Physics D--Chapter 21 Magnet Forces and and Magnetic Fields

Magnets North and South poleNorth and South pole –Like poles repel –Opposite poles attract

Magnetic Fields Surround magnetized materialSurround magnetized material Direction is the direction a compass pointsDirection is the direction a compass points –Away from N, towards S Strongest at the polesStrongest at the poles

Magnetic Fields

New vector symbols Into the pageInto the page Out of the pageOut of the page – – –     

Earth’s Magnetic Field

Charged Particles in a Magnetic Field Stationary chargeStationary charge Moving perpendicular to fieldMoving perpendicular to field Moving Parallel to fieldMoving Parallel to field A charge only experiences a force if a component of its velocity is perpendicular to the fieldA charge only experiences a force if a component of its velocity is perpendicular to the field –No force –Maximum force –Force is zero

Right-Hand Rule #2 (RHR2) Used to determine the direction of the magnetic forceUsed to determine the direction of the magnetic force

Magnitude of Magnetic Field Force exerted on a moving test charge at a certain pointForce exerted on a moving test charge at a certain point Symbol => BSymbol => B Units=>Tesla(T)=>(N  sec)/(C  m)Units=>Tesla(T)=>(N  sec)/(C  m) –1 gauss(G) = Tesla

Example Problem An electron experiences a downward force of 3.0* N while traveling through a magnetic field of 3.8*10 -3 T west. What is the direction and velocity of the electron?An electron experiences a downward force of 3.0* N while traveling through a magnetic field of 3.8*10 -3 T west. What is the direction and velocity of the electron?

Example Problem

Path of charge If a charge moves perpendicular to a magnetic field, it will travel in a circleIf a charge moves perpendicular to a magnetic field, it will travel in a circle

Path of charge

Magnetic Force on a Current Carrying Wire Current is a flow of charge particlesCurrent is a flow of charge particles A magnetic force is exerted on a moving chargeA magnetic force is exerted on a moving charge Then a magnetic force is exerted on current in a wireThen a magnetic force is exerted on current in a wire

Magnetic Force on a Current Carrying Wire Use your right hand rule to find the force on the wireUse your right hand rule to find the force on the wire

Example Problem A 4.0 m wire runs perpendicularly through a T field and carries a current of 3.0 A. The field points north and the current flows west. What is the force on the wire?A 4.0 m wire runs perpendicularly through a T field and carries a current of 3.0 A. The field points north and the current flows west. What is the force on the wire?

Example Problem

Magnetic Force on a Current Carrying Wire Most speakers work based on this conceptMost speakers work based on this concept

Torque on a Current Carrying Wire A loop of current carrying wire will experience a torque when placed properly in a magnetic fieldA loop of current carrying wire will experience a torque when placed properly in a magnetic field Maximum Torque when the normal of the loop is perpendicular to the field Minimum Torque (zero) when the normal of the loop is parallel to the field

Torque on a Current Carrying Wire

The greater the magnetic moment of a coil, the greater the torque that is applied will be.

Galvanometer

Galvanometer Used to measure a currentUsed to measure a current Consists of a loop of wire within a magnetic field and a spring to resist motionConsists of a loop of wire within a magnetic field and a spring to resist motion Current through the loop causes a force to rotate the loopCurrent through the loop causes a force to rotate the loop –Direction of rotation depends on direction of current

Motors Current is supplied and the magnetic field puts a force on the loop Current is supplied and the magnetic field puts a force on the loop Brushes allow current to be reversed every 180 degrees. This allows full rotation.

Magnetic Field of a Current Carrying Wire Concentric circles of a magnetic field form around a current carrying wireConcentric circles of a magnetic field form around a current carrying wire

Magnetic Field of a Current Carrying Wire Right-Hand Rule #1 (RHR1) Shows direction of magnetic field Thumb: direction of I Fingers: direction of B

Magnetic Field of a Current Carrying Wire Increased current yields increased magnetic field strengthIncreased current yields increased magnetic field strength Field weakens with distanceField weakens with distance Insert constantInsert constant Permeability of free spacePermeability of free space

Magnetic Field in a Current Loop Field formed by a loop can be found using the right hand ruleField formed by a loop can be found using the right hand rule –Inside of loop is always one direction –Outside of loop is always the opposite direction Forms N and S poleForms N and S pole

Magnetic Field in a Current Loop

Solenoids Several closely spaced parallel loopsSeveral closely spaced parallel loops Fields of individual loops combineFields of individual loops combine Iron core is inserted to make field strongerIron core is inserted to make field stronger –Electromagnet

Solenoids Reverse Right-Hand Rule #1: (rRHR1): determines field –Fingers: curl in direction of conventional current –Thumb: points to N pole of solenoid

Magnetic Domains Magnetism is caused by the spin of electronsMagnetism is caused by the spin of electrons –Usually paired up and cancels out Microscopic magnetic region composed of a group of atoms whose magnetic fields are alignedMicroscopic magnetic region composed of a group of atoms whose magnetic fields are aligned

Magnetic Domains Hard magnetic materials the domain alignment resists changeHard magnetic materials the domain alignment resists change Soft magnetic materials the domain alignment is easily changedSoft magnetic materials the domain alignment is easily changed

Permanent Magnets Hard Magnetism -Hard Magnetism - Soft Magnetism -Soft Magnetism - –Hard to magnetize but retain magnetism Cobalt and NickelCobalt and Nickel –Easy to magnetize, but do not retain magnetism IronIron

Two Parallel Wires Each wire carries a currentEach wire carries a current Each wire creates a magnetic fieldEach wire creates a magnetic field Each wire’s magnetic field will produce a force on the other wireEach wire’s magnetic field will produce a force on the other wire

Two Parallel Wires Current traveling in same directionCurrent traveling in same direction Current traveling in opposite directionsCurrent traveling in opposite directions –Attracts –Repels