Magnets and Magnetic Fields

1. Magnets and Magnetic Fields

2. A Magnet attracts certain materials to itself. A magnet will attract Iron, Steel, Nickel, Cobalt and some alloys of these. A magnet has no noticeable effect on other materials. A Bar Magnet is strongest at each end. Dip a bar magnet into iron filings or a box of pins. It attracts the filings or the pins to itself. Most cling on at each end of the magnet. The regions of greatest strength at each end are called Magnetic Poles.

3. If a Bar Magnet is suspended freely it will line up approximately North-South. The pole of the magnet that always points North is called the North-Seeking Pole or the North Pole. The pole that points south is called the South-Seeking Pole or theSouth Pole.

4. Magnetic Poles occur in Pairs. You cannot have a single pole on its own. For every north pole there is always a south pole. The strength of the north pole is the same as the strength of the south pole. Like poles repel and unlike poles attract The North Pole of one magnet repels the North Pole of another. The South Pole of one magnet repels the South Pole of another. The North Pole of one magnet is attracted to the South Pole of another magnet.

5. A Magnet causes some materials brought near it or touching it to become magnetised. This magnetism is called Induced Magnetism. If the magnet is taken away some materials (called permanent magnets) hold on to their magnetism but others (called temporary magnets) lose most of it. Hard steel holds onto magnetism very well whereas soft iron does not. Ordinary nails are usually made from soft iron and do not retain their magnetism very well.

6. What is a Magnetic Field? A Magnetic Field is any region of space where magnetic forces can be felt. What is the Direction of a Magnetic Field ? The Direction of the Magnetic Field at a point is the direction of the force on a north pole if it was placed at that point.

7. What is a Magnetic Field Line? A line drawn in a magnetic field so that the tangent to it at any point shows the direction of the magnetic field at that point is called a Magnetic Field Line. What are Magnetic Poles? Magnetic poles are the regions at each end of a magnet where the magnetic forces are greatest. Magnetic poles are always found in pairs.

8. A Plotting Compass is a small magnet that can rotate about a vertical axis. If no other magnets are nearby it will line up North-South. If another magnetic field is present it will deflect the compass needle from its N-S position. If the other field is strong enough the compass needle will line up almost parallel to the field rather than North-South.

9. The Magnetic Field around a Bar Magnet The magnetic field lines start at the north pole and end at the south pole. The magnetic field lines never cross each other. Near the poles - where the magnetic field is strongest - the lines are close together. Further away, where the field is weaker, the lines are far apart.

10. Iron Filings showing the Magnetic Field around a Bar Magnet

11. The magnetic field around a U-shaped Magnet Earth’s Magnetic Field, pointing approximately North

12. What is the Magnetic Effect of an Electric Current? Every Current-Carrying Conductor has a Magnetic Field around it caused by the current. As long as the current is flowing the Magnetic Field exists. If the current stops flowing the Magnetic Field disappears.

13. Experiment to show the Magnetic Effect of an Electric Current Set up the equipment with the wire lined up North-South. The plotting compass also lines up N-S. Close the switch, sending current through the wire. The compass needle will deflect. Reverse the direction of the current and the needle deflects in the opposite direction. Open the switch, no current flows, the magnetic field disappears and the compass again lines up N-S. Conclusion: Every current carrying conductor has a magnetic field around it caused by the current.

14. The Magnetic Field around a Long Straight Wire

15. The Right-Hand Grip Rule If a Right Hand clasps a conductor with the Thumb pointing in the direction of the Current Then the Fingers give the direction of the Magnetic Field around the conductor. The thumb points in the direction of conventional current, i.e. from + to -

16. A Circular Loop of Wire carries a current in the direction shown. Using the Right-Hand Grip Rule at a number of points on the wire shows us the shape of the magnetic field around the loop. The side of the loop facing us behaves like a South Pole (the magnetic field lines are going in) The other side is like a North Pole (the magnetic field lines are coming out).

17. The Magnetic Field around a Current-Carrying Coil (or a loop)

18. The Magnetic Field around a Current-Carrying Solenoid

19. What is an Electromagnet? A Solenoid carrying a current and containing a soft iron core is known as an Electromagnet. Electromagnets are used in: Scrap yard cranes Electric motors Electric bells Moving coil loudspeakers Induction coils By turning the current on or off the magnet can be turned on or off. By varying the size of the current the strength of the magnet can be varied.

20. A powerful Electromagnet lifting scrap Iron

21. A Magnetic Compass shows the direction of the Earth’s Magnetic Field and it is used in navigation. The Earth’s Magnetic Field forms a protective layer (from charged particles) around the Earth.

22. The Magnetic Compass has been used for hundreds of years in marine navigation, since it enables you to know direction. The angle between True North and Magnetic North is also of importance. Charts and maps used in navigation have its value in the locality of the chart noted on them since navigators must allow for it in their calculations.

23. To show that a Current-Carrying Conductor in a Magnetic Field experiences a Force Send a current through the tinfoil. The foil will move forwards. Reverse the current and the foil will move backwards. Conclusion: A current-carrying conductor in a magnetic field experiences a force.

24. The moving coil meter and the moving coil loudspeaker are based on the principle that a current-carrying conductor in a magnetic field experiences a force.

25. A simple d.c. Motor is based on the principle that a current-carrying conductor in a magnetic field experiences a force.

26. Why would you expect a current-carrying conductor placed in a magnetic field to experience a force? A current-carrying conductor has a magnetic field around it due to the current. When this conductor is placed in another magnetic field, the two magnetic fields interact (push off each other!). This causes the force on the current-carrying conductor.

