wound round an iron core is a strong magnet that can be switched on and off by the current.
Section 5 of 5Exam Checklist & Tips
1.9.1 – Key Points for the Exam
- Magnetic field The region around a magnet where a force acts on another magnet or on a magnetic material.
- Field line A line showing the direction of the force on a north pole placed in the field.
- Solenoid A long coil of wire that behaves like a bar magnet when a current flows.
- Electromagnet A solenoid with an iron core that is magnetic only while a current flows.
Common Mistakes
- Drawing field lines that cross each other or stop in the middle of nowhere.
- Putting the arrows from south to north; field lines always point from north to south outside the magnet.
- Saying that all metals are magnetic; only iron, steel, cobalt and nickel are.
- Forgetting that making the current bigger or adding turns makes an electromagnet stronger.
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.