Download Electricity & Magnetism

Magnetism and
Electromagnetism
The basics of magnetism
Named for Magnesia, an island in
the Aegean Sea >2000 years ago
Lodestones or magnetite, Fe2O3
Chinese used magnets as
compasses in the 12th century
The basics of magnetism
1750 in England, John Mitchell
shows that magnetic poles obey the
inverse-square law
1820 Oersted discovered that
magnetism is related to electricity…
Ampere later proposed that electric
currents are the source of all
magnetism
Magnetic Forces and Poles
Like the electrostatic force it can repel or
attract
“regions” of magnets called magnetic
poles give rise to magnetic forces
All magnets have both a north and south
pole
Like poles repel, opposite poles attract
No magnetic monopoles exist in nature
Magnetic Fields
The space around a magnet is
“altered”… the alteration is called a
magnetic field
Lines of force… always in closed
loops
Magnetic Field of a Bar Magnet
Earth has a magnetic field
Magnetic materials
Magnetic materials (cont’d)
Magnetic force is due to the motion of
electric charges
Magnetism is produced by two motions of
electrons… their spin and their orbiting
motion about the nucleus
The space around a moving electric charge
is “altered”… the alteration is called a
magnetic field
Magnetic materials (cont’d)
In many substances, magnetism
produced cancels out
In some materials… such cancellation
does not occur… called Ferromagnetic
materials such as iron, nickel, cobalt,
and alloys e.g. alnico… an alloy of
aluminum – nickel - cobalt
Electromagnetism
1820, Oersted finds that a current carrying
wire will cause a compass needle to deflect
Implication: the current carrying wire itself
behaves like a magnet (since it can exert a
force on another magnet… like a
permanent magnet would)
Let’s use field theory to understand what’s
happening
Magnetic Field for a coil of wire called a
solenoid
Magnetic Field for Solenoid with an
Iron Core
Electromagnetism (cont’d)
1820, Oersted finds that a current carrying
wire will cause a compass needle to
deflect… so there was a discovery waiting
to be made…
1832, Michael Faraday found that a
changing magnetic field could produce an
electric current… electromagnetic
induction
Principle underlying electric generators
Contrasting Electric motors and generators
Electric motors – convert electrical
energy into mechanical energy… the
motion of electrons into the motion of
something else
Electric generators – convert
mechanical energy into electrical
energy… the motion of something into
the motion of electrons
Electricity & Magnetism
Static, Currents, Circuits
Magnetic Fields & Electro Magnets
Motors & Generators
The world is filled with electrical
charges:
+
-
+ +
+
+ +
+
+
+
+
-
What is this electrical potential called?
Static Electricity
-
-
- -
+
++
++
Static Electricity
The build up of an electric charge on
the surface of an object.
The charge builds up but does not
flow.
Static electricity is potential energy.
It does not move. It is stored.
Static Discharge…
Occurs when there is a loss of static
electricity due to three possible
things:
Friction - rubbing
Conduction – direct contact
Induction – through an electrical
field (not direct contact)
Electricity that moves…
Current: The flow of electrons from
one place to another.
Measured in amperes (amps)
Kinetic energy
How can we control currents?
With circuits.
Circuit: is a path for the flow of
electrons. We use wires.
There are 2 types of currents:
Direct Current (DC) – Where
electrons flow in the same direction
in a wire.
There are 2 types of currents:
Alternating Current (AC) – electrons
flow in different directions in a wire
There are 2 types of circuits:
Series Circuit: the components are
lined up along one path. If the
circuit is broken, all components turn
off.
Series Circuit
There are 2 types of circuits:
Parallel Circuit – there are several
branching paths to the components.
If the circuit is broken at any one
branch, only the components on that
branch will turn off.
Parallel Circuit
Conductors vs. Insulators
Conductors – material through which
electric current flows easily.
Insulators – materials through which
electric current cannot move.
Examples
Conductors:
Metal
Water
Insulators:
Styrofoam
Rubber
Plastic
Paper
What is Resistance?
The opposition to the flow of an
electric current, producing heat.
The greater the resistance, the less
current gets through.
Good conductors have low
resistance.
Measured in ohms.
What Influences Resistance?
Material of wire – aluminum and copper
have low resistance
Thickness – the thicker the wire the
lower the resistance
Length – shorter wire has lower
resistance
Temperature – lower temperature has
lower resistance
What is Voltage?
The measure of energy given to the
charge flowing in a circuit.
The greater the voltage, the greater
the force or “pressure” that drives
the charge through the circuit.
Difference b/t Volts and Amps
Example – you could say that…
Amps measure how much water comes
out of a hose.
Volts measure how hard the water
comes out of a hose.
Ohm’s Law
Resistance = Voltage / Current
Ohms = Volts / Amps
Practice with Ohm’s Law
Ohms
4
15
2
9
6
Volts
100
150
30
45
48
Amps
25
10
15
5
8
What is an electromagnet?
Electromagnet – a magnet made
from a current bearing coil of wire
wrapped around an iron or steel core.
What is a generator?
Generator – a machine that changes
mechanical energy to electrical
energy
Usually use moving magnets to
create currents in coils of wire.
What is a motor?
Motor – a device that changes
electrical energy to mechanical
energy that can do work.