Topic 12 –Magnetic fields 6.3.1 State that moving charges give rise to magnetic fields. 6.3.2 Draw magnetic field patterns due to currents. 6.3.3 Determine the direction of the force on a current-carrying conductor in a magnetic field. 6.3.4 Determine the direction of the force on a charge moving in a magnetic field. 6.3.5 Define the magnitude and direction of a magnetic field. 6.3.6 Solve problems involving magnetic forces, fields and currents.

Main aims Students will: Describe the magnetic fields of permanent magnets and current-carrying conductors using lines of flux. Use the terms flux and flux density in connection with magnetic fields.

Prior knowledge Students should have met ideas relating to permanent magnets, the magnetic field due to a current, and motors. Students will probably have done some electromagnetic induction, probably including transformers but ideas in this area are likely to be much hazier. There are links to be made with other fields. Both electric and gravitational fields have been studied. When considering the force on charged particles, particle accelerators etc., students will need to be familiar with the equations governing circular motion.

Where this leads An understanding of the motion of charged particles in electric and magnetic fields is vital when studying particle physics. The relationship between electromagnetic fields and electromagnetic radiation (and the work of James Clerk Maxwell), this idea is very tricky. Electromagnetic theory is one of the cornerstones of physics, and a grasp of these ideas is fundamental for students who are going on to future studies of physics, chemistry or engineering.  

Magnetic fields 6.3.1 State that moving charges give rise to magnetic fields. 6.3.2 Draw magnetic field patterns due to currents. 6.3.3 Determine the direction of the force on a current-carrying conductor in a magnetic field. 6.3.4 Determine the direction of the force on a charge moving in a magnetic field. 6.3.5 Define the magnitude and direction of a magnetic field. 6.3.6 Solve problems involving magnetic forces, fields and currents.

Need to know These include the fields due to currents in a straight wire, a flat circular coil and a solenoid. Different rules may be used to determine the force direction on a conductor. Knowledge of any particular rule is not required.

Field shapes To do Investigate the field patterns for different magnets – Brushing up on magnetism experiment Fields near currents. – A more in depth investigation of fields produced by currents.

Magnetic field around a bar magnet

Magnetic field around a straight wire

Magnetic field around a flat coil

Magnetic field around a solenoid

Flux - To do

Extension - Measuring flux For a long, straight, current-carrying wire the field is proportional to the current The 1/r relationship for distance is not always easy to confirm. Use the equation B = µoI/2πr where µo = 4π  10-7 N A-2 is a constant known as the permeability of free space For a solenoid, you should be able to check the relationship of field to both current and the number of turns per unit length. Hence B = µoNI/L For a coil wound around iron field is given B = µNI/L where µ depends on the type of iron or other magnetic core material