1. 20.5 Applications of magnetic forces paper cone coil magnet Moving-coil loudspeakers  A moving-coil loudspeaker contains a coil – free to move in the magnetic field of a permanent magnet Varying current passes through the coil. The coil experiences varying magnetic force. varying I

2. 20.5 Applications of magnetic forces The coil moves inwards and outwards. Sound is produced. The paper cone attached vibrates.

3. 20.5 Applications of magnetic forces side CD When a current passes through the coil... current A B C D magnetic field downward force upward force side AB side BC no force The coil rotates. Turning effect on a current-carrying coil in a magnetic field

4. 20.5 Applications of magnetic forces  The turning effect on the coil can be increased by: increasing the current increasing the strength of magnetic field winding more turns of wire increasing the area of the coil

5. 20.5 Applications of magnetic forces Moving-coil galvanometers  A moving-coil galvanometer can measure a small current. makes use of the magnetic turning effect on a current- carrying coil.

6. 20.5 Applications of magnetic forces Structure of a moving-coil galvanometer scale pointer cylindrical magnet aluminium frame soft iron core hair spring coil

7. 20.5 Applications of magnetic forces I The current flows through the coil. The coil rotates. Magnetic turning effect balances restoring turning effect of the hair springs. The coil stop rotating. current deflection Simulation: How moving-coil galvanometers work

8. 20.5 Applications of magnetic forces Simple d.c. motor  The coil rotates clockwise. A B C D observer’s eye 1 A B C D 2 Why can’t a galvanometer rotate continuously in one direction?

9. 20.5 Applications of magnetic forces  There is no magnetic turning effect.  The coil keeps turning due to inertia. A B C D observer’s eye A B C D  The turning effect is reversed.  The coil rotates anti- clockwise. When the coil is at a vertical position... 3 When the coil passes the vertical position... 4

10. 20.5 Applications of magnetic forces A B C D observer’s eye 5  The coil comes to rest at vertical position eventually.

11. 20.5 Applications of magnetic forces + –  A d.c motor can rotate in one direction continuously. commutator brush coil magnet Two half-rings are attached to coil. connects the coil to the battery.

12. 20.5 Applications of magnetic forces  The coil rotates as the current flows.  The coil keeps turning due to inertia. At the start... 1 When the coil is at the vertical position... 2 + – A B D C + – A B D C

13. 20.5 Applications of magnetic forces The contacts between the half-rings and the brushes interchanges. When the coil passes the vertical position... 3 A B D C Current in the coil is reversed. Magnetic forces acting on the coil is reversed. The coil keeps on rotating in the same direction. Simulation: How d.c. motors work

14. 20.5 Applications of magnetic forces  It contains several coils spaced at an angle to each other.  Its wires are wound around a soft iron core. magnetic field strength A practical motor  Electromagnets are used instead of permanent magnets. magnetic field strength

15. 20.5 Applications of magnetic forces That’s the end of Section 20.5 Check Point Key Ideas Previous Page Exit