Chapter 20 Magnetism

Magnets The ends of a bar magnet are called poles Like poles repel and unlike poles attract Regardless of their shape, all magnets have a north and south pole

Magnetic Fields Magnetic Field lines point from the north pole to the south pole of the magnet The north pole of a compass needle always points in the direction of the field (from North to South)

Magnetic Field of the Earth The Earth’s geographic North pole is actually the magnetic south pole The north pole of a compass points towards geographic north and since opposites attract, we know that the Earth’s geographic pole is magnetic south

Magnetic Force A charge moving through a magnetic field experiences a force q= magnitude of charge v= speed of charge B= Strength of the magnetic field (measured in Tesla, T) θ= angle between v and B (F=0 if θ=0)

A second Right-Hand Rule Of course, force is a vector! To find the direction of the magnetic force use another right hand rule Fingers point in direction of the field Thumb points in direction of v Palm points in direction of magnetic force

Conventions for direction of field WARNING: The right hand rule is for the direction of the force acting on a POSITIVE CHARGE. To find the direction of the force acting on a negative charge, you’ll have to use the rule and change the sign! Direction of Field Symbol Into the page X Out of the page

Examples Direction of F Direction of v Direction of B Sign of Charge Out of the page East North + Into the page - West South west

Path of a charge in a magnetic field The path of a charged particle moving perpendicular to a magnetic field is a circle (p.595) The magnetic force acting on the particle acts like the centripetal force

Magnetic Field of a wire Moving charges produce magnetic fields If there is a current moving through a wire, a magnetic field is produced around the wire I is current, r is perpendicular to wire µo=4π x 10-7 Tm/A

Magnetic Field of a wire The “Right Hand Rule” for the magnetic field Point your thumb in the direction of the current and curl your fingers in the direction of the field

Force on a current carrying wire A magnet exerts a force on a current-carrying wire I= current l= length of wire B= magnitude of magnetic field Θ is the angle between the direction of current and the magnetic field If current is parallel to B, F=0 (F=0 if θ=0)

The Right-Hand Rule revised Of course, force is a vector! To find the direction of the magnetic force use another right hand rule Fingers point in direction of the field Thumb points in direction of I Palm points in direction of magnetic force I

Force between two current carrying wires Two current-carrying wires exert a force on each other If the currents are moving in the same direction the wires attract each other If the currents are moving in opposite directions, the wires repel 2.0x10-7 Tm/A= µo/2π I= current l= length of wire L= distance between wires

Electromagnetic Induction (Ch 20) Michael Faraday discovered the phenomenon of electromagnetic induction A changing magnetic field can produce an electric current (induced current) B must be changing for this to work Moving a magnet through a coil of wire produces a current

Magnetic Flux Magnetic Flux is proportional to the number of field lines passing through some area The angle θ is the angle between B and a line drawn perpendicular to the surface If θ is 90, no lines pass through the area, so flux is 0 Unit for flux is the Weber (1Wb= 1 Tm2 )

Faraday’s Law of Induction Recall what electromagnetic induction is. A changing magnetic field induces a current Faraday’s Law mathematically: N represents the number of loops in the wire ΔΦB is the change in magnetic flux

Lenz’s Law The negative sign indicates that the induced current’s magnetic field is always opposite to the original change in flux Changing flux induces an emf, which induces a current That current then produces its own magnetic field That magnetic field points in the opposite direction of the change in flux

Lenz’s Law Direction of the magnetic field produced by the induced current? A. Down B. Up

More Practice with Lenz’s Law In which direction is the current induced in the coil for each situation? Current produced will be counterclockwise to produce a field that points out of the page b. The area decreases, so flux decreases. Current will be clockwise to produce A field that points into the page c. Initially flux is out of the page. Moving the coil means the flux decreases. Induced current will be counterclockwise to produce a field out of the pge d. Field lines and surface are parallel so there is no flux, so no current is induced e. Flux will increase to the left so the current will be counterclockwise to produce a Field to the right

Induced EMF for moving conductor What if the magnet is stationary and the wire is moved instead? This is called motional emf B= magnetic field l= length of wire v= speed

Sample Problem p. 655 #15 B= 0.450 T R= 0.230 Ω v= 3.40 m/s Calculate the force required to pull the loop from the field at a constant velocity of 3.4 m/s

How do we get force? We have l, B what’s I? Ohm’s Law I= V/R. We have R…what’s V?? Law of induction!: V=Blv=0.5355 V I=V/R=0.5355V/0.230 Ω= 2.33 A F=IlB= (2.33A)(0.350m)(0.450 T)= 0.367 N