Physics 10 Grade Magnetic field Field in the conductors presentation

Field density of the infinitely straight and circular conductors with current. Right-hand screw rule.

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Lesson objectives

to explain the physical meaning of magnetic induction vector based on problem solving and modern technological advances (magnetic levitation train, etc.)

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What is B?

B is the magnetic field
It has units of Tesla (T)

This class & next: creating B fields
Next two classes: feeling B fields

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How Big is a Tesla?

Earth’s Field
Brain (at scalp)
Refrigerator Magnet
Inside MRI
Good NMR Magnet
Biggest in Lab
Biggest in Pulsars

5 x 10-5 T = 0.5 Gauss
~1 fT

3 T
18 T
150 T (pulsed)

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Notation Demonstration

OUT of page
“Arrow Head”

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

X

INTO page
“Arrow Tail”

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PRS: Cross Product

What is the direction of A x B given the following two vectors?

up
down
left
right
into page
out of page
Cross product is zero (so no direction)

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PRS Answer: Cross Product

Using your right hand, thumb along A, fingers along B, palm into page

Answer: 5. A x B points into the page

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PRS: Cross Product

What is the direction of A x B given the following two vectors?

up
down
left
right
into page
out of page
Cross product is zero (so no direction)

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PRS Answer: Cross Product

Using your right hand, thumb along A, fingers along B, palm out of page

Also note from before, one vector flipped so result does too

Answer: 6. A x B points out of the page

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Continuous charge distributions:Currents & Biot-Savart

^

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From Charges to Currents?

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The Biot-Savart Law

Current element of length ds carrying current I produces a magnetic field:

(Shockwave)

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The Right-Hand Rule #2

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Animation:Field Generated by a Moving Charge

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Demonstration:Field Generated by Wire

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Example : Coil of Radius R

Consider a coil with radius R and current I

Find the magnetic field B at the center (P)

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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

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Example : Coil of Radius R

In the circular part of the coil…

Biot-Savart:

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Example : Coil of Radius R

Consider a coil with radius R and current I

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Example : Coil of Radius R

Notes:
This is an EASY Biot-Savart problem:
No vectors involved
This is what I would expect on exam

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Example: Infinite Wire

Region 1: Outside wire (r ≥ R)

Cylindrical symmetry 
Amperian Circle
B-field counterclockwise

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Example: Infinite Wire

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Two Loops

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Two Loops Moved Closer Together

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Multiple Wire Loops

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Multiple Wire Loops –Solenoid

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Magnetic Field of Solenoid

loosely wound

tightly wound

For ideal solenoid, B is uniform inside & zero outside

Horiz.
comp.
cancel

Moving charges (currents)

Moving charges (electric currents) also produce a magnetic field

http://www.sciencebuddies.org

Conventional current – electrons flow in the opposite direction

Magnetic field around a straight wire

Stronger field closer to wire

Magnetic field around a flat circular coil

http://physicsed.buffalostate.edu

Magnetic field around a solenoid

The Motor Effect

When a current is placed in a magnetic field it will experience a force. This is called the motor effect.

The Motor Effect

The direction of the force on a current in a magnetic field is given by Flemming’s left hand rule.

Centre finger = Conventional Current

First finger = Field direction

Thumb = Motion

Sample question

In this example, which way will the wire be pushed? (red is north on the magnets)

Another sample question!

An electron approaches a bar magnet as shown. What is the direction of the force on the electron?

N

S

D.C.Motor

Commutator ensures that every half rotation the current direction reverses in the coil