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Quiz 1. Borderline. Trouble. Deep Trouble. From Last time.

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Quiz 1 l.jpg
Quiz 1



Deep Trouble

Slide2 l.jpg

From Last time

Temperature effects on resistancePermanent magnets and gaussmeter.Electromagnets - BigBite for ResearchForce on a wire in a magnetic field - DemoForce on a coil in a magnetic field - DemoCrossed E/B fields. Discovery of the electronLecture 9 starts here

Slide5 l.jpg


























Torques on current loops

Electric motors operate by connecting a coil in a magnetic field to a current supply,

which produces a torque on the coil causing it to rotate.







Above is a rectangular loop of wire of sides a and b carrying current i.

B is in the plane of the loop.

Equal and opposite forces

are exerted on the sides



No forces exerted on



Since net force is zero, we can evaluate

(torque) about any point. Evaluate it


about P (cm).

For N loops we mult by N


of loop

Slide6 l.jpg

Loop will rotate and stop when

the plane is perpendicular to B

Magnetic dipole moment is in the

same direction as the normal.

Electron moving with speed v in a crossed electric and magnetic field in a cathode ray tube l.jpg
Electron moving with speed v in a crossed electric and magnetic field in a cathode ray tube.



Electric field bends particle upwards

Magnetic field bends it downwards

Discovery of the electron by j j thompson in 1897 l.jpg
Discovery of the electron by J.J. Thompson in 1897 magnetic field

E=0, B=0 Observe spot on screen

2. Set E to some value and measure y the deflection

3. Now turn on B until spot returns to the original position

This ratio was first measured by Thompson

to be lighter than hydrogen by 1000

4 Solve for

Show demo of CRT

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How do you calculate the magnetic field

magnetic field from a moving charge

or from a current flowing in a wire?

What do the flux lines look like?

Lecture 9 magnetic fields due to currents ch 30 l.jpg
Lecture 9 Magnetic Fields due to Currents Ch. 30 magnetic field

  • Cartoon - Shows magnetic field around a long current carrying wire and a loop of wire

  • Topics

    • Magnetic field produced by a moving charge

    • Magnetic fields produced by currents. Big Bite as an example.

    • Using Biot-Savart Law to calculate magnetic fields produced by currents.

    • Examples: Field at center of loop of wire, at center of circular arc of wire, at center of segment of wire.

    • Amperes’ Law : Analogous to Gauss’ L:aw in electrostatics, Useful in symmetric cases.

    • Infinitely long straight wire of radius a. Find B outside and inside wire.

    • Solenoid and Toroid Find B field.

    • Forces between current carrying wires or parallel moving charges

  • Demos

    • Torque on a current loop(galvanometer)

    • Iron filings showing B fields around wires with currents.

    • Compass needle near current carrying wire

    • Big Bite as an example of using a magnet as a research tool.

    • Force between parallel wires carrying identical currents.

  • Polling

Magnetic fields due to currents l.jpg
Magnetic Fields due to Currents magnetic field

  • So far we have used permanent magnets as our source of magnetic field. Historically this is how it started.

  • In early decades of the last century, it was learned that moving charges and electric currents produced magnetic fields.

  • How do you find the Magnetic field due to a moving point charge?

  • How do you find the Magnetic field due to a current?

    • Biot-Savart Law – direct integration

    • Ampere’s Law – uses symmetry

  • Examples and Demos

Topic moving charges produce magnetic fields l.jpg

  • Magnitude of B is proportional to q, magnetic field, and 1/r2.

  • B is zero along the line of motion and proportional to sin at other points.

  • The direction is given by the RHR rotating into

  • Magnetic permeability

Topic: Moving charges produce magnetic fields



Determined from


Demo Sprinkle iron filings around a wire carrying current

Example l.jpg

10 magnetic field8 m/s



A point charge q = 1 mC (1x10-3C) moves in the x direction with v = 108 m/s. It misses a mosquito by 1 mm. What is the B field experienced by the mosquito?

