Electromagnetismo

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Electromagnetism

Definition: Magnet- object that exerts a force on a piece of iron

Definition: Magnetic- property where a material is affected by a magnetic field
-like poles repel
-unlike poles attract
-the force of attraction (or repulsion) varies inversely as the square of the distance between the poles

Difference between magnetism & electricity:
-electric charges can be isolated-magnetic poles cannot be isolated
ex: cut magnet in half --> 2 new magnets
examples of strongly magnetic materials: gadolinium, cobalt, nickel, iron, alnico (Al, Ni, Co)

Definition: Magnetic Domain - groups of aligned magnetic fields in elements
[pic]

-inducing magnetism on iron ( stroke it with a magnet
-magnetic effects can be lost by cooling, heating, or hammering material-magnitite (loadstone) is a special mineral found on Earth & is magnetized by exposure to Earth's magnetic field
[pic]
-soft magnetic materials-easily lose/gain magnetism, i.e. iron
-hard magnetic materials-difficult to lose/gain magnetism, i.e. nickel & cobalt
magnetic field ( surrounds a magnet
[pic]

-can be described with magnetic field lines
-lines can describe magnitude & direction{direction of MF ( direction in which north pole of compass needle points at a given position}

Magnetic Force on a Current-Carrying Conductor

-symbol for MF is "B"
-all magnetic fields are caused by the movement of charged particles
( i.e. electrons moving through a wire
-first discovered by Oersted (1820)

Right-hand Rule #1:
"if you point your thumb of your right hand in direction ofconventional current (+ to - ), the curl of your fingers will indicate direction of magnetic lines of force around wire"

-symbol for magnetic field going into page
[pic]
-symbol for magnetic field coming out of page
[pic]
[pic]

Magnetic Field of a Current-Carrying Coil (Solenoid)

Definition: Solenoid- long, straight wire bent into several closely-spaced loops
-aka"electromagnet"
-acts like a magnet only when it carries a charge
-MF inside solenoid is nearly uniform and strong
-outside, MF is non-uniform
-strength of solenoid can be increased by adding iron core
-one end acts like North Pole; other acts like South Pole
[pic]
Right-hand Rule #2
“if you grasp the coil with the right hand and curl your fingers around the loops in the direction of theconventional current, your thumb points towards the North pole of the electromagnet”

[pic]
[pic]

-strength of magnetic field of an electromagnet is proportional to current flowing in coil
-# of loops in coil proportional to strength of magnetic field

Forces Caused by Magnetic Fields

-when moving through a magnetic field, charged particles experience a force (a stationary charge doesnot!)
-first discovered by Faraday

Right-hand Rule #3:
"if you point the thumb of your right hand in the direction of the current velocity (v), then your fingers point in the direction of the magnetic field (B) and the magnetic force (Fm) on a +ve charge is directed out of the palm of your hand"

Assume +ve test charge (qo) with velocity (v):

[pic]

• Fm is magnetic force (measuredin N)
• B is strength of magnetic field or magnetic induction (measured in Tesla or T ( one N per ampere-meter is one Tesla)
• I is the current (measured in amperes)
• L is the length of straight wire (measured in m)
• [pic]is the angle between direction of conventional current and the direction of B
[pic]
Since [pic] then
[pic]
-max magnetic force occurs when anglebetween current (or v) and B is right angle
Using this formula..
[pic].

Electric Motors
-turns electrical energy into mechanical energy
-2 types: DC & AC
-when a current is passed through a wire loop (armature) in a magnetic field ( magnetic force on armature ( 180o turn
-to make 360o rotation, split-ring commutator (attached to armature) allows current to change direction in armature (due to...
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