Magnetic field of electric currents

1. Magnetic field of electric currents

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

3. Warm-up A There is wind blowing when the current flows along the wire. B A magnetic field, other than the Earth's field, exists around the compass when current flows along the wire. C Others: ______________________

4. Introduction 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.

5. Experiment 16b Magnetic fields from currents Use iron filings to show the magnetic fields.

6. 1 Field around a straight wire Circles around the wire Strongest close to the wire Current   strongerfield Reversing current  reverses the direction of field lines

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

8. Field around a straight wire Right-hand grip rule current direction 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 field direction

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

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

11. Field due to a solenoid • Insidethe solenoid: straight and evenly-spaced field lines; uniform field strength I I • Outside the solenoid: field pattern similar to that around a bar magnet

12. Field due to a solenoid To determine the poles of a solenoid: Right-hand grip rule for solenoid If right hand grips solenoid, N S 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

13. Experiment 16c Electromagnet Make an electromagnet, feel its force and look at its field pattern.

14. Electromagnets but can be switched on & off, • An electromagnet – behaves like apermanentmagnet, – 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 

15. Electromagnets • 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.

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

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

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

19. Example 2 Magnetic effect of coil The following 4 coils have the same I passing through them. Which coil(s) steel soft-iron A D B C (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.

20. Applications of electromagnets a Electric bells and buzzers b Telephones

21. Operation of electric bells: When switch is pressed, electro-magnet 1. Iflows a Electric bells and buzzers • electromagnet is magnetized contact breaker 2. pulls hammer across hits the gong hammer 3.breaks contact switches offI hit back gong 4. hammer springs back  contact closes 5. I flows again & process repeats

22. For a buzzer, a Electric bells and buzzers when I passes through, thin metal strip vibrates  produces the buzzing sound

23. carbon granules mouthpiece iron diaphragm carbon blocks Telephones I I aluminium diaphragm permanent magnet electromagnet of earpiece metal plate vibrates

24. carbon granules mouthpiece iron diaphragm carbon blocks b Telephones I I aluminium diaphragm permanent magnet electromagnet of earpiece carbon granules become closer to or further from the other resistance changes repeatedly

25. carbon granules mouthpiece iron diaphragm carbon blocks b Telephones I' I' aluminium diaphragm permanent magnet electromagnet of earpiece varying current

26. carbon granules mouthpiece iron diaphragm carbon blocks b Telephones I' I' aluminium diaphragm permanent magnet electromagnet of earpiece magnetic field strength changes diaphragm vibrates sound waves send into ear

27. > > > > I Example 3 Ding-dong door bell (a) Why a 'ding-dong' sound is heard when bell-push is pressed & released? When bell-push is pressed, I flows in coil  produces a magnetic field  soft iron rod is attracted & pulled into coil plastic tip plastic rod plastic tube plastic tip tip Qhits plateA  produces 'ding' sound P Q spring soft iron rod press bell-push coil metal plateB metal plateA

28. Example 3 Ding-dong door bell (a) Why a 'ding-dong' sound is heard when bell-push is pressed & released? When bell-push is released, no I flows in coil  magnetic field disappears  soft iron rod is no longer attracted plastic tip plastic rod plastic tube spring pushes rod out of coil plastic tip tip Phits plateB P Q  produces ‘dong’ sound spring soft iron rod bell-push coil metal plateB metal plateA

29. Example 3 Ding-dong door bell (b)(i) Will the door bell work if the polarity of the battery is reversed? Reverse the polarity of battery  reverse the polarity of magnetic field  rod is still attracted into the coil plastic tip plastic rod plastic tube plastic tip P Q spring soft iron rod bell-push coil metal plateB metal plateA

30. Example 3 Ding-dong door bell (b)(ii) Will the door bell work if the rod PQ is made of copper & plastic? copper rod is not magnetic   rod is not attracted into coil when current flows in the coil plastic tip plastic rod plastic tube plastic tip P Q spring soft iron rod bell-push coil metal plateB metal plateA

31. Q1 What happens to the magnet... What happens to the magnet if the switch is closed? A It is attracted by the coil. B It is repelled by the coil. C It is not affected.

32. Q2 What is the direction of the... What is the direction of the magnetic field at P? P wires are close togehter A Into paper. B Out of paper. C Upwards. D Downwards. E There is no magnetic field at P.

33. Q3 Which of the electromagnets... Which of the electromagnets below is the strongest? A B 2 A 2 A cardboard tubing D C 3 A 2 A

34. Q4 Which of the following... Which of the following devices CANNOT be an application of the electromagnet? A The mouth piece of telephones. B Electric bells. C Door locks. D Maglev trains (磁浮火車).

35. Q5 A wire carries a current... A wire carries a current flowing into the paper and gives a magnetic field around it. The current is then reversed and decreased. Which of the following best represents the new magnetic field? A B C D

36. Q6 True or false:Both of the... True or false:Both of the door bells X and Y work properly. (T/F) bell X bell Y