2. 1 16.2 Magnetic field of electric currents Warm-up Introduction Field around a straight wire Field around a flat coil Field due to a solenoid Electromagnets Applications of electromagnets Check-point 2 ? ?

3. 2 16.2 Magnetic field of electric currents Due to Earth's magnetic field, the compass needle will always point to a definite direction. Warm-up How would you guess about the reason of the following phenonmenon?

4. 3 16.2 Magnetic field of electric currents Warm-up AThere is wind blowing when the current flows along the wire. BA magnetic field, other than the Earth's field, exists around the compass when current flows along the wire. COthers: ______________________

5. 4 16.2 Magnetic field of electric currents 0Introduction Magnetism is produced not only by magnets but also by electric currents. We can carry out an experiment to find out about the magnetic fields set up by current through conductors of various shapes.

6. 5 16.2 Magnetic field of electric currents Experiment 16b Use iron filings to show the magnetic fields. Magnetic fields from currents Video

7. 6 16.2 Magnetic field of electric currents 1Field around a straight wire Circles around the wire Strongest close to the wire Current   stronger field Reversing current  reverses the direction of field lines

8. 7 16.2 Magnetic field of electric currents current: up out of paper current: down into page wire (end view)  Magnetic field lines around a wire Symbols for current direction: current direction

9. 8 16.2 Magnetic field of electric currents 1Field around a straight wire Right-hand grip rule If right hand grips the wire, so that thumb points in the same direction as the current  fingers curl in the same direction as the magnetic field lines current direction field direction

10. 9 16.2 Magnetic field of electric currents 2Field around a flat coil At the centre: magnetic field lines are straight & at right angles to the plane. Outside the coil: magnetic field lines run in loops. circular coil current direction

11. 10 16.2 Magnetic field of electric currents 3Field due to a solenoid Solenoid: a long coil of many turns of wire. When I flows through the solenoid, each turn acts as a single coil & produce a magnetic field. I I

12. 11 16.2 Magnetic field of electric currents 3Field due to a solenoid I I Inside the solenoid: straight and evenly-spaced field lines; uniform field strength Outside the solenoid: field pattern similar to that around a bar magnet

13. 12 16.2 Magnetic field of electric currents 3Field due to a solenoid NS To determine the poles of a solenoid: If right hand grips solenoid, so that fingers curl in the same direction as the current  thumb points to N-pole of solenoid Remarks:  magnetic field of solenoid by  current  no. of turns on coil Right-hand grip rule for solenoid

14. 13 16.2 Magnetic field of electric currents Experiment 16c Make an electromagnet, feel its force and look at its field pattern. Electromagnet Video

15. 14 16.2 Magnetic field of electric currents 4Electromagnets An electromagnet – behaves like a permanent magnet, but can be switched on & off, –has a coil of many turns of insulated copper wire,  no. of turns   magnetic field strength The coil winds round a core made of magnetic material (e.g. soft iron).  magnetic field 

16. 15 16.2 Magnetic field of electric currents 4Electromagnets Magnetism of soft-iron core dies away as soon as I is switched off. If the core is made of steel instead, it retains magnetism even after current has been switched off.

17. 16 16.2 Magnetic field of electric currents 4Electromagnets An electromagnet can be made stronger by  no. of turns of the coil  I through the coil inserting a soft-iron core into the coil Large electromagnets are used for lifting heavy iron objects. Small ones are used in electrical devices e.g. electric bells, telephones & loudspeakers.

18. 17 16.2 Magnetic field of electric currents e.g. 2 Magnetic effect of coil Question ? Example 2

19. 18 16.2 Magnetic field of electric currents 5Applications of electromagnets aElectric bells and buzzers bTelephones

20. 19 16.2 Magnetic field of electric currents aElectric bells and buzzers electro- magnet hit gong contact breaker hammer Operation of electric bells: 1. I flows 2.pulls hammer across  hits the gong  electromagnet is magnetized When switch is pressed, 3. breaks contact  switches off I 4. hammer springs back  contact closes repeats 5. I flows again & process repeats back

21. 20 16.2 Magnetic field of electric currents aElectric bells and buzzers For a buzzer, when I passes through,  produces the buzzing sound thin metal strip vibrates

22. 21 16.2 Magnetic field of electric currents bTelephones mouthpiece carbon granules carbon blocks iron diaphragm electromagnet of earpiece permanent magnet I I metal plate vibrates aluminium diaphragm

23. 22 16.2 Magnetic field of electric currents bTelephones mouthpiece carbon granules carbon blocks iron diaphragm electromagnet of earpiece permanent magnet I I aluminium diaphragm carbon granules become closer to or further from the other resistance changes repeatedly

