Презентация на тему Currents in мetals

The production and properties of magnetic fields.

moves easily.
Insulators are materials through which charge does

not move easily.
Semiconductors are materials intermediate to conductors and insulators.

of an electron in a conductor. The changes in

direction are the result of collisions between the electron and atoms in the conductor. The net motion – drift speed of the electron is opposite the direction of the electric field.

flow of electrons:



in reality there happens zigzag motion of

free electrons in the metal:

gives up one or more of its outer electrons.

These electrons are then free to move through the crystal, colliding at intervals with stationary positive ions, then the resistivity is:
ρ = m/(ne2τ)
n - the number density of free electrons,
m and e – mass and charge of electron,
– average time between collisions.

– the cross-sectional area of the conductor
vd – drift

speed
E = ρJ
ρ - resistivity

E are established in a conductor whenever a potential

difference is maintained across the conductor:


σ is conductivity:
σ = 1/ ρ.

the ratio of the current density to the electric

field is a constant σ that is independent of the electric field producing the current:
J = σE

Magnetic poles are always found in pairs.
The direction of

magnetic field is from the North pole to the South pole of a magnet.

lines repels from like poles.

exerted on the particle is proportional to the charge

q and to the speed v of the particle.
The magnitude and direction of FB depend on the velocity of the particle and on the magnitude and direction of the magnetic field B.
When a charged particle moves parallel to the magnetic field vector, the magnetic force acting on the particle is zero.
When the particle’s velocity vector makes any angle Θ≠0 with the magnetic field, the magnetic force acts in a direction perpendicular to both v and B.
The magnetic force exerted on a positive charge is in the direction opposite the direction of the magnetic force exerted on a negative charge moving in the same direction.
The magnitude of the magnetic force exerted on the moving particle is proportional to sin Θ, where Θ is the angle the particle’s velocity vector makes with the direction of B.

summarized as:




So the units for B are:

The magnetic force is perpendicular to both v and B.
FB=qVBsinΘ

the direction of v, with B coming out of

your palm, so that you can curl your fingers in the direction of B. The direction of , and the force on a positive charge, is the direction in which the thumb points.

the paper are indicated by dots, representing the tips

of arrows coming outward.

Magnetic field lines going into the paper are indicated by crosses, representing the feathers of arrows going inward.

on a single charge moving in a magnetic field.

A current-carrying wire also experiences a force when placed in a magnetic field. This follows from the fact that the current is a collection of many charged particles in motion; hence, the resultant force exerted by the field on the wire is the vector sum of the individual forces exerted on all the charges making up the current. The force exerted on the particles is transmitted to the wire when the particles collide with the atoms making up the wire.

q
vd is the drift speed of q
A – area

of the segment
L – the length of the segment
Then AL is the volume of the segment, and

nAL is the number of charged particles q.
Then the net force acting on all moving charges is:

of an arbitrary shaped wire is:

The total force is:


a

and b are the end points of the wire.

curved current-carrying wire in a uniform magnetic field is

equal to that on a straight wire connecting the end points and carrying the same current.

Curved Wire in a Uniform Magnetic field

a uniform magnetic field is:

is a vector multiplication.
Where L is a vector that points in the direction of the current I and has a magnitude equal to the length L of the segment. This expression applies only to a straight segment of wire in a uniform magnetic field.

Magnetic force on a straight wire

magnetic force acting on any closed current loop in

a uniform magnetic field is zero:



Then the net force is zero:
FB=0

Overhead view of a rectangular loop in a uniform

magnetic field.
Sides 1 and 3 are parallel to magnetic field, so only sides 2 and for experiences magnetic forces.
- Magnet forces, acting on sides 2 and 4 create a torque on the loop.

plane of the loop, the maximal torque on the

loop is:


ab is the area of the loop A:

magnetic field, i.e. the angle between A and B

is Θ < 90° then:





So the torque on a loop in a uniform magnetic field is:

This formula is correct not only for a rectangular loop, but for a planar loop of any shape.

A:
- Its direction is perpendicular

to the plane of the loop,
- its magnitude is equal to the area of the loop.

We determine the direction of A using the right-hand rule. When you curl the fingers of your right hand in the direction of the current in the loop, your thumb points in the direction of A.

Area Vector

magnetic moment is the same as the direction of

A.

be the magnetic dipole moment μ (often simply called

the “magnetic moment”) of the current loop:


Then the torque on a current-carrying loop is:

of a system having magnetic dipole μ in the

magnetic field B is:


Here we have scalar product μ B. Then the lowest energy is when μ points as B, the highest energy is when μ points opposite B:

Magnetic Field




When the velocity of a charged particle is

perpendicular to a uniform magnetic field, the particle moves in a circular path in a plane perpendicular to B. The magnetic force FB acting on the charge is always directed toward the center of the circle.

velocity


here v is perpendicular to B.

the force acting on a charged particle is:

here q

is the charge of the particle,
v – the speed of the particle,
E – electric field vector
B – magnetic field vector

in a magnetic field, a potential difference is generated

in a direction perpendicular to both the current and the magnetic field.

exerted on the carriers

has magnitude qvdB.
this force is balanced by the electric force qEH:



d is the width of the conductor:


n – charge density: .vd - charge carrier drift speed.

then we obtain the Hall voltage:

the conductor,
t – thickness of the conductor we can

obtain:




RH is the Hall coefficient:

RH = 1/(nq)

are negative, the upper edge of the conductor becomes

negatively charged, and c is at a lower electric potential than a.

When the charge carriers are positive, the upper edge becomes positively charged, and c is at a higher potential than a.