DHS 16 Electromagnetism (Notes & Tutorial)

Dunman High School (Senior High Physics) Page 1 of 18 Guiding Questions • What is a magnet? Are there “magnetic” charges? • Do magnetic fields have effects on electric charges? • What do field lines represent? Do field lines represent similar things for electric fields and magnetic fields? • Why is the word “flux” used when talking about a magnetic field? 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 charge Learning Outcomes Candidates should be able to: Magnetic Field (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 (g) define magnetic flux density (c) use 2 oB d= µ π I , 2 Iµ= oNB r and o Iµ=Bn 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 Magnetic force on current- carrying conductors (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 = BIL sin θ, with directions as interpreted by Fleming’s left-hand rule (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 Magnetic force on moving charges (j) predict the direction of a force on a charge moving in a magnetic field (k) recall and solve problems using F = BQv sin θ (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. 9749 H2 Physics Topic 16 : Electromagnetism Year 6 (2025) DUNMAN HIGH SCHOOL

Dunman High School (Senior High Physics) Page 2 of 18 1. Magnetic Fields due to Current (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 1.1 Magnetic Effect of a Current A magnetic field can be produced by: (i) Permanent magnets (ii) Current-carrying conductors The magnetic field around a permanent magnet 1 can be seen by sprinkling some iron filings around it. In 1820, Hans Christian Oersted, a Danish professor, discovered the magnetic effect of an electric current by chance. During a class demonstration, he noticed that when a current was flowing through a wire, it caused the needle of a compass nearby to be deflected. This indicated the presence of a magnetic field. Oersted’s observation eventually led to the discovery of electromagnetism – the relationship between electricity and magnetism. 1.2 Oersted’s experiment Fig. 1 shows the result of Oersted’s experiment. It showed that a magnetic field was present when a current flowed through wire XY. The wire XY was placed in the north-south direction. No current flowed through XY. The needles of both compasses pointed to the north Current flowed through XY. The needle of compass A (placed above the wire) pointed to the east. The needle of compass B (placed below the wire) pointed to the west. (a) Open Circuit (b) Closed Circuit Fig. 1: The positions of the needles of compasses A and B in Oersted’s experiment 1 By convention, magnetic field lines leave the north pole and enter the south pole of a magnet; the field lines do not start or end at the north or south poles of a magnet, they continue to go from south pole to the north pole in the magnet. Note: The assumption in the experiment in Fig. 1 is that the Earth’s magnetic field is negligible compared to the ones generated by the wire in the vicinity of the compass.

Dunman High School (Senior High Physics) Page 3 of 18 1.3 Properties of Fields 2 The following are characteristics of field lines: 1. They are imaginary. 2. A strong field is represented by lines that are closer to one another; a weak field is represented by lines that are further apart. 3. Field lines cannot cross one another (since this would imply that at the cross -over point, the field would have two directions). 4. The direction of a field at a point in space is along a tangent to the field line at that point. 5. Field lines can be directed out of the paper (represented by dots) or into the paper (represented by crosses). The field lines will be perpendicular to the paper. 1.4 Magnetic flux pattern around a long straight current-carrying wire An experiment can be conducted to plot the magnetic flux pattern around a long straight current-carrying wire (Fig. 2abc). The magnetic flux pattern obtained consisted of concentric circles (Fig 2c) The circles nearer the wire were closer to one another. This implies that the magnetic flux density 3 was greater at regions nearer the wire. (a) A wire threaded through a cardboard sheet 2 Michael Faraday (1791–1867) introduced field lines as a way to represent fields. This creative conceptual model provides us with a powerful tool for thinking and communication about interaction at a distance. 3 The term ‘magnetic field strength’ is not used nowadays. The term used to represent field strength in a magnetic field is called ‘magnetic flux density’. x x x x x x x x x x x x x x x x Field lines pointing into the paper. Field lines pointing out of the paper. B A non-uniform field Y X The field is stronger at X than at Y Strong Field Weak Field Uniform field A uniform field has the same field strength within it.

Dunman High School (Senior High Physics) Page 4 of 18 (b) The positions of the S and N ends of the compass needle are marked with pencil dots. (c) The magnetic flux pattern of a long straight current-carrying wire Fig. 2: Experiment to determine the magnetic flux pattern of a long straight current-carrying wire 1.5 Right-Hand Grip Rule (RHGR) The RHGR is a common mnemonic for understanding notation conventions for physical quantities in 3 dimensions; it states that Version 1: w hen the thumb of the right-hand points in the direction of the conventional current, the curled fingers point in the direction of the magnetic field lines (Fig. 3a). Version 2: w hen the thumb of the right-hand points in the direction of the magnetic field lines, the curled fingers point in the direction of the conventional current (Fig. 3b). Fig. 3a: Version 1 of RHGR Fig. 3b: Version 2 of RHGR Both versions of RHGR are correct and which version to use is a matter of convenience in different circumstances. It can be applied to Fig. 4: RHGR for a long straight current-carrying wire Fig. 5: RHGR for a flat circular current-carrying coil (a) a long straight current-carrying wire (Fig. 4), easier to use “Version 1 of RHGR” (b) a flat circular current-carrying coil (Fig. 5), and easier to use “Version 2 of RHGR”. (c) a long current-carrying solenoid (coils of wire) (Fig. 6). easier to use “Version 2 of RHGR”. Flat coil with current Magnetic field lines

Dunman High School (Senior High Physics) Page 5 of 18 Fig. 6: RHGR for a long current-carrying solenoid From Fig. 6, the following observa