Transcript Magneto Statics

Magneto statics
AP Physics
Magnetism
DWT
The properties of attraction possessed by magnets.
1. Permanent Magnets
–
Objects that retain their magnetic properties
•
Ex. the magnet in a compass, bar magnet, horseshoe magnet,
speakers, and of course the magnets on the fridge
2. Electromagnets
–
Objects that exhibit magnetic properties as a result of
electric current
•
Ex. electric motors,
disk drives, & speakers
Magnets
Magnets have two poles…
Similar to electric charge…opposites poles
Magnetic Fields (B) & Forces
1. A permanent magnet, a moving charge, or a current
creates a magnetic field B at all points in the
surrounding space.
2. The magnetic field exerts a force F on any other
permanent magnet, moving charge, or current that is
present in the field.
Magnetic Dipole
Magnetic Field of Earth
Magnetic Forces I
The force acting on a moving charge in a B field is:
1.
2.
3.
proportional to the magnitude of the
proportional to the magnitude of the
proportional to the component of the
B
θ
q
v
Right Hand Rule (RHR)
DWT
The direction and the magnitude of the magnetic force is determined
by a cross product computation (vector calculus, ugh!). We’ll cheat and
use our hands (correction, just right hand) rather than calculus.
Right Hand Rule #1
1. Fingers in direction of B Field
2. Thumb in direction of charge’s
velocity v
3a. Positive Charge
Direction of Force corresponds
to direction of the palm
3b. Negative Charge
Direction of Force corresponds
to direction of back of hand
RHR1 Examples
Notation used for Fields
x – into surface/page
· (dot) – out of surface/page
J.J. Thomson’s e/m Ratio
DWT Sir Joseph John “J.J.” Thomson (1856 – 1940) was a British physicist and Nobel laureate,
credited for the discovery of the electron (Physics Nobel Prize in 1906), and the
invention of the mass spectrometer.
Thomson used a cathode ray tube to fire a stream of electrons through magnetic
and electric fields (1897).
From his experiments he determined:
1. that there must be some small negatively charged particle (known today as
the electron, JJ called them corpuscles)
2. the charge to mass ratio (e/m ratio) of this new particle dubbed the electron.
J.J.’s Cathode Ray Tube (simplified)
Actual Apparatus
Velocity Selector
Mass Spectrometer
A scientific device that uses electric and
magnetic fields to determine the mass
of a substance.
How it Works
1. Substance is ionized and vaporized
2. Charged plates accelerate particles
3. Particles pass through magnetic and
electric fields (velocity selector)
4. Particles move in a curved path in the
presence of a magnetic field
Curved Motion Due to B Field
What is the radius R of the circular motion of the particle as
a result of the magnetic force?
Mars Rover
Scientists want to include a compact mass
spectrometer on a future Mars rover.
Suppose the instrument is designed to have
a magnetic field of 0.01 T, and selects
carbon ions that have a speed of 5 x 103 m/s
and are singly ionized (have a charge of +e).
What are the radii R12 and R13 of the orbits
of 12C and 13C ions in this spectrometer?
Particle in Magnetic Field
A particle of mass m and charge –q is accelerated through a
potential difference ε. It then passes into a uniform
magnetic field of magnitude B directed into the page as
shown below. Express your answers in terms of m, q, ε, and
B.
a) Determine the speed of the electron as it enters the
magnetic field.
b) Sketch the path of the electron in the magnetic field on
the diagram above.
c) Determine the radius of the path of the electron.
d) An electric field E is now established in the same region
as the magnetic field, so that the electron passes
through the field undeflected.
(i) Determine the magnitude of E.
(ii) Indicate the direction of E on the diagram above.
Magnetic Forces II
The force acting on a segment of conductor with length l,
carrying a current I in a uniform B field is:
B
θ
I
l
Right Hand Rule Dos (RHR2)
Right Hand Rule #2
1. Fingers in direction of B Field
2. Thumb in direction of
conductor’s current I
(remember, current is direction
of positive charges)
3. Direction of Force corresponds
to direction of the palm
B Force on Current Loop
How do the magnetic forces act on a current carrying loop?
B Force on Current Loop Cont.
Electric Motors
Motors operate because of the magnetic force on current carrying
conductors.
Force on Conducting Bar
Magnetic forces are used to accelerate current carrying
conductors.
B Fields From E Currents
An electric current creates a magnetic field.
Simplest example: Long straight wire
Right Hand Rule Trois (RHR3)
Right Hand Rule #3
1. Thumb in direction of
current
2. Curl fingers in direction
of B field
Current Carrying Wire & Compass
Force b/w Parallel Wires
How do current carrying wires affect one another.
Solenoids
Almost like the magneto static equivalent of electrostatics’
parallel plate capacitor