Chapter 20: Magnetism
ferromagnetic
Substances that can become magnetized; iron, nickel & cobalt. Their atoms will form magnetic domains.
period of rotation for a particle of charge q
T = 2πm / qB
SI unit for magnetic field
Tesla (T)
1 tesla
= 1 N/A·m
gauss (G)
= 10^-4 T
frequency of rotation
= qB / 2πm
electron spin
A property of the electron that makes it behave as though it were a tiny magnet. The electron behaves as if it were spinning on its axis; electron spin is quantized.
since the north pole (N) of a compass needle points north...
Earth's magnetic pole which is in the geographic north is magnetically a south pole
Force F2 exerted on B1 on a length l2 of wire 2
F2 = (I2)(B1)(l2)
angle of dip
The angle that the Earth's magnetic field makes with the horizontal at any point.
how does a compass work?
The compass functions as an indicator to "Magnetic North" because the magnetic bar at the heart of the compass aligns itself to one of the lines of the Earth's magnetic field.
the magnetic field ______ do work on a particle
cannot thus can't change kinetic energy of a particle
a charged particle moves in a _____ path with a .... in a uniform magnetic field
circular; constant magnitude of centripetal acceleration
right hand rule for current carrying wire
Thumb points in the direction of the current, fingers show the direction of the magnetic field
Hall equation
V(Hall) = Bvd
solenoid
Long coil of wires with many turns
Earth's magnetic field
Magnetic lines of force from Earth's polar north and south, acting like a giant magnet.
can a magnetic field be used to stop a charged particle?
No, because the force is always perpendicular to the velocity of the particle and thus can only change the direction, but not the magnitude of its velocity
how to make a magnet from an unmagnetized piece of iron
by placing it in a strong magnetic field
Newton's third law for magnets
electric current exerts a force on a magnet a magnet exerts a force on a current-carrying wire
an electric motor converts ______ energy into ________ energy
electric; mechanical
if currents in 2 parallel wires are in opposite direction...
exerts repulsive force on other
a compass needle placed near a straight section of current-carrying wire...
experiences a force, causing the needle to align tangent to a circle around the wire
one ampere is defined as
that current flowing in each of two long parallel wires, 1 m apart, which results in a force of exactly 2x10^-7N per meter of length of each wire
magnetic permeability
the ability of a material to attract the lines of magnetic field intensity
forces exerted on each other will be attractive as long as...
the currents are in the same direction
the direction of the magnetic field at a given location can be defined as...
the direction that the north pole of a compass needle would point if placed at that location
for galvanometer, magnets with curved pole pieces are used and coil is wrapped around a cylindrical iron core
-iron tends to concentrate the magnetic field lines so that B always points parallel to the face of the coil at the wire outside the core --> the force is then always perpendicular to the face of the coil, and torque will not vary with angle *thus deflection of pointer will be proportional to I, as required
1 C =
1 A·s
rules of magnetic field lines
1. the direction of the magnetic field is tangent to the field at any point 2. the number of lines per unit area is proportional to the strength of the magnetic field
magnetic field B1 produced by I1
B1 = (μ0/2π)(I1/d) field only due to I1
domains
behave like small magnets with a north and South Pole
Hall effect
When a current-carrying conductor is held fixed in a magnetic field, the field exerts a sideways force on the charges moving in the conductor -potential difference builds up until electric field E(H) that it produces exerts a force on the moving charges that is equal and opposite to the magnetic force
the path of a charged particle moving in a plane perpendicular to a uniform magnetic field is...
a circle
Uniform Magnetic Field
a field that has the same magnitude and the same direction at every point within some region of space
electric charges experience...
a force in a magnetic field
permanent magnet
a magnet that retains its magnetic properties in the absence of an inducing field or current.
Magnetic flux density
a measure of the strength of the magnetic field defined in terms of the force on a current-carrying conductor at right angles to the field lines Φ=B∆A cosθ
another device called a solenoid consist of...
a solenoid into which a rod of iron is partially inserted
if a motor is to turn continuously in one direction....
an alternating current is necessary
magnetic declination
angular difference between the direction of a compass needle (which points along magnetic field lines) at any location and true (geographical) north
saturation
as B0 increases, domains become more and more aligned until at point b, nearly all are aligned
how to break down velocity vectors of helical path
components parallel and perpendicular to the field -velocity component parallel to field lines experiences no force, so remains constant -velocity component perpendicular to the field results in circular motion about the field lines
magnetic field lines are
continuous closed loops
hysteresis
curve does not retrace itself on the same path
how can an alternating current for motor be achieved?
dc motor w/ commutators and brushes ac motor, with ac current as input
mass spectrometer
device to measure masses of atoms
Hall emf
difference of potential produced V(Hall) = E(H)d = v(d)Bd d is width of conductor
the deflection of the pointer is....
