Chapter 19 & 20

1. Chapter 19 & 20

2. 19.1 Magnets and Magnetic Fields Magnets • There are many types of magnets; horseshoe, bar and flat. • They will pick things up, attach things to a refrigerator, etc. • Magnets attract iron-containing materials like paperclips, nails and so on. • Items are attracted the strongest to the “ends” of the magnets or its poles. • North and South Poles

3. A compass is just a magnet that swings freely on a pivot. • Electromagnets are used to pick up heavy loads. • They are also used in meters, motors, loudspeakers and magnetic tape for sound and video recording. • Superconducting magnets are used to levitate modern trains.

4. Like poles repel each other, and unlike attract each other • Magnets can be marked with a “N” and “S” pole. • Unlike poles attract and like poles repel. • No matter how many times a permanent magnet is cut, each new piece will have a north and south.

5. Magnetic Domains • Atoms with many electrons, their electrons usually pair up with their spins opposite each other causing their fields cancel each other. • For this reason most substances are not magnetic. • Materials like iron, cobalt and nickel, the magnetic fields produce by the electron spins do not completely cancel – these are called ferromagnetic. • In these materials strong coupling occurs between nearby atoms to form large groups of atoms whose spins are aligned. • These groups are called magnetic domains.

6. When an external magnetic field is applied, the orientation of each magnetic field of each domain may change slightly to align closer with the applied magnetic field.

7. Some materials can be made into permanent magnets • An unmagnified piece of iron can become a permanent magnet by being exposed to a permanent magnet. • This process can be reversed by hitting the iron with a hammer or heating and cooling it. • Ether will cause the magnetic domains to go back to a random orientation. • Iron can be a “soft” magnetic material. They can be magnetized easily and lose their magnetism easily. • With “hard” magnetic materials, domain alignment persists after an external magnetic field is removed. These make good permanent magnets.

8. Magnetic Fields • Like an electric field, a magnetic field also has a force. • A magnetic fieldis a region in which a magnetic force can be detected. • A magnetic field B, is a vector quantity that has both magnitude and direction.

9. Magnetic field lines can be drawn with the aid of a compass • The magnetic field of a bar magnet can be explored using a compass. • Near a magnetic field, a compass needle will align itself with that field. • The direction of the magnetic field (B) travels out (away) from the north end and into the south pole. • These magnetic field lines actually never end. They pass within the magnet as well (not shown). • Magnetic Fields surround us without us seeing them.

10. Magnetic flux relates to the strength of a magnetic field • The quantity of magnetic field strength is called magnetic flux. This is show with a phi symbol ΦM • It is basically the number of field lines that cross a certain area. • A bar magnet has two circles of field lines that are perpendicular to the magnet’s axis. • More field lines are concentrated near each of the poles. Therefore the poles have the greatest magnetic flux.

11. Earth has a magnetic field similar to that of a bar magnet • If a compass needle is allowed to rotate both perpendicular to and parallel to the Earth’s surface, it will only be perpendicular at the Earth’s equator. • As we move the compass north, it will start to dip towards the Earth’s surface. • As we approach Canada, it will dip more and point towards Hudson Bay. • The Earth has a magnetic core, but it is slightly off from the Earth’s center of rotation, or true north. *Note the Earth’s south magnetic pole is near the north pole.

12. The angle the compass makes with lines of longitude, or true north, are called magnetic declination. • In the state of Washington, it is off by about 20 degrees. • It is likely the Earth’s magnetic field comes from the movement of charges in convective currents in its core. • There is further evidence that a planets rotational speed has a lot to do with the intensity of its magnetic field strength. • Jupiter rotates the fastest and has a very strong field.

13. An MRI machine can see inside you using strong magnets. It is strong enough to pull a pen from you pocket.It uses superconducting wires so no resistance or heat is produced.Its primary magnet cause hydrogen atoms in you to line up and then another magnet pulses to cause small orientation changes that can be scanned as images.