24. 23 16.2 Magnetic field of electric currents bTelephones mouthpiece carbon granules carbon blocks iron diaphragm electromagnet of earpiece permanent magnet I' aluminium diaphragm varying current

25. 24 16.2 Magnetic field of electric currents magnetic field strength changes bTelephones mouthpiece carbon granules carbon blocks iron diaphragm electromagnet of earpiece permanent magnet I' aluminium diaphragm diaphragm vibrates sound waves send into ear

26. 25 16.2 Magnetic field of electric currents e.g. 3 Ding-dong door bell Question ? Example 3

27. 26 16.2 Magnetic field of electric currents Check-point 2 1What happens to the magnet...What happens to the magnet... 2What is the direction of the...What is the direction of the... 3Which of the electromagnets…Which of the electromagnets… 4Which of the following…Which of the following… 5A wire carries a current…A wire carries a current… 6True or false: Both of the…True or false: Both of the…

28. 27 16.2 Magnetic field of electric currents Q1What happens to the magnet... What happens to the magnet if the switch is closed? AIt is attracted by the coil. BIt is repelled by the coil. CIt is not affected.

29. 28 16.2 Magnetic field of electric currents Q2What is the direction of the... What is the direction of the magnetic field at P? AInto paper. BOut of paper. CUpwards. DDownwards. EThere is no magnetic field at P. P wires are close togehter

30. 29 16.2 Magnetic field of electric currents Q3Which of the electromagnets... Which of the electromagnets below is the strongest? A B C D 2 A 3 A 2 A cardboard tubing

31. 30 16.2 Magnetic field of electric currents Q4Which of the following... Which of the following devices CANNOT be an application of the electromagnet? AThe mouth piece of telephones. BElectric bells. CDoor locks. DMaglev trains ( 磁浮火車 ).

32. 31 16.2 Magnetic field of electric currents Q5A wire carries a current... A wire carries a current flowing into the paper and gives a magnetic field around it. ABCD The current is then reversed and decreased. Which of the following best represents the new magnetic field?

33. 32 16.2 Magnetic field of electric currents Q6True or false: Both of the... True or false: Both of the door bells X and Y work properly. (T/F) bell Xbell Y

34. 33 16.2 Magnetic field of electric currents The End

35. 34 16.2 Magnetic field of electric currents Example 2 The following 4 coils have the same I passing through them. Which coil(s) Magnetic effect of coil (a) give(s) the weakest magnetic field? Why? Coil A: it has the smallest number of turns, and no steel or soft-iron core. A B C D steel soft-iron

36. 35 16.2 Magnetic field of electric currents Example 2 Magnetic effect of coil (b) have/has a N-pole at its left end? Why? Coil B: this is determined by the right-hand grip rule. The following 4 coils have the same I passing through them. Which coil(s) A B C D steel soft-iron

37. 36 16.2 Magnetic field of electric currents Example 2 Magnetic effect of coil (c) will still give a magnetic field when the current is switched off? Why? Coil B: the steel core retains its magnetism after the current has been switched off. The following 4 coils have the same I passing through them. Which coil(s) A B C D steel soft-iron

38. 37 16.2 Magnetic field of electric currents Return

39. 38 16.2 Magnetic field of electric currents Example 3 (a) Why a 'ding-dong' sound is heard when bell-push is pressed & released? Ding-dong door bell metal plate B plastic tip soft iron rod plastic rod spring coil metal plate A P Q plastic tube bell-push plastic tip When bell-push is pressed, I flows in coil  soft iron rod is attracted & pulled into coil tip Q hits plate A  produces 'ding' sound  produces a magnetic field press >> >> I

40. 39 16.2 Magnetic field of electric currents Example 3 (a) Why a 'ding-dong' sound is heard when bell-push is pressed & released? Ding-dong door bell metal plate B plastic tip soft iron rod plastic rod spring coil metal plate A P Q plastic tube bell-push plastic tip When bell-push is released,no I flows in coil  soft iron rod is no longer attracted spring pushes rod out of coil  produces ‘dong’ sound  magnetic field disappears tip P hits plate B

41. 40 16.2 Magnetic field of electric currents Example 3 (b)(i) Will the door bell work if the polarity of the battery is reversed? Ding-dong door bell metal plate B plastic tip soft iron rod plastic rod spring coil metal plate A P Q plastic tube bell-push plastic tip Reverse the polarity of battery  reverse the polarity of magnetic field  rod is still attracted into the coil

42. 41 16.2 Magnetic field of electric currents Example 3 (b)(ii) Will the door bell work if the rod PQ is made of copper & plastic? Ding-dong door bell metal plate B plastic tip soft iron rod plastic rod spring coil metal plate A P Q plastic tube bell-push plastic tip  copper rod is not magnetic  rod is not attracted into coil when current flows in the coil

43. 42 16.2 Magnetic field of electric currents Return