College Physics B - PHY2054C

Transcription

1 College - PHY2054C 09/22/2014 My Office Hours: Tuesday 10:00 AM - Noon 206 Keen Building

2 Outline 1 2 3

3 When current passes through one resistor and then another, the resistors are said to be in series: E I R 1 I R 2 = 0 Kirchhoff s Loop Rule Any number of resistors can be connected in series. The resistors will be equivalent to a single resistor with: R equiv = R 1 + R 2 + R

4 Review Question 1 Two light bulbs, A and B, are connected in series to a constant voltage source. When a wire is connected across B as shown, bulb A A burns more brightly. B burns as brightly. C burns more dimly. D goes out.

5 Review Question 1 Two light bulbs, A and B, are connected in series to a constant voltage source. When a wire is connected across B as shown, bulb A A burns more brightly. B burns as brightly. C burns more dimly. D goes out.

6 Applying the Junction Rule (Kirchhoff s Junction Rule) For path 1, +E I 1 R 1 = 0 For path 2, +E I 2 R 2 = 0 The total current is: I 3 = I 1 + I 2 = E R 1 + E R 2 = E ( 1 R R 2 )

7 Equivalent Resistance - Parallel

8 Circuit Analysis 1 Some complex circuits can be solved by combinations of series and parallel rules. 2 Other circuits must be analyzed directly by Kirchhoff s Rules. Loop Rule: The total change in the electric potential around any closed circuit path must be zero. Junction Rule: The current entering a circuit junction must equal the current leaving the junction. 3 Connecting resistors in series always gives a total resistance larger than the resistance of any of the component resistors. 4 Connecting resistors in parallel always gives a total resistance smaller than the resistance of any of the component resistors.

9 Ammeters An Ammeter is a device that measures current. An ammeter must be connected in series with the desired circuit branch. An ideal ammeter will measure current without changing its value. Must have a very low resistance.

10 Voltmeters A Voltmeter is a device that measures the voltage across a circuit element. It must be connected in parallel with the element. An ideal voltmeter should measure the voltage without changing its value. Should have a very high resistance.

11 At very low temperatures, the linearity of resistance breaks down. The resistivities of metals approach a nonzero value at very low temperatures. In some metals, resistivity drops abruptly and is zero below a critical temperature. These metals for which the resistivity goes to zero are the called superconductors.

12 John Robert Schrieffer Nobel Laureate Emeritus Professor at Florida State Bardeen, Cooper, and Schrieffer received the Nobel Prize in 1972 for the development of the theory of superconductivity. The BCS Theory is one of the greatest discoveries of the 20th century.

13 Outline 1 2 3

14 Magnetism The first observations of magnetic fields involved permanent magnets. Many ancient cultures discovered natural magnetic properties of materials. Permanent magnetic applications include: Compass needles Speakers Computer hard disks

15 Poles

16 A bar magnet is a permanent magnet in the shape of a bar. The symbol for the magnetic field is B. SI unit of the magnetic field is the Tesla (T) The magnetic field lines can be deduced from the pattern of the iron filings. Field Lines Some properties of the magnetic field: The iron filings align parallel to the magnetic field line. The magnetic field lines go from the north pole toward the south pole.

17 A bar magnet is a permanent magnet in the shape of a bar. The symbol for the magnetic field is B. SI unit of the magnetic field is the Tesla (T) The magnetic field lines can be deduced from the pattern of the iron filings. Field Lines Some properties of the magnetic field: The magnitude of the field decreases as you move farther from a pole. The magnetic field lines form closed loops!

18 Field Lines The magnetic field lines always form closed loops. A general property of magnetic fields, not just bar magnets. The magnetic poles are analogous to positive and negative charges.

19 Question 2 al charges and magnetic poles have many similarities, but one important difference is: A Opposite magnetic poles repel. B One magnetic pole cannot create magnetic poles in other materials. C A magnetic pole cannot be isolated. D poles do not produce magnetic fields.

20 Question 2 al charges and magnetic poles have many similarities, but one important difference is: A Opposite magnetic poles repel. B One magnetic pole cannot create magnetic poles in other materials. C A magnetic pole cannot be isolated. D poles do not produce magnetic fields.

21 Can be made by bending a bar magnet. There are poles at the ends of the horseshoe magnet. The field is largest in the horseshoe gap. The field is directed across the gap. Horseshoe Magnet iron yoke to strengthen field

22 Outline 1 2 3

23 Connection between ity and Magnetism Sources of Charge Sources of

24 Capacitor Michael Faraday ( ) Static Point Charges

25 Faraday s Cage

26 Connection between ity and Magnetism Sources of Charge Sources of Moving Charge

27 Christian Oersted ( ) Field around a currentcarrying wire is fairly weak

28 Field from Current Moving charges produce magnetic fields: An electric current consists of moving charges, so it will produce a magnetic field. The iron filings show the magnetic field pattern due to the current.

29 Question 3 A current in a long, straight wire produces a magnetic field. The magnetic field lines A go out from the wire to infinity. B come in from infinity to the wire. C form circles that pass through the wire. D form circles that go around the wire.

30 Question 3 A current in a long, straight wire produces a magnetic field. The magnetic field lines A go out from the wire to infinity. B come in from infinity to the wire. C form circles that pass through the wire. D form circles that go around the wire.

31 Question 3 A current in a long, straight wire produces a magnetic field. The magnetic field lines D form circles that go around the wire. Ampère s Law: B L = µ 0 I enclosed closed path B = µ 0 I 2π r for a straight wire The constant µ 0 is called the permeability of free space: µ 0 = 4π 10 7 T m/a

32 Right-Hand Rule For a straight wire, the magnetic field lines form circles: The direction of the field is always tangent to the circles. The magnitude of the field decreases as the distance from the wire increases. The direction of the field is given by the right-hand rule.

33 Right-Hand Rule Point the thumb of your ight hand in the direction of the current: Your thumb will be parallel to the wire. Curling the fingers of your right hand around the wire gives the direction of the magnetic field.

34 Question 4 Two current-carrying wires are parallel as shown below; the current is the same in both wires. The current in both wires is flowing to the right. At a point midway between the wires, the direction of the net magnetic field is A to the right B to the left C into the screen D out of the screen E The field is zero. P

35 Question 4 Two current-carrying wires are parallel as shown below; the current is the same in both wires. The current in both wires is flowing to the right. At a point midway between the wires, the direction of the net magnetic field is A to the right B to the left C into the screen D out of the screen E The field is zero.

36 Plotting Field Lines Field lines are three-dimensional. 1 A large dot ( ) indicates the tip of the vector when it points out of the plane. 2 A cross ( ) denotes the tail of the vector when it points into the plane.

37 Charges and The electric current can be modeled as a collection of positive electric charges. The charges would be moving with a velocity parallel to the current direction. The direction of the magnetic field is given by the right-hand rule. A positive charge moving to the left produces the same magnetic field as a negative charge moving to the right. Principle of Superposition The Principle of Superposition states the total magnetic field produced by two or more different sources is equal to the sum of the fields produced by each source individually.

38 Treat the loop as many small pieces of wire: Apply the right-hand rule to find the field from each piece of wire. Applying superposition gives the overall pattern shown on the right. At the center of the loop: B = µ 0 I 2R

39 Solenoids By stacking many loops close together, the field along the axis is much larger than for a sinle loop. A helical winding of wire is called a solenoid. More practical than stacking single loops.