14. Questions1. Items are attracted the strongest to the “ends” of the magnets, also called the ______. 2.T / F No matter how many times a permanent magnet is cut, each new piece will have a north and south.3.Atom’s electrons usually pair up with their spins ________ each other causing their fields cancel each other. For this reason most substances are not magnetic.4. An unmagnified piece of iron can become a magnet by being exposed to a permanent _______. 5. More field lines are concentrated near each of the poles. Therefore the poles have the greatest magnetic _____. poles true opposite magnet flux

15. 19-2 Magnetism from Electricity Magnetic Field of a Current-Carrying Wire • In 1820, Danish physicist Hans Christian Oersted demonstrated that when a compass is near a current carrying wire, it deflected from its north-south orientation. • This showed that electricity and magnetism were related. 

16. Magnetic Field of a Current-Carrying Wire

17. A long, straight, current carrying wire has a cylindrical magnetic field • As shown on the previous slide, current carrying wires produce a magnetic field. • The compass needles deflect in directions tangent to concentric circles around the wire. • This shows the direction of B, the magnetic field induced by the current. • When the current is reversed, the needles reverse direction too.

18. The right-hand rule can be used to determine the direction of the magnetic field • The direction of B is consistent with a simple rule for conventional current, known as the right hand rule. • As shown here, grasping the wire with the right hand so the thumb is in the direction of the current, the fingers curl in the direction of B. • B is proportional to the current in the wire and inversely proportional to the distance from the wire.

19. Magnetic Field of a Current Loop • The right hand rule can also be applied to find the direction of the magnetic field of a current-carrying loop. • As shown below, regardless of where on the loop you apply the right-hand rule, the field within the loop points in the same direction – upward. • The field lines of the current-carrying loop resemble those of a bar magnet. Magnetic Field of a Current Loop

20. Solenoids produce a strong magnetic field by combining several loops • A long spiral wound coil of wire makes a solenoid. • A solenoid acts as a magnet when it carries a current. • The magnetic field strength inside the coil increases with current and is proportional to the number of coils per length. • The magnetic field strength can be further increased by inserting an iron rod through the center of the coil. • The rod then becomes an electromagnet.

21. Here you can see that the field lines inside the solenoid point in the same direction. • They are nearly parallel inside, point in the same direction and are very close together. • This indicates the field inside the solenoid is strong and nearly uniform.

22. Satellites • Small satellites use torque coils to align themselves in space. • Since the Earth’s magnetic field is present in these low Earth orbits, satellites can use this field to sense direction. • These coils, once activated, can pull the satellite in any direction to orientate it to where the operator needs it. • For an even stronger solenoid, ferromagnetic rods are placed in the center making an electromagnet. These are even more effective at turning the satellites.

23. Questions1. Hans Christian Oersteddemonstrated that electricity and __________ were related.2. What does this photo tell us about current?3.As shown above, grasping the wire with the ( right / left ) hand so the thumb is in the direction of the current, the fingers curl in the direction of B.4. The magnetic field strength inside a solenoid is proportional to the number of _____ per length.5.Magnetic strength can be further increased by inserting an iron rod in the center of a solenoid, creating an ____________. magnetism A magnetic field is created around it ___ coils electromagnet

24. 20 -1 Electricity from Magnetism Electromagnetic Induction • So far, all electric circuits that you have studied have used a battery or electric source to create potential difference. • It is possible to induce a current in a circuit without the use of a power supply? • The last chapter showed current in a circuit is the source of a magnetic field.

25. Therefore, current also results when a closed electric circuit moves through a magnetic field, as shown below… • The process of inducing a current in a circuit using a changing magnetic field is called electromagneticinduction. • If the circuit moves toward or away from the magnet, current is induced as long as there is relative motion between the two.

