Lecture 33: WED 11 NOVLecture 33: WED 11 NOVReview Session B: Midterm 3Review Session B: Midterm 3

Physics 2113

Jonathan Dowling

EXAM 03: 6PM WED 11 NOV in Cox Auditorium

The exam will cover: Ch.27.4 through Ch.30The exam will be based on: HW08–11

The formula sheet and practice exams are here:http://www.phys.lsu.edu/~jdowling/PHYS21133-FA15/lectures/index.html

Note this link also includes tutorial videos on the various right hand rules.

You can see examples of even older exam IIIs here:http://www.phys.lsu.edu/faculty/gonzalez/Teaching/Phys2102/Phys2102OldTests/

The Biot-Savart LawThe Biot-Savart LawFor B-FieldsFor B-Fields

• Quantitative rule for computing the magnetic field from any electric current

• Choose a differential element of wire of length dL and carrying a current i

• The field dB from this element at a point located by the vector r is given by the Biot-Savart Law

μ0 =4π×10–7 T•m/A(permeability constant of free space)

μ0 =4π×10–7 T•m/A(permeability constant of free space)

Jean-Baptiste Biot (1774-1862)

Felix Savart (1791-1841)

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Field of a Straight WireField of a Straight Wire

Biot-Savart LawBiot-Savart Law• A circular arc of wire of radius R

carries a current i. • What is the magnetic field at the

center of the loop?

Direction of B?? Not another right hand rule?!

TWO right hand rules!:If your thumb points along the CURRENT, your fingers will point in the same direction as the FIELD.

If you curl our fingers around direction of CURRENT, your thumb points along FIELD!

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Example, Magnetic field at the center of a circular arc of a circle pg 768:

ICPP: What is the direction of the B field at point C?

✔

Example, Magnetic field at the center of a circular arc of a circle.:

A length of wire is formed into a closed circuit with radii a and b, as shown in the Figure, and carries a current i.

                                           

(a) What are the magnitude and direction of B at point P?                                            

(b) Find the magnetic dipole moment of the circuit.                                                

μ=NiA

Right Hand Rule & Biot-Savart: Given i Find B 

Magnetic field due to wire 1 where the wire 2 is,

d

I2I1L

Force on wire 2 due to this field,F

Forces Between WiresForces Between Wires

eHarmony’s Rule for Currents: Same Currents – Attract!Opposite Currents – Repel!

eHarmony’s Rule for Currents: Same Currents – Attract!Opposite Currents – Repel!

29.3: Force Between Two Parallel Wires:

29.3.3. The drawing represents a device called Roget’s Spiral. A coil of wire hangs vertically and its windings are parallel to one another. One end of the coil is connected by a wire to a terminal of a battery. The other end of the coil is slightly submerged below the surface of a cup of mercury. Mercury is a liquid metal at room temperature. The bottom of the cup is also metallic and connected by a wire to a switch. A wire from the switch to the battery completes the circuit. What is the behavior of this circuit after the switch is closed?

a) When current flows in the circuit, the coils of the wire move apart and the wire is extended further into the mercury.

b) Nothing happens to the coil because there will not be a current in this circuit.

c) A current passes through the circuit until all of the mercury is boiled away.

d) When current flows in the circuit, the coils of the wire move together, causing the circuit to break at the surface of the mercury. The coil then extends and the process begins again when the circuit is once again complete.

29.3: Force Between Two Parallel Wires, Rail Gun:

The circulation of B (the integral of B scalar ds) along an imaginary closed loop is proportional to the net amount of current traversing the loop.

The circulation of B (the integral of B scalar ds) along an imaginary closed loop is proportional to the net amount of current traversing the loop.

i1

i2i3

ds

i4

Thumb rule for sign; ignore i4

If you have a lot of symmetry, knowing the circulation of B allows you to know B.

AmpereAmpere’’s law: Closed s law: Closed Loops Loops

The circulation of B (the integral of B scalar ds) along an imaginary closed loop is proportional to the net amount of current piercing the loop.

The circulation of B (the integral of B scalar ds) along an imaginary closed loop is proportional to the net amount of current piercing the loop.

