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Physics 106 Lesson #20
Magnetism: Relay and Buzzer
Dr. Andrew Tomasch
2405 Randall Lab
Review: Magnets• All magnets have two
poles: North and South
• Like poles repel each other; Unlike poles attract each other
• Magnetic poles are always found in pairs
• Isolated magnetic poles have never been found
Review: The Magnetic Field• The north pole of a
compass needle is a magnetic north pole
• The direction of the magnetic field at any point in space is the direction indicated by the north pole of a small compass needle (test magnet) placed at that point
The magnetic field is avector and denoted bythe letter B
B
Review: Magnetic Field Lines• Magnetic field lines
surround magnets• The magnetic field is
always tangent to the magnetic field line
• The number of lines per unit area (density) is proportional to the magnetic field strength
• Outside of the magnet, magnetic field lines point away from north poles and toward south poles
B
Review: The Magnetic Field of a Straight Wire• The magnetic field
due to the current in a long straight wire has circular field lines around the wire
• The direction of the field is given by a right hand rule Right Hand
Review: Electromagnets• Current flowing in a loop of wire creates a
magnetic field• The current loop can be visualized as a
“phantom bar magnet”
=N S
N S
• Another Right Hand Rule!
Review: Finding the North Pole
N
S
Magnets: More Key Points
• Only moving charges (currents!) produce magnetic fields
• Only moving charges experience magnetic forces → forces exerted on wires carrying current
• Magnetic field lines always form closed loops → no isolated poles → no monopoles
The Field of a Solenoid
Current Into Page
Current Out of Page
Inside a solenoid(away from the ends)the magnetic field is constant.
B
F
B Fingers point in the
direction of the current flowing in the wire
The Magnetic Force on a Wire
ILRight Hand
•The force on a wire carrying current is perpendicular to both the wire and the magnetic field•The magnitude of the force is the product of the current, the wire length and the component of the field perpendicular to the wire•The force on a wire carrying current parallel to the magnetic field is therefore zero!
F ILB B
for to the wireDemonstration
Permanent Magnetism Arises From Internal Atomic Current Loops • The current loops of a
solenoid coil produce a bar magnet (dipole) field
• The motion of atomic electrons inside a permanent magnet is equivalent to many microscopic loops of current (moving charges)
• When the fields from the atomic current loops within the material align in one direction the resulting magnetic field is the sum of the many individual atomic fields and the result is a permanent magnet
Permanent Magnet
Solenoid Coil
Demonstration
Current Loop
