College Physics B - PHY2054C. Transformers & Electromagnetic Waves 10/08/2014. My Office Hours: Tuesday 10:00 AM - Noon 206 Keen Building

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1 College - PHY2054C & Electromagnetic Waves 10/08/2014 My Office Hours: Tuesday 10:00 AM - Noon 206 Keen Building

2 PHY2054C Second Mini-Exam next week on Wednesday!! Location: UPL 101, 10:10-11:00 AM Exam on chapters 22, 23 & 25 (HW 5, 6, 7 & 8) Magnetic forces & fields (right-hand rules) Induction, Faraday s Law, Lenz s Law Generator & Eletromagnetic Spectrum & Waves Equation sheet will be provided. Do not forget to bring your student ID!

3 Review Question 1 The three loops of wire shown in the figure are all subject to the same uniform magnetic field that does not vary with time. Loop 1 oscillates back and forth as the bob in a pendulum, loop 2 rotates about a vertical axis, and loop 3 oscillates up and down at the end of a spring. Which loop, or loops, will have an induced emf? A Loop 1 B Loop 2 C Loop 3 D Loops 1 and 3 E Loops 2 and 3

4 Review Question 1 The three loops of wire shown in the figure are all subject to the same uniform magnetic field that does not vary with time. Loop 1 oscillates back and forth as the bob in a pendulum, loop 2 rotates about a vertical axis, and loop 3 oscillates up and down at the end of a spring. Which loop, or loops, will have an induced emf? A Loop 1 B Loop 2 C Loop 3 D Loops 1 and 3 E Loops 2 and 3

5 Review Question 2 The two identical bar magnets in the figure are dropped from rest along a vertical line passing through the center of the rings, as shown. The two rings are identical in every respect except that the ring on the right has a small break in it. Calling a L and a R the magnitude of the downward accelerations of the magnets on the left and right, respectively, you observe that A It is not possible to predict the outcome of this experiment with the data given. B a L = a R. C a L > a R. D a L < a R.

6 Review Question 2 The two identical bar magnets in the figure are dropped from rest along a vertical line passing through the center of the rings, as shown. The two rings are identical in every respect except that the ring on the right has a small break in it. Calling a L and a R the magnitude of the downward accelerations of the magnets on the left and right, respectively, you observe that A It is not possible to predict the outcome of this experiment with the data given. B a L = a R. C a L > a R. D a L < a R.

7 Review Question 3 The wire in the figure carries a current I that is increasing with time at a constant rate. The induced emf in each of the loops is such that A loop A has clockwise emf, loop B has no induced emf, and loop C has counterclockwise emf. B loop A has counterclockwise emf, loop B has no induced emf, and loop C has clockwise emf. C loop A has counterclockwise emf, loop B clockwise emf, and loop C has clockwise emf. D all loops experience counterclockwise emf. E no emf is induced in any loop.

8 Review Question 3 The wire in the figure carries a current I that is increasing with time at a constant rate. The induced emf in each of the loops is such that A loop A has clockwise emf, loop B has no induced emf, and loop C has counterclockwise emf. B loop A has counterclockwise emf, loop B has no induced emf, and loop C has clockwise emf. C loop A has counterclockwise emf, loop B clockwise emf, and loop C has clockwise emf. D all loops experience counterclockwise emf. E no emf is induced in any loop.

9 Outline 1 2

10 are devices that can increase or decrease the amplitude of an applied AC voltage: A simple transformer consists of two solenoid coils with the loops arranged such that all or most of the magnetic field lines and flux generated by one coil passes through the other coil.

11 are devices that can increase or decrease the amplitude of an applied AC voltage: 1 An AC current in one coil will induce an AC voltage across the other coil. 2 An AC voltage source is typically attached to one of the coils called the input coil. 3 The other coil is called the output coil.

12 Faraday s Law applies to both coils: V in = Φ in t and V out = Φ out t If the input coil has N in turns and the output coil has N out turns, the flux in the coils is related by: Φ out = N out N in Φ in V out = N out N in V in cannot change DC voltages!

13 Most practical transformers have central regions filled with a magnetic material. This produces a larger flux, resulting in a larger voltage at both the input and output coils. However: V out V in = constant Φ out = N out N in Φ in V out = N out N in V in cannot change DC voltages!

