Magnetic Effects

Chapter 19

Natural magnets known since ancient times, called lodestones, made of magnetite, an iron ore
All magnetic effects caused by moving electrical charges
Ferromagnetic materials highly attracted to magnets: Fe, steel, Co, Ni, Nd, Pa

Due to spinning electrons in atoms
In most materials, magnetic effects cancel -- paired electrons with opposite spin
Fe, Ni, Co, Gd, Dy, Nd have unpaired electrons in outer valence shells with same spin causing atoms to become small magnets (Fe has 4)

Domains are microscopic groups of atoms with same magnetic orientation
When placed in strong external mag. field, domains aligned with ext. field grow in size
Other domains turn towards ext. field
If domains remain aligned after ext. field removed, permanent magnet results
Domain alignment can be destroyed by heat or excessive vibration
Curie Point: Temperature at which magnetic domains disappear and material becomes paramagnetic

Paramagnetism: some materials have slight natural magnetic moment and are weakly attracted by strong external magnetic field (Al, Pt, O₂, wood)
Effects of orbiting and spinning electrons don't cancel out and atoms are slightly magnetic
Diamagnetism:In a strong external magnetic field, some materials experience induced magnetic moment (become slightly magnetic) and are weakly repelled (Zn, Bi, Au, Hg, NaCl)
Strong ext. field opposes motion of electrons causing slight repulsive force

Most magnets have 2 poles: North (north seeking) and South (south seeking)
Named because of effect due to earth's magnetic field
Opposite poles attract; like poles repel
Since magnetic N poles point north, earth's magnetic pole in the north is actually a S pole
Some magnets have more than one N or S pole
Some circular magnets have no poles
If magnet is cut, each piece has N and S pole
Magnetic unit pole (single N or S pole) never been found but is used in theory

Magnetic forces of attraction and repulsion obey inverse square law similar to those of gravitation and electrostatics
Force is directly proportional to product of strength of poles & inversely prop. to square of distance between them

Represented like electrical field: lines of flux showing magnitude and direction of force on N unit pole
Arrows point from N to S, lines continue through magnet
Magnetic flux (Φ): number of lines passing through a surface; unit is weber (Wb)
Flux Density: number of lines of flux per unit area, often called magnetic field strength; symbol B, a vector; unit: tesla (T)
B = Φ/A ; 1T = 1Wb/m²
Magnetic field can be mapped by using tiny magnets or iron filings

The amount a material changes flux density of external field; symbol: μ; ratio of flux densities
μ of free space is 1, paramagnetic >1, ferromagnetic >>1, diamagnetic <1
High m means mag. flux enters material, is intensified internally, blocking flux passage through material
Soft iron has high permeability, becomes temporary magnet in ext. field - induced magnetism

Cause is convection currents of ions in molten core (theory)
Magnetic axis not in line with rotational axis
Difference between true north and magnetic north is called declination
Dipping needle
(3 dimensional compass) can measure inclination, angle with horizontal
Magnetic field has shifted over historical time
Geologic record shows field has reversed polarity many times, even disappeared for long periods

Sun emits many charged particles in solar wind, most are deflected by earth's mag. field
Magnetosphere: area where charged particles are affected - about 57,000 km up on side facing sun; greatly elongated away from sun
Many particles are concentrated in 2 regions called Van Allen radiation belts

1820: Oersted shows current in wire will deflect compass needle
Magnetic field around conductor is in concentric circles
Direction of B found using left hand rule: point left thumb in direction of electron flow; curled fingers point in direction of field
B = 2kI/r ; k = 10^-7N/A²

Current in parallel wires create forces on wires
Currents in same direction - attractive force; opposite direction - repel
F = (2klI₁I₂)/d: k = 10^-7N/A²; l = length of wires; I₁I₂=currents in wires; d = distance between wires

If straight conductor is bent into loop, magnetic field is concentrated inside loop creating magnetic dipole
Use left hand rule to find direction of B through loop: fingers point in direction of electrons flow, thumb points in direction of mag field through loop
To increase mag. strength, add more loops
Long series of coils makes solenoid with strong mag field inside coils
If iron core is added to solenoid, electromagnet results whose strength depneds on number of ampere-turns
If long solenoid is bent into circle (donut shape) result is called toroid

Detects current with coil of wire on soft iron core between poles of permanent magnet
Current in coil creates electromagnet whose polarity depends on direction of current
Electromagnet tries to align itself with permanent magnet, held back by springs
Needle attached to coil shows deflection

Galvanometer adapted with high resistance in series with coil
Can be calibrated by knowing R_m, the meter resistance; R_s, the series resistance; and V, the desired full scale voltage reading
V = I_mR_m + I_mR_s ; I_m ,the meter current, is calculated for galvanometer full scale deflection

Galvanometer adapted for higher currents by placing low resistance shunt in parallel with meter coil
Calibrate meter for desired current range
Total current I_t= shunt current I_s+ meter current I_mand I_mR_m = R_sI_s
So R_s= I_mR_m/(I_t- I_m)

Modified voltmeter-ammeter combination
Circuit powered by internal battery
Has enough internal resistance to provide full scale deflection on short circuit (no resistance)

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