The Magnetic Field Chapter 30
Magnetic Forces • Magnetic Force - A force present when an electric charge is in motion. • A moving charge is said to produce a magnetic field . • Magnetic fields exert forces on moving charges.
Magnetic Fields • Represented by field lines . • By definition: • or more commonly: • Where q is the angle between v and B.
Magnetic Field Units • Standard Unit = Tesla (T) • 1 T = 1 N/A•m • 1 T = 104 gauss
Force on Moving Charges • The diagram below shows a uniform magnetic field with several charges in motion. v v v v
Force on Moving Charges • The magnitude of the force on each charge can be found by qvXB or qvBsinq. • The direction of the force is found by a right hand rule.
Right Hand Rule • 1) Place your fingers in the direction of the velocity. • 2) Curl your fingers toward the direction of the field. You might need to turn your hand. • 3) Your thumb points in the direction of the force.
Direction of Force v F F = 0 v F F v v
Magnetic Field Lines • NOT lines of force. • Force on charges is not in the direction of the magnetic field. • Force is always perpendicular to the velocity of the charge. • Force is always perpendicular to the magnetic field. • RHR & LHR
Permanent Magnets • Magnetic field lines point away from north poles • and toward south poles.
Magnetic Flux • The amount of a magnetic field passing through a given area. • Proportional to the number of magnetic field lines which pass through an area.
Magnetic Flux Maximum Flux A A A No Flux
Flux Units • Weber • 1 Wb = 1 T/m2
Gauss's Law for Magnetism • The magnetic flux through any closed surface must be zero. N S
Example • Exercise 4
homework • E 1, 2, 7
Motion of Charges in a Magnetic Field • Two possible paths can result for the motion of the charge: • 1) If vo is perpendicular to B, a circular path will result. • 2) If vo is not perpendicular to B, the charge will travel in a spiral path.
Motion of Charges • As a charge circles or spirals in a magnetic field, the radius of its path is dependent on the perpendicular component of its velocity.
Velocity Selector • Only allows charges with a specific velocity to pass through undeflected. • FB is opposite of FE • E is perpendicular to B
Velocity Selector FE FB
Velocity Selector • For a specific value of v, the electric force and the magnetic force will be equal to each other and opposite in direction. • FB = FE • qvB = qE • vB = E
Current-Carrying Wire • Since a current is moving charges, a current-carrying wire experiences a force in a magnetic field. (B into screen) F X X X X X X X X X X X X X X X X
Example • Exercise 14
homework • E 19, 20
Sources of Magnetic Fields Chapter 31
Long, straight wire mo is equal to 4p x 10–7 T•m/A.
Current Carrying Wire • Shape of the field is circular. • Concentric circles • The direction is given a Right Hand Rule: • Thumb in the direction of the current. • Curl your fingers and they give the direction of the field.
Moving Charge + • v
I B Wire • • • • • • • • • I x x x x x x x x x
Parallel conductors • Each creates a magnetic field that produces a force on the other • Can calculate force per unit length • To find direction, use both right hand rules
Definition of Ampere • Comes from force exerted by two parallel conductors • 1 A is the current necessary in each conductor (if 1 m apart) to produce a force of 2 x 10-7 N.
Field of a circular loop or coil • At center of loop • Direction found with right hand rule – like current in straight wire
Field of a Solenoid • Long Spring-like Coil • Uniform field in the interior:
Examples • Exercises 1 and 7
homework • E 2, 6, 10, 12
Ampere’s Law • Like Gauss’s law I
Example • A wire has a radius of R and carries a current I that is uniformly distributed across its area. • Determine how to calculate the magnitude of the magnetic field inside and outside the conductor.
R r Inside • The current inside a circle of radius r would be a fraction of the total current. • Same ratio as areas. • With total current, I:
Outside • A circle of radius r, where r > R, encloses all the current. r R
Example • Determine the field inside a solenoid
Solenoid • Vertical sides – zero because B is perpendicular to sides • Side outside solenoid – if it is far away from the solenoid, B is zero
Paramagnetic materials • Can become magnetized • An external magnetic field causes atoms to line up so their currents add to the external field
Ferromagnetic materials • Atomic currents line up even when no external field is present • Permanent magnets