14 At the power plant Supply voltage of about V Cross-country lines Voltage of about V

15 and Power are used in the transmission of electric power over long distances: Many household appliances use transformers to convert the AC voltage at a wall socket to the smaller voltages needed in many devices. The output voltage of a transformer can also be made much larger by arranging the number of coils. According to the principle of energy conservation, the energy delivered through the input coil must either be stored in the transformer s magnetic field or transferred to the output circuit: The power delivered to the input coil must equal the output power.

16 and Power According to the principle of energy conservation, the energy delivered through the input coil must either be stored in the transformer s magnetic field or transferred to the output circuit: Since P = V I, if V out is greater than V in, then I out must be smaller than I in. P in = P out only in ideal transformers In real transformers, the coils always have a small electrical resistance causing some power dissipation. For a real transformer, the output power is always less than the input power. Power carried by the power line: P avg = V rms I rms

17 Example An AC power line operates with a voltage V rms = 500, 000 V and carries an AC current with I rms = 1000 A. What is the average (rms) power carried by the power line? P avg = V rms I rms = (500, 000 V)(1000 A) = 500 MW

18 Example An AC power line operates with a voltage V rms = 500, 000 V and carries an AC current with I rms = 1000 A. What is the average (rms) power carried by the power line? P avg = V rms I rms = (500, 000 V)(1000 A) = 500 MW If 10 % of the power is dissipated in the power line itself, what is the resistance of the power line? P line = I 2 rms R line = (0.10) P avg R line = (0.10) P avg I 2 rms = (0.10)(5 108 W) (1000 A) 2 = 50 Ω

19 Example An AC power line operates with a voltage V rms = 500, 000 V and carries an AC current with I rms = 1000 A. The same power line is now operated with a reduced voltage of V rms = 250, 000 V and current I rms = 2000 A. The product V rms I rms is still the same, so the power carried by the line is the same. What percentage of this power is now dissipated in the power line? P line = I 2 rms R line = (2000 A) 2 (50 Ω) = W The percentage of the total power now dissipated in the line is: P line 100 = W P total = 40 % W compared to the initial 10 %.

20 Outline 1 2

21 Connection between Electricity and Magnetism Sources of Electric Fields Sources of Magnetic Fields Electric Charge

22 Electric Fields Capacitor Michael Faraday ( ) Static Point Charges

23 Connection between Electricity and Magnetism Sources of Electric Fields Electric Charge Sources of Magnetic Fields Moving Electric Charge

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

25 Connection between Electricity and Magnetism Sources of Electric Fields Electric Charge Sources of Magnetic Fields Moving Electric Charge Changing Magnetic Fields

26 Transformer Alternating current in one circuit induces an alternating current in a second circuit. Transfers power between the two circuits. Doesn t transfer charge between the two circuits.

27 Connection between Electricity and Magnetism Sources of Electric Field Electric Charge Changing Magnetic Fields Sources of Magnetic Fields Moving Electric Charge Changing Electric Fields

28 Connection between Electricity and Magnetism Sources of Electric Field Electric Charge Changing Magnetic Fields Sources of Magnetic Fields Moving Electric Charge Changing Electric Fields Electric fields that change with time produce magnetic fields. Electromagnetic Waves

29 James Clerk Maxwell Scottish Physicist ( )

30 James Clerk Maxwell Can these fields have a life on their own? Can they propagate independently of the charges? Scottish Physicist ( )

31 James Clerk Maxwell Can these fields have a life on their own? Can they propagate independently of the charges? Maxwell formulated a complete theory on electromagnetism. Also predicted electromagnetic waves

32 Experimental Evidence of Electromagnetic Waves Heinrich Hertz German Physicist ( ) 1 What is the nature of electric and magnetic fields? 2 What is the idea of action at a distance? 3 How fast do the field lines associated with a charge react to a movement in the charge?

33 The History of Wireless Communication 1865 Prediction of radio waves (James Clerk Maxwell) 1886 Experimental evidence of radio waves (Heinrich Hertz) 1895 Signal transmission over 10 m 1899 Signal transmission over the English Channel (Giulielmo Marconi) 1901 Signal transmission over the Atlantic Ocean

34 The History of Wireless Communication 1865 Prediction of radio waves (James Clerk Maxwell) 1886 Experimental evidence of radio waves (Heinrich Hertz) 1895 Signal transmission over 10 m 1899 Signal transmission over the English Channel (Giulielmo Marconi) 1901 Signal transmission over the Atlantic Ocean