Transcript Electricity and Magnetism

Electricity and Magnetism
Chapters 11 and 12
Central High School
Physical Science
Electric Charge
• Rule 1: Like charges repel one another
• Rule 2: Unlike charges attract one another
Electric Charge
• Every atom has a positively charged
nucleus surrounded by negatively charged
electrons
• The electrons of all atoms are identical
• Protons and neutrons make up the
nucleus. Protons are 1800x heavier than
an electron
Electric Charge
• A charged atom is called an ion. Ions are
made when electrons are moved.
• Adding electrons make a negative (-) ion.
• Removing electrons make a positive (+)
ion.
• Ions may have charges ranging from -4 to
+6. (-4, -3, -2, -1, +1, +2, +3, +4, +5, +6)
• Hydrogen without an electron is a +1 ion.
Coulomb’s Law
• The force between the two charged
particles varies directly as the product of
their charges and inversely as the square
of the separation distance. (Remember
Inverse-Square Law?)
• F= k q1q2/d2
where k is the proportionality constant equal to
9,000,000,000 Nm2/C2; q is the charge in
coulombs (C); and d is in m
Electric Charge
• The charge on 1 electron is -1.602x10-19 C
• The charge on 1 proton is +1.602x10-19 C
• What is the force on an electron on a
Helium atom with a radius of
128 x 10-12 m? (Helium will have 2 protons
in the nucleus for a charge of
+3.204 x 10-19 C)
Charge Polarization
• A polarized object has no net electrical
charge. The “centers of charge” moves to
the actual charge that it nears.
Electric Current
• The flow of electrons is what makes a
current of electricity.
• Rate of the electrons is measured in
Amperes and is 1.00 Coulombs of charge
in 1.00 seconds.
Voltage
• Voltage is a pressure that forces electrons
in a circuit.
• V=PEele/charge
or V=J/C or a Volt
• Voltage does not flow through a circuit but
is applied across a circuit from something
like a battery that stores electro-chemical
potential energy.
Electrical Resistance
• The resistance is the friction of a circuit.
This is the force that causes electrons to
use energy in the form of heat, light, or
mechanical motion.
• Any substance with a resistance of zero is
called a superconductor.
• Scientists measure resistance in Ohm’s or
the symbol W or omega.
Ohm’s Law
• The relationship between voltage,
resistance, and current is found in the
equation R=V/I Where I is the current
V=IR is a more familiar rearrangement of
this equation. Units are Volts, Amps, and
Ohm’s.
Direct Current
• Direct current (dc) is a flow of electrons
between the negative terminal to the
positive terminal of a circuit. The electrons
flow in only one direction. The current is
usually very slow.
Alternating Current
• Alternating current (ac) is the periodic
switching of a terminal from positive to
negative that also switches the flow of
electrons. Household electrical wiring uses
ac current for the efficiency and speed
with which electrons may be made to
power useful appliances. 60 cycles of
these switches per second or 60Hertz is
the most common US power arrangement.
Series Circuit
• A series circuit is one in which all
resistance or voltage follows a single path.
Series Circuit
In a series circuit
The current has but one pathway.
The current is resisted by all devices in a
connected link one behind the other.
The current, resistance, and voltage follow
Ohm’s Law.
The resistance around the circuit is the
sum of all the resistance.
Parallel Circuit
• A parallel circuit is a circuit in which the
paths diverge (branch) from the source to
the end.
Parallel Circuit
A parallel circuit will
The device connects the same two points
A and B of the circuits. The voltage is the
same across the devices.
The total current is divided by each of the
branches. Each branch is proportional to
the inverse of the resistance.
As the overall number of branches
increases the resistance is reduced.
Parallel Circuit
• To find the overall resistance of a parallel
circuit the following equation applies.
• 1
1
1
1
• ____ = _____ + ____ + ______
• Rtotal
R1
R2
R3
Overloading Circuits
• An overloaded circuit carries a current too large
to be safely delivered. Once a circuit reaches a
threshold a breaker is deployed to shut off all
power to the circuit.
• Ground Fault Disrupters GFD are found in the
actual outlet and are located in rooms with
readily available water supplies.
• Fuse boxes and circuit breakers are set up for
whole areas of a house to provide safety and
prevent fires.
Magnetism
• Magnetic forces are similar to electrical
forces. They cause attractions and
repulsions of objects without actually
touching.
• Magnetic poles produce magnetic forces.
Magnetism
• Like poles repel and opposite poles attract
• If you break a magnet in half; two poles
will form
Magnetism
• A magnet is formed by like atoms aligning
with one another. These are called
magnetic domains.
• In order for a magnet to lose its force, the
magnetic domains must be disordered.
Electromagnets
• Electrical current through a coil of wire will
acquire a magnetic field. The pattern that
is produced is influenced by the direction
of the current.
• Large electromagnets are used in junk
yards to lift automobiles.
• Long superconducting electromagnets are
used for transportation and high energy
particle physics. (TiNb)
Faraday’s Law
• A magnetic field can be used to create a
charge.
• The induced voltage in a coil is
proportional to the number of loops
multiplied by the rate at which the
magnetic field changes within those loops.
• Measurement for magnetically induced
charges is a galvanometer.