General Physics (PHYS )

Chapter 21 - 22

Electricity and Magnetism

Resistors in series and parallel

Kirchoff’s rules

Magnetism

Magnets

Announcement

Exam: Question # 11 & Last problem

Science FunLand Volunteer

Units of Chapter 22

• The Magnetic Field

• 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)

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.

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?

1. 2.1 kW

2. 1.0 kW

3. 1.1 kW

4. 2.6 kW

0%

0%

0%

0%

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 2nd question

1. 4.0 A

2. 4.6 A

3. 3.0 A

4. 2.6 A

0%

0%

0%

0%

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?

1. 2

2. 1

3. 4

4. 3

0%

0%

0%

0%

Example # 10 Three resistors R1 = 100 Ω, R2 = 150 Ω , R3 = 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.

Example # 11 Three resistors R1 = 100 Ω, R2 = 150 Ω , R3 = 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.

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.

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.

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).

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.

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:

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)

• Does this remind you of a similar case in electrostatics?

NS

Magnetic Field Lines of a bar magnet

Electric Field Lines of an Electric Dipole

NS

Chapter 22 Cont.

• http://medschoolodyssey.wordpress.com/2010/03/30/some-statistics-on-the-mcat-and-your-undergraduate-major/

Magnetism

Earth’s Magnetic Field

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:

• 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!

N S N N S S

Even an individual electron has a magnetic “dipole”!

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)

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

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)

The Magnetic Force on Moving Charges

This is an

experimental result

– we observe it to

be true.

Force on a Charge Moving in a Magnetic Field

is the magnetic force

is the charge

is the velocity of the charge

is the magnetic field

This equation defines B, just as defines .

vB

BF

q

EqFE

E

B

Magnitude of FB.

• The magnitude of FB = |q| v B sin q

– q is the smaller angle between and

– FB is zero when and are parallel or antiparallel

• q = 0 or 180o

– FB is a maximum when and are perpendicular

• q = 90o

v

v

v

B

B

B

The Magnetic Force on Moving Charges

The magnetic force on a moving charge is

actually used to define the magnetic field:

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.

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.