Ch21 magnetic fields and forces 23 july 2015

Chapter 21

Magnetic Forces and Magnetic Fields

FieldField

Gravitational

Directed towards the Centre of the

earth

Electric

Directed from positive charge

to negative

Magnetic

Directed from north pole to south pole

21.1 Magnetic Fields

The needle of a compass is permanent magnet that has a north magnetic pole (N) at one end and a south magnetic pole (S) atthe other.


The behavior of magneticpoles is similar to that oflike and unlike electric charges.


Surrounding a magnet there is a magnetic field. The directionof the magnetic field at any point in space is the direction indicatedby the north pole of a small compass needle placed at that point.


The magnetic field lines and pattern of iron filings in the vicinity of abar magnet and the magnetic field lines in the gap of a horseshoe magnet.

The magnetic field lines are directed from the north pole to the south pole

21.2 The Force That a Magnetic Field Exerts on a Charge

When a charge is placed in an electric field, it experiences a force, according to EF

q


The following conditions must be met for a charge to experience a magnetic force when placed in a magnetic field:

1. The charge must be moving, no magnetic force acts on a stationary charge2. The velocity of the charge must have a component that is perpendicular to the direction of the magnetic field.


Right Hand Rule No. 1. Extend the right hand so the fingers pointalong the direction of the magnetic field and the thumb points alongthe velocity of the charge. The palm of the hand then faces in the direction of the magnetic force that acts on a positive charge.

If the moving charge is negative,the direction of the force is oppositeto that predicted by RHR-1.

Right-Hand Rule No. 2.Commonly called: Right hand grasp wire rule

Curl the fingers of the right hand into the shape of a half-circle.

Point the thumb in the direction of the conventional current, and the tips of the fingers will point in the direction of the magnetic field.

Any current carrying wire produces its own magnetic field.

The linking of the motion of electric charges with the creation of magnetic field marked the of an important discipline called: Electromagnetism.

Right hand 2 solenoid

For each electromagnet below in the drawing, will it be attracted to or repelled from the permanent magnet immediately to its right?

Identifications of the polarity of magnet and current direction


Right Hand Rule No. 1. Extend the right hand so the fingers pointalong the direction of the magnetic field and the thumb points alongthe velocity of the charge. The palm of the hand then faces in the direction of the magnetic force that acts on a positive charge.

If the moving charge is negative,the direction of the force is oppositeto that predicted by RHR-1.


The magnitude of the magnetic field at any point in space is definedas

sinvqFB

o

where the angle (0<θ<180o) is the angle between the velocity ofthe charge and the direction of the magnetic field.

SI Unit of Magnetic Field: T tesla1metercoulomb

secondnewton

tesla10gauss 1 4

Do example 1 page 641

21.3 The Motion of a Charged Particle in a Magnetic Field

Charged particle in an electric field.

Charged particle in a magnetic field.


Conceptual Example 2 A Velocity Selector

A velocity selector is a device for measuring the velocity of a charged particle. The device operates by applying electric and magnetic forces to the particle in such a way that these forces balance.

How should an electric field be applied so thatthe force it applies to the particle can balancethe magnetic force?


The electrical force can do work on acharged particle.

The magnetic force cannot do work on acharged particle.

Note: for work to be done, the force must cause an object to move in its direction


The magnetic force always remainsPerpendicular(sin 90 =1) to the velocity and is directed toward the center of the circular path.

rvmFc

2

rvmqvB

2

qBmvr

Do example 3 page 644

21.4 The Mass Spectrometer

eBmv

qBmvr eVmv 2

21

magnitude ofelectron charge

KE=PE

22

2B

Verm

21.5 The Force on a Current in a Magnetic Field

The magnetic force on themoving charges pushes thewire to the right.


sinqvBF

sinBtvtqF

LI

sinILBF

Hence the magnetic force on a current carrying wire of length L is given by sinILBF


Example 5 The Force and Acceleration in a Loudspeaker

The voice coil of a speaker has a diameter of 0.0025 m, contains 55 turns ofwire, and is placed in a 0.10-T magnetic field. The current in the voice coil is2.0 A. (a) Determine the magnetic force that acts on the coil and thecone. (b) The voice coil and cone have a combined mass of 0.0200 kg. Find their acceleration.


