What is being taught lesson by lesson:
- Poles of magnets, attractions and repulsion, drawing magnets.
- Permanent and induced magnets.
- Magnetic fields, forces and plotting.
- Fleming’s left-hand rule.
- Electric motors and the motor effect.
Key Terms for this topic (Tier 3 vocabulary)
Magnetic field – induced – solenoid – Fleming – magnetic flux density.
Are you ready for your assessment in this topic? Try out this simple quiz.
What everyone needs to know:
Magnetic forces are strongest at the poles, they exert forces on other magnets – like poles repel and opposite poles attract. This is a non-contact force. Permanent magnets are always producing a magnetic field based on the domains within. Electromagnets have their fields induced by the current flowing around them.
The four common magnetic metals are iron, steel, cobalt and nickel. The region aroud these magnets is called the magnetic field. The further away you get from a magnet, the weaker the force. A compass has a tiny bar magnet that is free to rotate which aligns itself with the Earth’s magnetic field.
You need to be able to explain how to draw a magnetic field using a bar magnet and a small plotting compass.
When current flows through a wire that is wrapped around an iron core, it produces an electromagnet. The force, like with a permanent magnet is stronger the closer you are to it. You can also make the magnetic force higher by adding more turns of wire around the core or increasing the current. The wire on its own is called a solenoid. You need to draw the field lines again and show the directions North to South.
Extra topics needed for the Higher papers:
Fleming’s left-hand rule allows us to show how conductors in a magnetic field move based on the direction of current flow. This movement of a current carrier in a field is called the motor-effect.
- First Finger Field (points from North to South)
- SeCond Finger Current (current flows from the hand towards the finger tip)
- ThuMb Movement (Wire moves from the hand towards the tip of the tumb).
When this conductor is conducting at right angles to the field, we can not only work out the direction it will move but also the force applied to it.
Force (N) = Magnetic Flux Density (T) x Current (A) x Length (m)
F = B I l
Electric motors rely on this principle, the current on either sides of the coil is travelling in opposite directions so the force is “up” on one side and “down” on the other which causes it to rotate. You need to be able to explain how one works.