EJC Physics H216 Electromagnetism -1. Notes (2024)_for print

Magnetically levitated (maglev) trains exploit the physics of electromagnetism to (i) lift a train off the tracks to minimize friction and (ii) accelerate or perform regenerative braking of the train. Content • Concept of a magnetic field • Magnetic fields due to currents • Force on a current-carrying conductor • Force between current-carrying conductors • Force on a moving charge Learning Objectives: Candidates should be able to: (a) show an understanding that a magnetic field is an example of a field of force produced either by current -carrying conductors or by permanent magnets (b) sketch flux patterns due to currents in a long straight wire, a flat circular coil and a long solenoid (c) use 00 0 an 2 2 d BB ,d Br  = = =I NI nI for flux densities of the fields due to currents in a long straight wire, a flat circular coil and a long solenoid respectively (d) show an understanding that the magnetic field due to a solenoid may be influenced by the presence of a ferrous core (e) show an understanding that a current-carrying conductor placed in a magnetic field might experience a force (f) recall and solve problems using the equation sin FB = Il , with directions as interpreted by Fleming's left -hand rule (g) define magnetic flux density (h) show an understanding of how the force on a current-carrying conductor can be used to measure the flux density of a magnetic field using a current balance (i) explain the forces between current-carrying conductors and predict the direction of the forces (j) predict the direction of the force on a charge moving in a magnetic field (k) recall and solve problems using sin F BQv = (l) describe and analyse deflections of beams of charged particles by uniform electric and uniform magnetic fields (m) explain how electric and magnetic fields can be used in velocity selection for charged particles.

Earlier in gravitational fields and electric fields, we described the directions along which bodies will experience forces, and quantitatively found the field strengths. Back at the secondary level we could deduce the direction of force that will act on a current-carrying conductor subject to a magnetic field. At the A-Levels, we will go on further to quantify how strong a magnetic field is. Curiously the definition reads as “ may experience a force”; recall that if the direction of current is parallel to the magnetic field lines, there will be no force due to the magnetic field. Strength of a magnetic field is represented by a vector quantity called the magnetic flux density B. uniform magnetic field non-uniform magnetic field Parallel and equally -spaced lines. Number of lines passing through an area (for comparison) is the same. As with gravitational fields and electric fields, we represent magnetic fields using magnetic field lines. Field lines do not intersect. When two or more fields interact, the resultant is a vector addition.