General Physics (PHYS )

Transcription

1 General Physics (PHYS ) Chapter 22 Magnetism Magnetic Force Exerted on a current Magnetic Torque Electric Currents, magnetic Fields, and Ampere s Law Current Loops and Solenoids Magnetism in Matter

2 GOT IT 4 Figure shows a flexible wire passing through a magnetic field that points out of page. The wire is deflected upward, as shown. Is the current flowing 1. To the left 2. To the right 0% 0%

3 GOT IT 5 Power Line A power line runs along Earth s equator, where B points horizontally from south to north; the line carries current flowing from west to east. What s the direction of the magnetic force on the power line? F I L B 1. Upward 2. Downward 3. Right 4. Left 0% 0% 0% 0%

4 Example 3: Making the Connection Earth s equatorial field strength is 30 T, and the power line carries 500 A. What s the magnetic force on a kilometer of the line. F I L B

5 Loops of Current and Magnetic Torque In the current loop shown, the vertical sides experience forces that are equal in magnitude and opposite in direction. They do not operate at the same point, so they create a torque around the vertical axis of the loop.

6 Loops of Current and Magnetic Torque The total torque is the sum of the torques from each force: Or, since A = hw,

7 Loops of Current and Magnetic Torque If the plane of the loop is at an angle to the magnetic field,

8 Loops of Current and Magnetic Torque To increase the torque, a long wire may be wrapped in a loop many times, or turns. If the number of turns is N, we have

9 Loops of Current and Magnetic Torque The torque on a current loop is proportional to the current in it, which forms the basis of a variety of useful electrical instruments. Here is a galvanometer:

10 Application: Electric Motors Rotating loop CW Commutator CCW Battery Brushes Always CCW

11 Example 4: Hybrid Car Motor Toyota s Prius gas-electric hybrid car uses a 50-kW electric motor that develops a maximum torque of 400 N m. Suppose you want to produce this same torque in the motor just discussed, with a 950-turn rectangular coil of wire measuring 30 cm by 20 cm, in a uniform field of 50 mt. How much current does the motor need? Max torque at sin = 1 : 950-turn coil

12 12 GOT IT : A rectangular loop is placed in a uniform magnetic field with the plane of the loop parallel to the direction of the field. If a current is made to flow through the loop in the sense shown by the arrows, the field exerts on the loop: 1. a net force. 2. a net torque. 3. a net force and a net torque. 4. neither a net force nor a net torque.

13 Electric Currents, Magnetic Fields, and Ampère s Law Experimental observation: Electric currents can create magnetic fields. These fields form circles around the current.

14 Electric Currents, Magnetic Fields, and Ampère s Law To find the direction of the magnetic field due to a current-carrying wire, point the thumb of your right hand along the wire in the direction of the current I. Your fingers are now curling around the wire in the direction of the magnetic field.

15 Electric Currents, Magnetic Fields, and Ampère s Law The magnetic field is inversely proportional to the distance from the wire:

16 Electric Currents, Magnetic Fields, and Ampère s Law Ampère s Law relates the current through a surface defined by a closed path to the magnetic field along the path:

17 Ampère s Law WARNING!! B and L need to be parallel

18 Ampère s Law We can use Ampère s Law to find the magnetic field around a long, straight wire:

19 Example 5 Problem 46. A long, straight wire carries a current of 7.2 A. How far from this wire is the magnetic field it produces equal to the Earth s magnetic field, which is approximately 5.0 X 10-5 T?

20 Example 6 Problem 52. In Oersted s experiment, suppose that the compass was 0.25 m from the current-carrying wire. If a magnetic field of half the Earth s magnetic field of 5.0 X 10-5 T was required to give a noticeable deflection of the compass needle, what current must the wire have carried?

