ASRJC Electromagnetism Notes

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 16- 1 Additional Notes Topic 16: Electromagnetism Content: A Concept of a Magnetic Field B Magnetic fields due to currents C Effects of ferrous core on magnetic field of a solenoid D Force on a current-carrying conductor E Force between current-carrying conductors F Force on a moving charge Learning Outcomes: 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 B = µoI / 2πd, B = µoNI / 2r, and B = µonI for the 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 F = B Il sin, with directions as interpreted by Fleming’s left hand rule. (g) define magnetic flux density. (h) show an understanding on 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 F = BQv sin. (l) describe and analyse deflections of beams of charged particles by uniform elect ric and uniform magnetic fields. (m) explain how electric and magnetic fields can be used in velocity selection for charged particles. μ0 = 4  10 -7 H m-1 Electromagnetism Lecture Plan Lect Concept Est. duration of videos Qns 1 A, B, C,D 43 min Eg1-3; CYU1 2 E Eg2-5 43 min Eg4,5; CYU2 3 F 48 min Eg6,7; CYU3,4

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 16-2 Additional Notes • Recall: ➢ A gravitational field i s a region in space where a mass would experience gravitational force acting on it. ➢ An electric field is a region in space where a force acts on a stationary charge. • Similarly, ➢ A magnetic field will be a region where a magnetic pole will experience a force. o However, it is not possible to have an isolated magnetic monopole as they always exist in pairs (i.e. North and South poles existing within same object), unlike charges which can exist as isolated positive or negative • Hence, a magnetic field is defined as: A region of space where a magnetic pole, a current-carrying conductor or a moving charged particle will experience a force. • The magnetic field may exist at a point as a result of the presence of: (a) either a permanent magnet or (b) a conductor carrying an electric current in the vicinity of that point. Magnetic Lines of Force • Just like gravitational and electric fields, a magnetic field can be represented by lines of force or field lines. • Magnetic field lines run from the north-pole to the south-pole outside the magnet. • The direction of a magnetic field at a point is taken as the direction where the north- seeking pole of a compass would point when the compass is placed there (only if the field is much stronger than Earth’s magnetic field). • If the field lines are parallel and equally spaced, the associated field is uniform. Otherwise the field will vary in its magnitude from one point to another. A magnetic field may be distinguished from an electric field in that a charged particle experiences a force in a magnetic field only when it is moving (not parallel) to the field. In an electric field, electric forces acts on charges whether it is moving or not. A Concept of a Magnetic Field

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 16- 3 Additional Notes B at point X X • The field is stronger if the lines of force are relatively closer together and weaker if they are relatively widely separated from one another. • Field lines do not cross one another. • If two or more magnets are present, their field strength at a point in the field is the vector sum of their individual field strengths. Magnetic Flux Density • The magnetic flux density, B, is a measure of the strength of the magnetic field in a particular region of space, i.e. B is high if many lines passing normally through a unit area. • Unit of the magnetic flux density is tesla (T) or weber per metre squared (Wb m-2). • The direction of magnetic flux density at a point in space is along a tangent to the magnetic field at that point. • Some typical values of magnetic flux density: ➢ Earth’s magnetic field (at Earth’s surface): 24 - 66 T ➢ A large permanent magnet: 0.1 T ➢ Electromagnet: approximately 1 T ➢ INUMAC, world’s most powerful MRI: 11.75 T strong uniform field weak uniform field X non-uniform field – stronger at X than at Y Y Seeing links: Recall that electric field lines are also drawn such that a) the electric field direction at any point is tangent to the electric field line at that point. b) Electric field lines never touch or cross. c) The closeness of the lines indicates the strength of the field. Refer to pg 13 for the definition of magnetic flux density.

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 16-4 Additional Notes • A conductor carrying an electric current produces a magnetic field. Different configurations of current carrying conductors give rise to different field patterns and associated magnetic flux densities. • Convention for representing current / field directions: represents a current / field directed into the plane of paper represents a current / field directed out of the plane of the paper View from Point Y View from Point Z Representing a current Current flowing from Y to Z in a conductor Representing a field Magnetic field lines in the direction from Y to Z I Y Z I I B B Z B Y B Magnetic Fields due to Currents X X X X X X X X X

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 16- 5 Additional Notes (a) Long straight wire • Magnetic flux pattern The field lines in the vicinity of a long, current -carrying wire take t he form of concentric circles. The direction of the field lines, indicated by arrows, can be predicted using the Right Hand Grip Rule. Right Hand Grip Rule • Grip the wire using the right hand, with the thumb pointing in the direction of the current. • The fingers will point in the direction of the B-field. • Drawing Guidelines: ➢ The circles are concentric. ➢ The spacing between the circles should increase as distance from wire increases. ➢ You should draw at least 3 field lines to show the difference in spacing. Spacing between the circles should increase due to B = 0 2 r   I B Current out of paper (viewed from top of cardboard) B Current into paper (viewed from bottom of cardboard)

ANDERSON SERANGOON JUNIOR COLLEGE PHYSICS 9749 16-6 Additional Notes Check your Understanding 1 What is the direction of the magnetic flux density due to each wire at point Q? Direction of magnetic flux density due to wire Z Direction of magnetic flux density due to wire Y A into the page eastwards B out of the page eastwards C out of the page westwards D into the page westwards • Expression for magnetic f