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W06D2 Magnetic Forces and Sources of Magnetic FieldsPowerPoint Presentation

W06D2 Magnetic Forces and Sources of Magnetic Fields

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Presentation Transcript

### Announcements

### Demonstration:Series or Parallel Current Carrying Wires G9

### What Creates Magnetic Fields: Moving Charges

### Demonstration:Field Generated by Wire G12

### Continuous Moving Charge Distributions:Currents & Biot-Savart

### Repeat Demonstration:Parallel & Anti-Parallel Currents G9

### Animation: Magnetic Field Generated by a Current Loop

W06D2 Magnetic Forces and Sources of Magnetic Fields

W06D2 Magnetic Force on Current Carrying Wire, Sources of Magnetic Fields: Biot-Savart Law

Reading Course Notes: Sections 8.3, 9.1-9.2

Review Week 6 Tuesday from 9-11 pm in 26-152

PS 6 due Week 6 Tuesday at 9 pm in boxes outside 32-082 or 26-152

W06D3 PS05: Calculating Magnetic Fields and Magnetic Force Reading Course Notes: Sections 8.9, 8.10, 9.10.1, 9.11.1-.3, 9.11.7-.8

Exam 2 Thursday March 21 7:30 - 9:30 pm

Conflict Exam 2 March 22 9-11 am or 10-12 noon

Magnetic Force on Current-Carrying Wire

Current is moving charges, and we know that moving charges feel a force in a magnetic field

Magnetic Force on Current-Carrying Wire

If the wire is a uniform magnetic field then

If the wire is also straight then

Demonstration:Wire in a Magnetic Field (Jumping Wire) G8

http://tsgphysics.mit.edu/front/?page=demo.php&letnum=G%208&show=0

http://tsgphysics.mit.edu/front/?page=demo.php&letnum=G%209&show=0

You tube video of experiment showing parallel wires attracting

http://youtu.be/rU7QOukFjTo

or repelling:

http://youtu.be/bnbKdbXofEI

Group Problem: Current Loop

A conducting rod of uniform mass per length l is suspended by two flexible wires in a uniform magnetic field of magnitude B which points out of the page and a uniform gravitational field of magnitude g pointing down. If the tension in the wires is zero, what is the magnitude and direction of the current in the rod?

http://youtu.be/JmqX1GrMYnU

Electric Field Of Point Charge

An electric charge produces an electric field:

: unit vector directed from the charged object

to the field point P

Magnetic Field Of Moving Charge

Moving charge with velocity v produces magnetic field:

P

unit vector directed from charged object to P

permeability of free space

http://tsgphysics.mit.edu/front/?page=demo.php&letnum=G%2012&show=0

The Biot-Savart Law

Current element of length ds pointing in direction of current I produces a magnetic field:

http://web.mit.edu/viz/EM/visualizations/magnetostatics/MagneticFieldConfigurations/CurrentElement3d/CurrentElement.htm

Concept Question: Biot-Savart

The magnetic field at P points towards the

- +x direction
- +y direction
- +z direction
- -x direction
- -y direction
- -z direction
- Field is zero

Concept Question Answer: Biot-Savart

Answer: 6. is in the negative z direction (into page)

The vertical line segment contributes nothing to the field at P (it is parallel to the displacement). The horizontal segment makes a field pointing into the page by the right-hand rule # 2 (right thumb in direction of current, fingers curl into page at P.

Concept Question: Bent Wire

The magnetic field at P is equal to the field of:

- a semicircle
- a semicircle plus the field of a long straight wire
- a semicircle minus the field of a long straight wire
- none of the above

Concept Question Answer: Bent Wire

Answer: 2. Semicircle + infinite wire

All of the wire makes into the page. The two straight parts, if put together, would make an infinite wire. The semicircle is added to this to get the magnetic field

http://tsgphysics.mit.edu/front/?page=demo.php&letnum=G%209&show=0

Concept Question: Parallel Wires

Consider two parallel current carrying wires. With the currents running in the opposite direction, the wires are

- attracted (opposites attract?)
- repelled (opposites repel?)
- pushed another direction
- not pushed – no net force
- I don’t know

Concept Q. Answer: Parallel Wires

Answer: 1. The wires are repelled

I1 creates a magnetic field into the

page at wire 2. That makes a force

on wire 2 to the right.

I2 creates a magnetic field into the

page at wire 1. That makes a force

on wire 1 to the left.

Can we understand why?

Whether they attract or repel can be seen in the shape of the created B field

http://youtu.be/5nKQjKgS9z0

http://youtu.be/nQX-BM3GCv4

You tube videos of field line animations showing why showing parallel wires attract: or repel:

Concept Question: Current Carrying Coils

- parallel currents that attract
- parallel currents that repel
- opposite currents that attract
- opposite currents that repel

The above coils have

Concept Q. Answer: Current Carrying Coils

Look at the field lines at the edge between the coils. They are jammed in, want to push out. Also, must be in opposite directions

Answer: 4. Opposite currents that repel

http://web.mit.edu/viz/EM/visualizations/magnetostatics/ForceOnCurrents/MagneticForceRepel/MagneticForceRepel.htm

Example : Coil of Radius R

Consider a coil with radius R and current I

Find the magnetic field B at the center (P)

Example : Coil of Radius R

Consider a coil with radius R and current I

- 1) Think about it:
- Legs contribute nothing

- I parallel to r
- Ring makes field into page

- 2) Choose a ds
- 3) Pick your coordinates
- 4) Write Biot-Savart

http://web.mit.edu/viz/EM/visualizations/magnetostatics/calculatingMagneticFields/RingMagInt/RingMagIntegration.htm

Group Problem:B Field from Coil of Radius R

Consider a coil with radius R and carrying a current I

WARNING:

This is much harder than the previous problem. Why??

What is B at point P?