27. What is the Direction of the Force on a Current-Carrying Conductor in a Magnetic Field? The direction of the force is always:  Perpendicular to the current  Perpendicular to the magnetic field NOTE: A Current-Carrying Conductor in a magnetic field experiences noforce if the conductor is parallel to the magnetic field.

28. Fleming’s Left-Hand Rule: If the thumb, first finger and second finger of the left hand are held at right angles, with the first finger in the direction of the magnetic field and the second finger in the direction of the current, then the thumb points in the direction of the force.

29. What determines the Size of the Force on a current-carrying conductor in a magnetic field? The size of the Current The Length of the conductor How strong the Magnetic Field is The Angle between the conductor and the magnetic field

30. Magnetic Flux Density (B) Is the magnitude of the magnetic field strength(B): FI and Fl It follows that: FI l F = I l B where B is a constant. The value of B depends on how strong the magnetic field is. In a strong magnetic field B is large and in a weak field B is small. Thus B is a measure of how strong the magnetic field is. B is called the Magnetic Flux Density.

31. Define Magnetic Flux Density • At a point in a magnetic field the Magnetic Flux Density ( B ) is a vector whose: • direction is the direction of the force on a north pole placed at that point • magnitude is the value of B from the equation F = I l B • The SI unit of magnetic flux density is the tesla (T) • or put another way: • The magnetic Flux Density (B) at a point in a magnetic field is a vector whose: • magnitude is equal to the force that would be experienced by a conductor of length 1 m carrying a current of 1 A at right angles to the field at that point. Its direction is the direction of the force on a north pole placed at that point.

32. If the conductor is not perpendicular to the field resolve the B into two perpendicular components - one parallel to the conductor and the other at right angles to the conductor. It is the component of B that is perpendicular to the conductor that causes the force on it. The parallel component has no effect on the wire. F = B I l Sin 0o

33. The coil is free to rotate about the axis. Convince yourself that the directions of the forces on the sides of the coil are correct and that the coil will begin to rotate.

34. To Show the Force on a current-carrying coil in a magnetic field Use the equipment above. The coil is free to rotate about the axis. When the current is switched on the coil starts to rotate as shown.

35. A Beam of Electrons in a cathode ray tube is an Electric Current A beam of electrons in a cathode ray tube moves in a vacuum. The beam passes close to a fluorescent screen and shows up as a beam of light. The moving electrons have negative charge and thus are an electric current. They, therefore, have a magnetic field around them.

36. Force on a Moving Charge in a Magnetic Field This magnetic field, due to the beam of moving charges (the electrons), will interact with any other magnetic field placed near it. The picture shows the beam of electrons deflecting due to the presence of a bar magnet.

37. The Size of the Force on a Moving Charge in a Magnetic Field A charge of q coulombs moving with a speed of v metres per second at right angles to a magnetic field of flux density B teslas experiences a force of F newtons, givenby; F = q v B

38. A charged particle moving at constant speed enters a uniform magnetic field and moves at right angles to the field. Explain why the particle moves in a circle?. When the charged particle enters the magnetic field there is a force on it. The force is at right angles to its direction of motion. Therefore its speed does not change. Only its direction of motion changes. The force on it has a constant magnitude (F = q v B.). As it turns the force always remains at right angles to the direction of motion. Thus the particle moves in a circular path.

39. Electric Current and Electric Charge An Electric Current is a flow of charge. The ampere (A). The coulomb (C). 1 coulomb is the amount of charge that passes any point in a circuit when a current of 1 ampere flows for 1 second. What is an Electric Current? What is the SI Unit of electric current? What is the SI Unit of electric charge? Define the coulomb.

40. Electric Current and Electric Charge What is the relationship between Electric Current and Electric Charge? The current (I) is the amount of charge (Q) passing per second Q = I t Where: Q is charge gone past I is the steady current t is the time taken.

41. Magnetic Forces between Currents Two parallel conductors carry current in opposite directions. Each current creates a magnetic field around itself. The magnetic fields interact with each other and cause a force on each conductor, pushing the conductors apart. If the conductors carry current in the same direction the force between them is attractive.

42. State the principle on which the definition of the ampere is based. The definition of the ampere is based on the principle that: Two current carrying conductors exert a force on each other due to their magnetic fields.

43. The Ampere The ampere is that current which: • if maintained in two infinitely long parallel wires, is of negligible cross section • is placed 1 metre apart in a vacuum • would produce a force on each wire of 2 × 10-7 newtons per metre of length

44. What is Magnetic Flux? Magnetic fluxis defined as:  = B A Where:  = magnetic flux B = magnetic flux density A = area Magnetic flux through Area A is equal to Magnetic Flux Density × Area

45. What is the SI Unit of Magnetic Flux? The SI Unit of Magnetic Flux is the weber (Wb). Magnetic flux is a Scalar Quantity.

46. B = 3 T What is the magnetic flux through a loop of area 0.5 m2 placed at right angles to a magnetic field of flux density 3 T?  = B A = (3)(0.5) = 1.5 Wb A = 0.5 m2

47. What if the magnetic field is not perpendicular to the area? Resolve the magnetic flux density B into components parallel and perpendicular to the area. Flux through A = Component of B perp to A × (area A) In the diagram : Component of B perp. to coil = B Sin 30o = 2 Sin 30o = 1 T Flux through coil = B× A = (1)(0.4) = 0.4 Wb

48. What is Electromagnetic Induction? Whenever the magnetic field passing through a coil changes an emf appears in the coil. This is Electromagnetic Induction.

49. To Demonstrate Electromagnetic Induction Move the magnet towards (or away from) the coil. The galvanometer deflects, indicating that current flows and that an emf appears. When the magnet is not moving the meter reads zero.

50. State Faraday’s Law of Electromagnetic Induction. Faraday’s Law states that the induced emf is directly proportional to the rate of change of magnetic flux.