General Physics (PHYS )

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1 General Physics (PHYS ) Chapter Electricity and Magnetism Resistors in series and parallel Kirchoff s rules Magnetism Magnets

2 Announcement Exam: Question # 11 & Last problem Science FunLand Volunteer

3 The Magnetic Field Units of Chapter 22 The Magnetic Force on Moving Charges The Motion of Charged Particles in a Magnetic Field The Magnetic Force Exerted on a Current-Carrying Wire Loops of Current and Magnetic Torque Electric Currents, Magnetic Fields, and Ampère s Law Current Loops and Solenoids Magnetism in Matter (no cover in class, but you need to read it)

4 Energy Use: Energy and Power in Electric Circuits When the electric company sends you a bill, your usage is quoted in kilowatt-hours (kwh). They are charging you for energy use, and kwh are a measure of energy.

5 Example # 8 Electric utilities measure energy in kilowatt-hours (kwh), where 1 kwh is the energy consumed if you use energy at the rate of 1 kw for 1 hour. If your monthly electric bill (30 days) is $100 and you pay 12.5c/kWh, what s your home s average power consumption and average current, assuming a 240-V potential difference between the wires supplying your home? kw kw kw kw 0% 0% 0% 0%

6 Example # 8 Electric utilities measure energy in kilowatt-hours (kwh), where 1 kwh is the energy consumed if you use energy at the rate of 1 kw for 1 hour. If your monthly electric bill (30 days) is $100 and you pay 12.5c/kWh, what s your home s average power consumption and average current, assuming a 240-V potential difference between the wires supplying your home? Response for 2 nd question A 0% A 0% A 0% A 0%

7 Example # 9 Several male students in the same dorm room want to dry their hair. Having taken PHYS 1402 at UTPA, they have set their hair dryers to the low, 1000-W settings. Assuming a standard 120-V, how many hair dryers can they operate simultaneously without tripping the 20-A circuit breaker? % 0% 0% 0%

8 Example # 10 Three resistors R 1 = 100 Ω, R 2 = 150 Ω, R 3 = 300 Ω are connected in series. (a) Find the equivalent resistance. (b) When the circuit is connected across a 12.0-V battery, find the total current supplied by the battery. ( c ) The current in each resistor. ( d ) The potential difference across each resistor.

9 Example # 11 Three resistors R 1 = 100 Ω, R 2 = 150 Ω, R 3 = 300 Ω are connected in parallel. (a) Find the equivalent resistance. (b) When the circuit is connected across a 12.0-V battery, find the total current supplied by the battery. ( c ) The current in each resistor. ( d ) The potential difference across each resistor.

10 Kirchhoff s Rules More complex circuits cannot be broken down into series and parallel pieces. For these circuits, Kirchhoff s rules are useful. The junction rule is a consequence of charge conservation; the loop rule is a consequence of energy conservation.

11 Kirchhoff s Rules The junction rule: At any junction, the current entering the junction must equal the current leaving it. The algebraic sum of all currents meeting at any junction in a circuit must equal zero.

12 Kirchhoff s Rules The loop rule: The algebraic sum of the potential differences around a closed loop must be zero (it must return to its original value at the original point).

13 Magnetism Magnetic effects from natural magnets have been known for a long time. Recorded observations from the Greeks more than 2500 years ago. The word magnetism comes from the Greek word for a certain type of stone (lodestone) containing iron oxide found in Magnesia, a district in northern Greece. Properties of lodestones: could exert forces on similar stones and could impart this property (magnetize) to a piece of iron it touched. Small sliver of lodestone suspended with a string will always align itself in a north-south direction it detects the earth s magnetic field.

14 The Magnetic Field Permanent bar magnets have opposite poles on each end, called north and south. Like poles repel; opposites attract. If a magnet is broken in half, each half has two poles:

15 Bar Magnet Bar magnet... two poles: N and S Like poles repel; Unlike poles attract. Magnetic Field lines: (defined in same way as electric field lines, direction and density) S N Does this remind you of a similar case in electrostatics?

16 Electric Field Lines of an Electric Dipole Magnetic Field Lines of a bar magnet S N

17 Chapter 22 Cont. Magnetism 0/03/30/some-statistics-on-the-mcat-andyour-undergraduate-major/

18 Earth s Magnetic Field

19 Magnetic Monopoles? Perhaps there exist magnetic charges, just like electric charges. Such an entity would be called a magnetic monopole (having + or - magnetic charge). How can you isolate this magnetic charge? Try cutting a bar magnet in half: S N S N S N Many searches for magnetic monopoles the existence of which would explain (within framework of QM) the quantization of electric charge (argument of Dirac) No monopoles have ever been found! Even an individual electron has a magnetic dipole!

20 Source of Magnetic Fields? What is the source of magnetic fields, if not magnetic charge? Answer: electric charge in motion! e.g., current in wire surrounding cylinder (solenoid) produces very similar field to that of bar magnet. Therefore, understanding source of field generated by bar magnet lies in understanding currents at atomic level within bulk matter. Orbits of electrons about nuclei Intrinsic spin of electrons (more important effect)

21 Magnetic Fields in analogy with Electric Fields Electric Field: Distribution of charge creates an electric field E(r) in the surrounding space. Field exerts a force F=q E(r) on a charge q at r Magnetic Field: Moving charge or current creates a magnetic field B(r) in the surrounding space. Field exerts a force F on a charge moving q at r (emphasis this chapter is on force law) 22

22 Applications of magnetic forces electric motor, electric car, electric generator, electric drill, solenoid actuator. tape recorder, magnetic hard drive, CRT for oscilloscopes and TV magnetic levitation for trains science (Nmr, mass spectrometer) medicine (MRI; magnetic navigation systems for catheter)

23 The Magnetic Force on Moving Charges This is an experimental result we observe it to be true.

24 Force on a Charge Moving in a Magnetic Field B F B q v B is the magnetic force is the charge is the velocity of the charge is the magnetic field This equation defines B, just as qe defines. F E E

25 Magnitude of F B. The magnitude of F B = q v B sin q q is the smaller angle between v and B F B is zero when and are parallel or antiparallel q = 0 or 180 o F B is a maximum when and are perpendicular q = 90 o v v B B

26 The Magnetic Force on Moving Charges The magnetic force on a moving charge is actually used to define the magnetic field:

27 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.

28 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.

General Physics (PHYS )

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 Magnetism