directly proportional to the current I flowing in the coil, but also depends on the angle the coil makes with B *prob
the Earth's magnetic poles...
do not coincide with the geographic poles, which are on the Earth's axis of rotation
a piece of iron is made up of tiny regions called __________
domains
force between two parallel wires
each current produces a magnetic field that is "felt" by the other, so each must exert a force on the other
right hand rule for magnetic force
fingers first point straight alone current, then bend along magnetic field thumb points in direction of force
right hand rule for force on electric charge due to magnetic field (POSITIVE CHARGE)
fingers point along particle's velocity, then along B (curve) -thumb points in direction of force
the field strength at any given point is _____ if the current flowing in the wire were _______
greater; greater
the current in the coil reverses every ________ as required for...
half revolution; continuous rotation
for a long straight wire, the field is
inversely proportional to the distance from the wire
if B' is fixed, the radius of the circular path...
is found from their mark on film or by detectors
torus/toroid
long solenoid bent into the shape of a donut
a solenoid acts like a ________
magnet - one end can be considered north/South Pole, depending on direction of current in the loops (rhr)
Curie temperature
magnet cannot be made
when the current is perpendicular to the field lines (θ=90°)...
magnetic force is strongest
an electric generator converts ______ energy into ________ energy
mechanical; electrical
an isolated single magnetic pole (magnetic monopole) has...
never been observed
When the wire is parallel to the magnetic field lines (θ=0°)...
no force at all
magnetic fields due to a circular loop of wire and straight current-carrying wire are..
no uniform
magnetic field lines always point out from the _____ pole and in toward the ______ pole of a magnet
north; south
galvanometer consists
of a coil of wire (with attached pointer) suspended in the magnetic field of a permanent magnet
direction of force on a negative charge
opposite to what positive charge direction would be with right hand rule
helical path
path of a charged particle in a uniform magnetic field if its velocity is not perpendicular to the magnetic field
the direction of the force is always ______ to the direction of the current, and ______ to the direction of the magnetic field
perpendicular; perpendicular
domains in a magnet
preferentially aligned in one direction
the current in each loop of a solenoid...
produces a magnetic field
an electric current...
produces a magnetic field (Oersted)
domains in an unmagnetized piece of iron
randomly arranged *magnetic effects of the domains cancel out, so this piece is not a magnet
Ampère's Law
relates the magnetic field around a closed loop to the current flowing through the loop *can be used to calculate the magnetic fields in situations with a high degree of symmetry
the force one magnet exerts on another can be described as...
the interaction between one magnet and the magnetic field of another
if a piece of iron is placed inside a solenoid, the magnetic field is increased greatly because...
the iron becomes a magnet --> electromagnet
if a piece of ferromagnetic material like iron is placed inside a solenoid to form an electromagnet...
the magnetic field increases greatly over that produced by the current in the solenoid coils alone
Hall: the voltage depends on...
the magnetic field put there, velocity of the charges, and separation (width of wire)
when an electric current flows in a closed loop of wire placed in an external magnetic field...
the magnetic force on the current can produce a torque
because the force is always perpendicular to v,...
the magnitude of v does not change; electron moves at constant speed
if the force on a particle is always perpendicular to its velocity v,..
the particle moves in a circle and has a centripetal acceletration
magnetic force on a particle is greatest when...
the particle moves perpendicular to the magnetic field
geomagnetic north
the point on the earth where a compass would lead to the Earth's magnetic field North Pole geographically
magnetic dipole moment
the product of the strength and distance between the poles M=NIA *a vector considered perpendicular to the coil
magnetic field
the region around a magnet where the magnetic force is exerted
the total field inside the solenoid will be...
the sum of the fields due to each current loops
magnetic field lines due to a circular loop of current carrying wire
thumb grips wire, fingers curl around it
a motor, works on same principle as a galvanometer (torque is exerted on a current-carrying loop in a magnetic field) except that the coil must....
turn continuously in one direction
magnetic poles
two ends of a magnet where the magnetic forces are strongest
only ions whose speed is ________ will pass through undeflected and emerge through second slit
v=E/B (velocity selector)
if the current-carrying wires are made of superconducting material kept below the transition temperature,...
very high fields can be produced, and no electric power is needed to maintain the large current in the superconducting coils --> energy is required to refrigerate the coils at the low temps where they superconduct
Outside a solenoid the magnetic field is
weaker *field lines are spread out in space
if the solenoid has many loops and they are close together, the field inside...
will be nearly uniform and parallel to the solenoid axis except at the ends
the force is ____ if the particle moves parallel to the magnetic field lines
zero
permeability of free space
μ0