Thumb rule for sign; ignore i3

If you have a lot of symmetry, knowing the circulation of B allows you to know B.

AmpereAmpere’’s law: Closed s law: Closed Loops Loops

enc Calculation of . We curl the fingers

of the right hand in the direction in which

the Amperian loop was traversed. We note

the

i

direction of the thumb.

AmpereAmpere’’s Law: Example 2s Law: Example 2

• Infinitely long cylindrical wire of finite radius R carries a total current i with uniform current density

• Compute the magnetic field at a distance r from cylinder axis for:r < a (inside the wire)r > a (outside the wire)

• Infinitely long cylindrical wire of finite radius R carries a total current i with uniform current density

• Compute the magnetic field at a distance r from cylinder axis for:r < a (inside the wire)r > a (outside the wire)

iCurrent out

of page, circular field

lines

AmpereAmpere’’s Law: Example 2 s Law: Example 2 (cont)(cont)

Current out of page, field

tangent to the closed

amperian loop

For r < R For r>R, ienc=i, soB=μ0i/2πR = LONG WIRE!

For r>R, ienc=i, soB=μ0i/2πR = LONG WIRE!

Need Current Density J!

R r

B

O

B-Field In/Out Wire: J is B-Field In/Out Wire: J is ConstantConstant

29.4.1. A copper cylinder has an outer radius 2R and an inner radius of R and carries a current i. Which one of the following statements concerning the magnetic field in the hollow region of the cylinder is true?

a) The magnetic field within the hollow region may be represented as concentric circles with the direction of the field being the same as that outside the cylinder.

b) The magnetic field within the hollow region may be represented as concentric circles with the direction of the field being the opposite as that outside the cylinder.

c) The magnetic field within the hollow region is parallel to the axis of the cylinder and is directed in the same direction as the current.

d) The magnetic field within the hollow region is parallel to the axis of the cylinder and is directed in the opposite direction as the current.

e) The magnetic field within the hollow region is equal to zero tesla.

29.4.3. The drawing shows two long, straight wires that are parallel to each other and carry a current of magnitude i toward you. The wires are separated by a distance d; and the centers of the wires are a distance d from the y axis. Which one of the following expressions correctly gives the magnitude of the total magnetic field at the origin of the x, y coordinate system?

a)

b)

c)

d)

e) zero tesla

0

2

i

d

μ

0

2

i

d

μ

0

2

i

d

μπ

0i

d

μπ

29.5: Solenoids:

Fig. 29-19 Application of Ampere’s law to a section of a long ideal solenoid carrying a current i. The Amperian loop is the rectangle abcda.

Here n be the number of turns per unit length of the solenoid

29.5.5. A solenoid carries current I as shown in the figure. If the observer could “see” the magnetic field inside the solenoid, how would it appear?

29.5.1. The drawing shows a rectangular current carrying wire loop that has one side passing through the center of a solenoid. Which one of the following statements describes the force, if any, that acts on the rectangular loop when a current is passing through the solenoid.

a) The magnetic force causes the loop to move upward.

b) The magnetic force causes the loop to move downward.

c) The magnetic force causes the loop to move to the right.

d) The magnetic force causes the loop to move to the left.

e) The loop is not affected by the current passing through the solenoid or the magnetic field resulting from it.

Magnetic Field of a Magnetic DipoleMagnetic Field of a Magnetic Dipole

All loops in the figure have radius r or 2r. Which of these arrangements produce the largest magnetic field at the point indicated?

A circular loop or a coil currying electrical current is a magnetic dipole, with magnetic dipole moment of magnitude μ=NiA. Since the coil curries a current, it produces a magnetic field, that can be calculated using Biot-Savart’s law:

29.6: A Current Carrying Coil as a Magnetic Dipole:

29.6: A Current Carrying Coil as a Magnetic Dipole:

Lenz’s Law

Induction and Induction and InductanceInductance

• Faraday’s law: or

• Inductance: L=NΦ/i – For a solenoid: L=μ0n2Al=μ0N2A/l

• Inductor EMF: EL= L di/dt

• RL circuits: i(t)=(E/R)(1–e–tR/L) or i(t)=i0e–tR/L

• RL Time Constant: τ = L/R Units: [s]• Magnetic energy: U=Li2/2 Units: [J]

• Magnetic energy density: u=B2/2μ0 Units: [J/m3]

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SummarySummaryTwo versions of Faradays’ law:

– A time varying magnetic flux produces an EMF:

–A time varying magnetic flux produces an electric field:

Changing B-Flux Induces EMF

Flux Up Flux DownRL Circuits

RL Circuits

E/2

t=?