21 FROM BOOK: Active Example 22-2 I hope you worked on this example.

22 Electric Currents, Magnetic Fields, and Ampère s Law Since a current-carrying wire experiences a force in a magnetic field, and a magnetic field is created by a current-carrying wire, there is a force between current-carrying wires. Let s study this phenomenon

23 Electric Currents, Magnetic Fields, and Ampère s Law Since a current-carrying wire experiences a force in a magnetic field, and a magnetic field is created by a current-carrying wire, there is a force between current-carrying wires:

24 Magnetic Field of a current loop Magnetic field produced by a wire can be enhanced by having the wire in a loop. Dx 1 B I Dx /20/2013

25 Current Loops The magnetic field of a current loop is similar to the magnetic field of a bar magnet. In the center of the loop,

26 What can we do to create a stronger magnet? Well, we add more turns/loops 26 10/20/2013

27 Solenoid Solenoid magnet consists of a wire coil with multiple loops. It is often called an electromagnet /20/2013

28 Solenoid Magnet Field lines inside a solenoid magnet are parallel, uniformly spaced and close together. The field inside is uniform and strong. The field outside is non uniform and much weaker. One end of the solenoid acts as a north pole, the other as a south pole. For a long and tightly looped solenoid, the field inside has a value: 28 10/20/2013

29 Current Loops and Solenoids We can use Ampère s Law to find the field inside the solenoid:

30 Solenoids Solenoid: a long, tightly wounded coil. N turns per unit length.. B 0 n I B outside = 0 Application: magnetic switches, valves,.

31 Field of a solenoid is very similar to that of a bar magnet.

32 Example 7 MRI Scanner The solenoid used in an MRI scanner is 2.4 m long & 95 cm in diameter. It s wounded from superconducting wire 2.0 mm in diameter, with adjacent turns separated by an insulating layer of negligible thickness. Find the current that will produce a 1.5-T magnetic field inside the solenoid. wire diameter = 1/500 m n = 500 turns/m

33 Example 8: Magnetic Field inside a Solenoid. Consider a sole noid consisting of 100 turns of wire and length of 10.0 cm. Find the magnetic field inside when it carries a current of A /20/2013

34 Comparison: Electric Field vs. Magnetic Field Electric Magnetic Source Charges Moving Charges Acts on Charges Moving Charges Force F = Eq F = q v B sin( ) Direction Parallel E Perpendicular to v,b Field Lines Opposites Charges Attract Currents Repel 34 10/20/2013

35 MAGNETS MAGNETS QGytW_C6hR8

36 Magnetism in Matter The electrons surrounding an atom create magnetic fields through their motion. Usually these fields are in random directions and have no net effect, but in some atoms there is a net magnetic field. If the atoms have a strong tendency to align with each other, creating a net magnetic field, the material is called ferromagnetic.

37 Magnetism in Matter Ferromagnets are characterized by domains, which each have a strong magnetic field, but which are randomly oriented. In the presence of an external magnetic field, the domains align, creating a magnetic field within the material.

38 Permanent magnets are ferromagnetic; such materials can preserve a memory of magnetic fields that are present when the material cools or is formed. Magnetism in Matter

39 Magnetism in Matter Many materials that are not ferromagnetic are paramagnetic they will partially align in a strong magnetic field, but the alignment disappears when the external field is gone.

40 Magnetism in Matter Finally, all materials exhibit diamagnetism an applied magnetic field induces a small magnetic field in the opposite direction in the material.

41 Summary of Chapter 22 All magnets have two poles, north and south. Magnetic fields can be visualized using magnetic field lines. These lines point away from north poles and toward south poles. The Earth produces its own magnetic field. A magnetic field exerts a force on an electric charge only if it is moving: A right-hand rule gives the direction of the magnetic force on a positive charge.

42 Summary of Chapter 22 If a charged particle is moving parallel to a magnetic field, it experiences no magnetic force. If a charged particle is moving perpendicular to a magnetic field, it moves in a circle: If a charged particle is moving at an angle to a magnetic field, it moves in a helix.

43 Summary of Chapter 22 A wire carrying an electric current also may experience a force in a magnetic field; the force on a length L of the wire is: A current loop in a magnetic field may experience a torque:

44 Summary of Chapter 22 Electric currents create magnetic fields; the direction can be determined using a right-hand rule. Ampère s law: Magnetic field of a long, straight wire: Force between current-carrying wires:

45 Summary of Chapter 22 The magnetic field of a current loop is similar to that of a bar magnet. The field in the center of the loop is: Magnetic field of a solenoid:

46 Summary of Chapter 22 A paramagnetic material has no magnetic field, but will develop a small magnetic field in the direction of an applied field. A ferromagnetic material may have a permanent magnetic field; when placed in an applied field it develops permanent magnetization. All materials have a small diamagnetic effect; when placed in an external magnetic field they develop a small magnetic field opposite to the applied field.