Electricity

1
Electricity Magnetism

• Magnetism
• Magnetic poles
• Magnetism currents
• Motors and generators

• Electrostatics
• Electric charge
• Direction Rules
• Coulomb's Law

• Atomic Structure

• Electric Circuits
• Fundamentals
• Ohm's Law

• AC Current

2
Electrostatics

• Historically, electric forces were first noticed
  when different materials were rubs together.
• In some cases, these would then acquire the
  ability to exert forces on other bodies from a
  distance.

3
Electric Charge

• Electric charge (like mass, length, and time) is
  a fundamental property.
• The SI unit for electric charge is called the
  coulomb.
• Unlike gravity (which is always attractive),
  electric forces can be either attractive or
  repulsive.
• This happens because electric charge (unlike)
  mass, comes in two varieties.

4
Direction rule

• Charges of the same type repel one another,
  unlike charges attract.
• Because of this, the two types offset one another
  when placed on the same object.
• We therefore label them positive and negative.

5
Atomic Structure

• Negatively charged electrons orbit are attracted
  by a positively charged nucleus and orbit it much
  like planets orbit the sun
• The net charge of the atom is therefore zero.

6
Coulomb's Law

• The electric force between two charges is
  directly proportional to the product of their
  magnitudes and inversely proportional to the
  square of the distance between them.

Q1 q2
7
Conductors and Insulators

• In some materials, electrons can be moved between
  atoms fairly easily. These are known as
  conductors.
• In other materials, electrons are difficult or
  impossible to move. These are known as
  insulators.
• Generally, metals are conductors and nonmetallic
  materials are insulators.

8
Electric Current

• An electric current exists when electrons move
  through a conductor.
• The current is the rate at which charge flows
  through a cross-section of the conductor.

9
Current Definition

• Although current in a conductor is due to
  negatively charged electrons, the sign is ignored
  in electric circuits.

10
SI Unit for Current

• The SI unit for current is called the ampere

11
Battery

• A battery is a device that causes electrons to
  move in a conductor.
• Batteries are characterized by an electromotive
  force (emf or voltage).
• For any battery, this value remains constant over
  a wide range of conditions.

12
SI Unit of Voltage

• The SI unit of voltage is called the volt.

13
Ohm's Law

• The current that flows in a conductor is
  proportional to the applied voltage.
• The factor R is called the resistance of the
  conductor.

14
SI Unit for Resistance

• The SI unit of resistance is called the Ohm.

15
Electric Power

• Electric power is a measure of the rate at which
  an electric device does work (or transforms
  energy).

16
Cost of Electricty

• Utility companies sell energy, not power.
• The energy is measured in kilowatt-hours, not
  joules
• To calculate energy in kW-h, express power in
  kilowatts and time in hours.

17
Magnetic Poles

• At first glance, magnetic poles appear to obey
  rules similar to electric charges.
• Poles of the same type repel, opposites attract.

S N
N S
Force
S N
S N
Force
18
Magnetic Monopoles

• Unlike electric charge, isolated magnetic poles
  do not appear to exist in nature.
• All naturally occuring magnetic effects result
  from moving electric charges.

N
S
19
Motors and Generators

• The magnetic force on moving charges can be used
  to makes both electric motors and electric
  generators.

20
AC Current

• The voltage (and therefore the current) vary
  periodically with time.
• Standard US voltage levels are 120 V and 240 V.
• The standard US frequency is 60 Hz.

• In the graph, negative voltages indicate that
  the current has reversed direction.

21
Reasons for AC

• AC voltages are the natural output of a
  generator.
• AC voltages can be easily changed with a
  transformers.
• A method of changing voltages easily is needed
  because high voltages transmit power more
  efficiently, but lower voltages are safer to use.