Make or Break?At t=0 all L’s make breaks so throw out all loops with a break and solve circuit. At t=∞ all L’s make solid wires so replace L’s with wire and solve circuit.

Checkpoints/QuestionsCheckpoints/Questions

Magnitude of induced emf/current? Magnitude/direction of induced current?Magnitude/direction of magnetic field inducing current?

Given |∫E∙ds| , direction of magnetic field?

Current inducing EL?

Current through the battery?Time for current to rise 50% of max value?

Given B, dB/dt, magnitude of electric field?

Largest current?

Largest L?

R,L or2R,L orR, 2L or2R,2L?

30.4.1. Consider the situation shown. A triangular, aluminum loop is slowly moving to the right. Eventually, it will enter and pass through the uniform magnetic field region represented by the tails of arrows directed away from you. Initially, there is no current in the loop. When the loop is entering the magnetic field, what will be the direction of any induced current present in the loop?

a) clockwise

b) counterclockwise

c) No current is induced.

30.4.3. A rigid, circular metal loop begins at rest in a uniform magnetic field directed away from you as shown. The loop is then pulled through the field toward the right, but does not exit the field. What is the direction of any induced current within the loop?

a) clockwise

b) counterclockwise

c) No current is induced.

ICPP ICPP • 3 loops are shown.

• B = 0 everywhere except in

the circular region I where B

is uniform, pointing out of

the page and is increasing

at a steady rate.

• Rank the 3 loops in order of

increasing induced EMF.

– (a) III < II < I ?

– (b) III < II = I ?

– (c) III = II = I ?

• III encloses no flux so EMF=0

• I and II enclose same flux so

EMF same.• Are Currents in Loops I & II

Clockwise or Counterclockwise?

• III encloses no flux so EMF=0

• I and II enclose same flux so

EMF same.• Are Currents in Loops I & II

Clockwise or Counterclockwise?

III

III

B

The RL circuitThe RL circuit• Set up a single loop series

circuit with a battery, a resistor, a solenoid and a switch.

• Describe what happens when the switch is closed.

• Key processes to understand:– What happens JUST AFTER

the switch is closed?– What happens a LONG TIME

after switch has been closed?

– What happens in between?

Key insights:• You cannot change the

CURRENT in an inductor instantaneously!

• If you wait long enough, the current in an RL circuit stops changing!

At t = 0, a capacitor acts like a solid wire and inductor acts like break in the wire.At t = ∞ a capacitor acts like a break in the wire and inductor acts like a solid wire.

ICPPICPPImmediately after the switch is closed, what is the potential difference across the inductor?(a) 0 V(b) 9 V(c) 0.9 V

• Immediately after the switch, current in circuit = 0.• So, potential difference across the resistor = 0!• So, the potential difference across the inductor = E = 9 V!

10 Ω

10 H9 V

Fluxing Up The InductorFluxing Up The Inductor

• How does the current in the circuit change with time?

Time constant of RL circuit: τ = L/RTime constant of RL circuit: τ = L/R

E/R

i(t) Fast = Small τ

Slow= Large τ

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Fluxing Down an Fluxing Down an InductorInductor

E/RExponential defluxing

i(t)

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The switch is at a for a long time, until the inductor is charged. Then, the switch is closed to b.

What is the current in the circuit?

Loop rule around the new circuit walking counter clockwise:

When the switch is closed, the inductor begins to get fluxed up, and the current is i=(E/R)(1e–t/τ).When the switch is opened, the inductors begins to deflux.

The current in this case is then i= (E/R) e–t/τ

Example, RL circuit, immediately after switching and after a long time: SP30.05