adftdagenda for today - u of t physicsxingxing/phy132s17/pre_lect12.pdf · elements of physics ii...

Physics 132: Lecture 12ys cs 3 ectu eElements of Physics II

A d f T dAgenda for Today Magnets and the Magnetic Field Magnetic fields caused by charged particles B-field from a current-carrying wire

f f Magnetic fields and forces B-fields Magnetic forces on moving particles Magnetic forces on moving particles

Physics 201: Lecture 1, Pg 1

Magnetism

Tape a bar magnet to a piece of cork and allow it topiece of cork and allow it to float in a dish of water.

It always turns to align itself y gin an approximate north-south direction.

The end of a magnet that points north is called the north-seeking pole, or simply the north g p , p ypole.

The end of a magnet that points south is called


the south pole.

Magnetism Like poles repel, opposite poles attract


MagnetismMagnetism Magnets can pick up some

objects, such as paper clips, j , p p p ,but not all.

If an object is attracted to one end of a magnet it isone end of a magnet, it is also attracted to the other end.Most materials including Most materials, including copper (a penny), aluminum, glass, and plastic experience no forceplastic, experience no force from a magnet.


Magnetism

Cutting a bar magnet in half produces two weaker but still complete magnets, each with a p gnorth pole and a south pole.

No matter how small the magnets are cut, even down to microscopic sizes each piece remains adown to microscopic sizes, each piece remains a complete magnet with two poles.


Magnetism1. Magnetism is not the same as electricity.2. Magnetism is a long range force.3 All magnets have two poles called north and3. All magnets have two poles, called north and

south poles. Two like poles exert repulsive forces on each other; two opposite poles attractattract.

4. The poles of a bar magnet can be identified by using it as a compass. The north pole tends to

t t t i t i t l throtate to point approximately north.5. Materials that are attracted to a magnet are

called magnetic materials. The most common gmagnetic material is iron.


Mapping Magnetic Field


B-field

Similar to electric charges a magnet will create a Similar to electric charges a magnet will create a magnetic field in the area around it

Due to convention we will call the magnetic field a B-field

The force magnets apply on each other can be described as an interaction between a magnet and another magnet’s B-field


B-field lines

We will also draw magnetic field lines We will also draw magnetic field lines

(1) The direction of a magnetic field is tangent to ( ) e d ec o o a ag e c e d s a ge oa magnetic field line at any point

(2) The number of lines per area is proportional to the magnitude of the B-field


B-Field Lines


Earth’s Magnetic Field

The Earth’s magnetic field resembles that achieved by burying aachieved by burying a huge bar magnet deep in the Earth’s interior


B-field of Current-Carrying Wire

In 1819 Hans Christian Oersted discovered that In 1819 Hans Christian Oersted discovered that an electric current in a wire causes a compass to turn.


B-field of Current-Carrying Wirey g

The right-hand rule determines the orientation of the compass The right-hand rule determines the orientation of the compass needles to the direction of the current.


B-field of Current-Carrying Wire

Magnetic field lines are Magnetic field lines are imaginary lines drawn through a region of space

th tso that: A tangent to a field line is

in the direction of thein the direction of the magnetic field.

The field lines are closer t th h thtogether where the magnetic field strength is larger.


RHRRHR


The Source of the Magnetic Field: Moving ChargesCharges

The magnetic field of a charged The magnetic field of a charged particle q moving with velocity vis given by the Biot-Savart law:


The Magnetic Field

The constant 0 in the Biot-Savart law is called the permeability constant:

0 = 4× 10-7 T m/A = 1.257 × 10-6 T m/A

The SI unit of magnetic gfield strength is the

tesla, abbreviated as T:

1 tesla = 1 T = 1 N/A m


Currents Create B-Fields

Current I OUT

IMagnitude:

B

•7

B 0I2r

rB

r = distance from wire

0 4 10 7 Tm / A

Lines of BDirection of b-field from current carrying wire:

Curly right hand ruleCurly right hand rule

Thumb along current I, direction of B along curled fingers


direction of B along curled fingers.

Give it a try:

Compared to the magnetic field at point A, the magnetic field at point B is p

A. Half as strong, same direction.B. Half as strong, opposite direction.C. One-quarter as strong, same

di tidirection.D. One-quarter as strong, opposite

directiondirection.E. Can’t compare without knowing I.


Give it a try: What is the direction of the magnetic field at the

position of the dot?

A Into the screenA. Into the screen.B. Out of the screen.C UpC. Up.D. Down.E LeftE. Left.


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