2. 1 16.3 Magnetic force on current-carrying wires Warm-up Introduction Force on a current-carrying conductor Combined magnetic field Moving-coil loudspeakers Check-point 3 ?

3. 2 16.3 Magnetic force on current-carrying wires Warm-up 1What happens if a current flows along…What happens if a current flows along… 2Try to draw the magnetic field…Try to draw the magnetic field…

4. 3 16.3 Magnetic force on current-carrying wires Warm-up 1In 16.2, we learnt that a fixed current-carrying wire deflects a movable compass needle (Fig a). AThe wire moves. BThe wire does not move. CIt may or may not move. It cannot be predicted. What happens if a current flows along a movable wire between fixed magnets (Fig b)? Fig a Fig b

5. 4 16.3 Magnetic force on current-carrying wires Warm-up 2Below shows a side-view of: Try to draw the magnetic field lines of (a) the magnet, What happens when the field lines from the magnet and the wire 'cross' each other. (b) the wire.

6. 5 16.3 Magnetic force on current-carrying wires 0Introduction If a current flows in a conductor and the conductor makes an angle with magnetic field lines, then a force acts on the conductor. This explains why a loudspeaker paper cone vibrates and an electric motor rotates. We can do an experiment to find out about the force on a current-carrying conductor.

7. 6 16.3 Magnetic force on current-carrying wires Experiment 16d Investigate the force acting on a current- carrying wire in a magnetic field. Force on a current-carrying conductor Video slab-shaped magnets on steel yoke to low-voltage power supply

8. 7 16.3 Magnetic force on current-carrying wires 1Force on a current- carrying conductor When wire carries a current I, wire is moved upwards by an upward force. I force I If the direction of either I or B-field is reversed, wire is moved downwards by a downward force.

9. 8 16.3 Magnetic force on current-carrying wires 1Force on a current- carrying conductor How to determine the direction of force on the current-carrying wire? I force I

10. 9 16.3 Magnetic force on current-carrying wires Fleming’s left-hand rule: Hold the thumb, first and second finger of LEFT HAND at right angles. force F magnetic field B current, I F B I NOT This is a rule to help you to remember their directions. (NOT a physics principle!)

11. 10 16.3 Magnetic force on current-carrying wires Fleming’s left-hand rule: Remarks: (1) If current & field NOT at right angles,  force  (2) If current & field parallel,  NO force (3) Force  if I  use stronger magnet length of wire inside magnetic field 

12. 11 16.3 Magnetic force on current-carrying wires 2Combined magnetic field Video (00:42-) This apparatus is used to show 1circular field lines due to I in a straight wire 2 straight field lines of 2 slab-shaped magnets 3the combined field of (1) & (2) 10-turn coil of enamelled copper wire demonstration meter 0  5 A low voltage supply slab-shaped magnets plastic plate

13. 12 16.3 Magnetic force on current-carrying wires 2Combined magnetic field Results:  + neutral point It appears: the wire is being ‘catapulted’ in a direction from strong field region to week field region. =

14. 13 16.3 Magnetic force on current-carrying wires 3Moving-coil loudspeakers

15. 14 16.3 Magnetic force on current-carrying wires 3Moving-coil loudspeakers Wire  field Contains a short-coil: free to move inside a cylindrical permanent magnet. coil circular pole short coil stiff paper or plastic cone radial magnetic field

16. 15 16.3 Magnetic force on current-carrying wires 3Moving-coil loudspeakers coil circular pole short coil stiff paper or plastic cone radial magnetic field As a.c. passes through the coil,  coil is pushed in & out  gives out sound waves I flows backwards & forwards

17. 16 16.3 Magnetic force on current-carrying wires e.g. 4 Interaction between current-carrying wires Question ? Example 4

18. 17 16.3 Magnetic force on current-carrying wires Check-point 3 1What is the direction of...What is the direction of... 2Describe the direction of...Describe the direction of... 3Redraw the diagram so that...Redraw the diagram so that... 4A magnetic force acts on the...A magnetic force acts on the... 5A magnet exerts a force on...A magnet exerts a force on...

19. 18 16.3 Magnetic force on current-carrying wires Q1What is the direction of... What is the direction of the magnetic force acting on the wire MN and OP? P O M N Segment MN Segment OP A upwardsupwards B downwardsdownwards C upwardsdownwards D downwardsupwards

20. 19 16.3 Magnetic force on current-carrying wires Q2Describe the direction of... Describe the direction of magnetic force acting on the conductor AB. AB The current is parallel to the magnetic field and no magnetic force acts on AB.

21. 20 16.3 Magnetic force on current-carrying wires Q3Redraw the diagram so that... Redraw the diagram so that the magnetic force acting on the conductor AB is greatest and pointing out of the paper. A B A B

22. 21 16.3 Magnetic force on current-carrying wires Q4A magnetic force acts on the... A magnetic force acts on the current-carrying metal rider and makes the rider move (see Experiment 16d). Does the magnetic force do work on the rider? (Yes / No)

23. 22 16.3 Magnetic force on current-carrying wires Q5A magnet exerts a force on... A magnet exerts a force on a current-carrying wire. For two parallel current-carrying wires, does one of them exert a force on the other? (Yes / No)

24. 23 16.3 Magnetic force on current-carrying wires The End

25. 24 16.3 Magnetic force on current-carrying wires Example 4 Interaction between current-carrying wires 2 parallel wires A & B, carry the same size of current. (a)Draw a few magnetic field lines due to the current through wire A. AB

26. 25 16.3 Magnetic force on current-carrying wires Example 4 Interaction between current-carrying wires 2 parallel wires A & B, carry the same size of current. (b)Mark the direction of this magnetic field at wire B with an arrow. AB field at B due to current through A

27. 26 16.3 Magnetic force on current-carrying wires Example 4 Interaction between current-carrying wires 2 parallel wires A & B, carry the same size of current. (c)Mark the direction of magnetic force acting on B due to magnetic field of the current through A. AB field at B due to current through A magnetic force

28. 27 16.3 Magnetic force on current-carrying wires Example 4 Interaction between current-carrying wires 2 parallel wires A & B, carry the same size of current. (d)Deduce the direction of magnetic force acting on A due to magnetic field of the current through B. AB field at B due to current through A magnetic force field at A due to current through B

29. 28 16.3 Magnetic force on current-carrying wires Example 4 Interaction between current-carrying wires 2 parallel wires A & B, carry same size of current. (e)Do A & B attract or repel each other? AB field at B due to current through A magnetic force field at A due to current through B They attract each other.

30. 29 16.3 Magnetic force on current-carrying wires Return