(a)

N 86.0

90sinT 10.0m 0025.055A 0.2

sin

ILBF

(b) 2sm43kg 020.0N 86.0

mFa

Tutorial Questions

Chapter 21 Problem no 9, 15, 27 and 40

21.7 Magnetic Fields Produced by Currents

Right-Hand Rule No. 2.Commonly called: Right hand grasp wire rule

Curl the fingers of the right hand into the shape of a half-circle.

Point the thumb in the direction of the conventional current, and the tips of the fingers will point in the direction of the magnetic field.

Any current carrying wire produces its own magnetic field.

The linking of the motion of electric charges with the creation of magnetic field marked the of an important discipline called: Electromagnetism.

21.7 Magnetic Fields Produced by CurrentsA LONG, STRAIGHT WIRE

rIB o

2

AmT104 7 owherepermeability of free space

Experimentally, it has been proved that:• The magnetic field produced by infinitely long, straight wire is proportional to current and

inversely proportional to radial distance r from the wire• Mathematically • To convert it to equation, we introduce a constant:


sin2

sin

rIqvqvBF o

rIB o

2

Do examples 7 and 8 page 652 to 653


Current carrying wires can exert forces on each other.

Check Conceptual Example 9 page 655

Magnetic Force Between Two Parallel Conductors

• Two parallel wires each carry steady currents

• The field due to the current in wire 2 exerts a force on wire 1 of F1 = I1ℓ B2

• Substituting the equation for gives

2B

I Ia

1 21 2

oμFπ

The force per unit length on the wire is

I Ia

1 2

2oB μFπ

And this formula defines the current unit Ampere.Definition of an Ampere: The current in each of two long parallel wires, 1m apart, which results in the force of of length of each wire


A LOOP OF WIRE

center of circular loop

RIB o

2


Example 10 Finding the Net Magnetic FieldA long straight wire carries a current of 8.0 A and a circular loop ofwire carries a current of 2.0 A and has a radius of 0.030 m. Find themagnitude and direction of the magnetic field at the center of the loop.

RI

rI

RI

rIB ooo 2121

222

T101.1

m 030.0A 0.2

m 030.0A 0.8

2AmT104 5

7

B


The field lines around the bar magnet resemble those around the loop.

Magnetic Field of a Solenoid• A solenoid is a long wire wound in the

form of a helix. • Each loop produces a magnetic field

that adds together to form the total field.

• A reasonably uniform magnetic field can be produced in the space surrounded by the turns of the wire

• The field lines in the interior are – nearly parallel to each other– uniformly distributed

• Outside the solenoid, the field is weak as field lines from neighboring wires tend to cancel.


A SOLENOID

Interior of a solenoid nIB onumber of turns perunit length

Check you understanding page 659

Tutorial Exercises problems 59 and 64

21.6 The Motor effect: The movement of a Current-Carrying conductor in a magnetic field

Any conductor carrying current in a magnetic field region will experience a force. The force is caused by the interaction of magnetic field caused by Magnet field from the magnet and magnetic field around the conductor

The two forces on the loop have equal magnitude but an applicationof RHR-1 shows that they are opposite in direction.

21.6 The Torque on a Current-Carrying Coil

The basic components of a dc motor.

21.6 The Torque on a Current-Carrying Coil

21.8 Ampere’s Law

AMPERE’S LAW FOR STATIC MAGNETIC FIELDS

For any current geometry that produces a magnetic field that does not change in time,

IB o ||

net currentpassing throughsurface bounded by path

21.8 Ampere’s Law

Example 11 An Infinitely Long, Straight, Current-Carrying Wire

Use Ampere’s law to obtain the magnetic field.

IB o ||

IB o

IrB o 2

rIB o

2

21.9 Magnetic Materials

The intrinsic “spin” and orbital motion of electrons gives rise to the magnetic properties of materials.

In ferromagnetic materials groups of neighboring atoms, forming magnetic domains, the spins of electrons are naturally aligned witheach other.

21.9 Magnetic Materials

Ch21 magnetic fields and forces 23 july 2015

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Transcript of Ch21 magnetic fields and forces 23 july 2015