Recall the e field of a charge distribution l.jpg

Topic: A current produces a magnetic field magnetic field

Recall the E field of a charge distribution

“Coulombs Law”

Biot-Savart Law

To find the field of a current distribution use:

This Law is found from experiment

Slide16 l.jpg

i magnetic field



is a vector coming out of the paper The angle between dl and r is constant and equal to 90 degrees.



Find B field at center of loop of wire lying in a plane with radius R and total current i flowing in it.

Magnitude of B field at center of loop. Direction is out of paper.

Example17 l.jpg

i magnetic field


Loop of wire of radius R = 5 cm and current i = 10 A. What is B at the center? Magnitude and direction

Direction is out of the page.

What is the b field at the center of a segment or circular arc of wire l.jpg
What is the B field at the center of a segment or circular arc of wire?


Total length of arc is S.

where S is the arc length S = R0

0 is in radians (not degrees)




Why is the contribution to the B field at P equal to zero from

the straight section of wire?

Find magnetic field at center of arc length l.jpg

Suppose you had the following loop arc of wire?.

Find magnetic field at center of arc length

What is the magnitude and direction of B at the origin?

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Current enclosed by the path arc of wire?

Ampere’s Law is

Next topic: Ampere’s Law

Allows us to solve certain highly symmetric current problems for the magnetic field as Gauss’ Law did in electrostatics.

This dl is along the Amperian loop not

along the wire. dl is the same as ds below


Slide21 l.jpg

i arc of wire?



By symmetry


i = 10 A

R = 10 cm

Example: Use Ampere’s Law to find B near a very long, straight wire. B is independent of position along the wire and only depends on the distance from the wire (symmetry).

Show Fe fillings around a straight wire with current,

current loop, and solenoid.

Force between two current carrying wires l.jpg

Suppose one of the currents is in the opposite direction? What direction is ?

Find the force due to the current element of the first wire and the magnetic field of the second wire. Integrate over the length of both wires. This will give the force between the two wires.

Force between two current carrying wires

(Given by Ampere’s Law)

The force is directed towards the wire (wires are attracted).

Show Demo

Slide24 l.jpg
Example What direction is ?: What is the magnetic field inside the wire? Find the magnetic field inside a long, thick wire of radius a

IC = current enclosed by the circle whose radius is r

Example: Find field inside a solenoid. See next slide.

Solenoid l.jpg
Solenoid What direction is ?

n is the number of turns per meter

Slide26 l.jpg

d What direction is ?





IC is the total current enclosed by the path

n is the the number of loops of current per meter and h is the length of one side.

First evaluate the right side of the integral , it’s easy

Right side =

Slide27 l.jpg

d What direction is ?





Evaluate left side of the integral :

n = the number of loops or turns per meter

Toroid l.jpg

a < r < b What direction is ?




N is the total

number of turns

Tokamak Toroid at Princeton

I = 73,000 Amps for 3 secs

B = 5.2 T

Magnetic dipole inverse cube law l.jpg
Magnetic dipole inverse cube law What direction is ?




Magnetic field lines for a bar magnet

and a current loop are similar

Chapter 29 problem 5 l.jpg
Chapter 29 Problem 5 What direction is ?

In Figure 29-36, two circular arcs have radii a = 13.1 cm and b = 10.6 cm, subtend angle θ = 74.0°, carry current i = 0.388 A, and share the same center of curvature P. What is the magnitude and direction of the net magnetic field at P? (Take out of the page to be positive.)

Figure 29-36

Chapter 29 problem 41 l.jpg
Chapter 29 Problem 41 What direction is ?

A 180 turn solenoid having a length of 22 cm and a diameter of 10 cm carries a current of 0.39 A. Calculate the magnitude of the magnetic field B inside the solenoid.

Chapter 29 problem 49 l.jpg
Chapter 29 Problem 49 What direction is ?

A student makes a short electromagnet by winding 150 turns of wire around a wooden cylinder of diameter d = 7.0 cm. The coil is connected to a battery producing a current of 4.0 A in the wire.

  • What is the magnetic moment of this device?

    (b) At what axial distance z d will the magnetic field of this dipole have the magnitude 5.0 µT (approximately one-tenth that of Earth's magnetic field)?