26. The separation of charges by the magnetic force induces an emf • A moving charge can be deflected by a magnetic field. • This deflection can be used to explain how an emf occurs in a wire that moves through a magnetic field. • Notice how the conducting wire is moved through the magnetic field here…

27. According to the right hand rule, this force will be perpendicular to both the magnetic field and the motion of the charges. • For positive charges in the wire, the force is directed downward along the wire. • As long as the conducting wire moves through the magnetic field, the emf will be maintained. • The direction the wire is moved through the field determines the direction the current will move. • The magnitude of the emf is also determined by the size of the wire, the magnetic field strength and the velocity the wire moves.

28. The angle between a magnetic field and a circuit affects induction • One way to induce an emf in a closed loop of wire is to move the loop into and out of a magnetic field. • If the loop stops moving, no emf is produced. • The magnitude of the induced emf is determined on the orientation of the loop. • As the loop passes through perpendicular, there is noemf at that instant.

29. Changes in the number of magnetic field lines induces a current • Changing the size of the loop or the strength of the magnetic field also will induce an emf in the circuit. • Rotating the loop changes the number of filed lines that it passes through. So does changing the magnetic field strength and direction. Induced EMF’s

30. Characteristics of Induced Current • If a bar magnet is pushed into a coil of wire, the strength of the magnetic field within the coil increases. • This allows a current to be induced in the circuit. • This induced current then produces its own magnetic field (found by using the right-hand rule). • As the magnet approaches, the magnetic field passing through the coil increases its strength.

31. The induced current in the coil is in a direction that produces a magnetic field that opposes the increasing strength of the approaching field. • The induced magnetic field is in the opposite direction of the increasing magnetic field. *Note the repelling N/N pushing • The coil and the approaching magnet create a pair of forces that repel each other.

32. If the magnet is moved away from the coil, the magnetic field passing through the coil decreases in strength. • Again, the current induced in the coil produces a magnetic field that opposes the decreasing strength of the receding field. *Note the pulling (attracting) force N/S Self-Induction

33. Heinrich Lenz 1804-1865 • The rule for finding the direction of the induced current is called Lenz’s Law… The magnetic field of the induced current is in a direction to produce a field that opposes the change causing it. • Note that the field of the induced current does not oppose the applied field but rather the charge in the applied field.

34. Faraday’s law of induction predicts the magnitude of the induced emf • Lenz’s law allows you to determine the direction of an induced current in a circuit. • To calculate the magnitude of the induced emf, you must use Faraday’s law of magnetic induction. • Magnetic flux ФMcan be written as AB cos θ. • The term B cos θrepresents the component of the magnetic field perpendicular to the plane of the loop. 

35. The minus sign (-) in front of the equation is included to indicate the polarity of the induced emf. • N is the number of tightly wound loops. • The S.I. unit for magnetic field strength is the tesla(T). Nikola Tesla 1856-1943

36. Practice AInduced emf and Current • A coil with 25 turns of wire is wrapped around a hollow tube with an area of 1.8 m2. A uniform magnetic field is applied to a right angle to the plane of the coil. • If the field increases uniformly from 0.00 T to 0.55 T in 5 s, find the magnitude of the induced emf in the coil. • If the resistance in the coil is 2.5Ω , find the magnitude of the induced current in the coil. Unknown: emf = ? I = ?

37. answer  -29 V

38. Given: V = -29 V R = 2.5 Ω I = ? V = IR  Answer -12 A

39. Questions1. The process of inducing a current in a circuit using a changing magnetic field is called _____________ induction.2. The direction the wire is moved through the field determines the direction the ______ will move.3. If a bar magnet is pushed into a coil of wire, the strength of the magnetic field within the coil ( increases / decreases ).4. The induced magnetic field is in the ( opposite / same ) direction of the increasing magnetic field.5.The following is ______ law…The magnetic field of the induced current is in a direction to produce a field that opposes the change causing it. electromagnetic current ________ _______ Lenz’s

40. End