Transcript Electric Charge & Current

Electric Charge &
Current
Chapter 20
Types of Electric Charge
 What are the two types of charge?
 Positive
 Negative
 Ben Franklin named these charges.
 Where do these charges come from?
 Protons
 Electrons
Interactions Between
Charges
Electric Field
 Region around a charged particle
where a force is exerted.
 Similar to magnetic field
 Draw with lines to show what kind of force
would be on a test charge (always +)
 Remember, what is a force?
Electric Fields around
Single Charge
Electric Fields Around
Multiple Charges
Static Charge
 What is the charge of most objects?
 Trick question??
 Most objects are neutral.
 What does that mean?
 Protons equal Electrons or +
=-
So, how do things become
charged?
 Can all materials become charged?
 Insulators: electrons cannot be lost easily
 Conductors: electrons free to move
 Electrons can be lost or gained.
 Less electrons means…
 Less negative or a overall positive charge
 More electrons means…
 More negative or a overall negative charge
Static Electricity
 A buildup of charges on an
object is called Static
Electricity.
 Charges buildup but do not
flow.
Transferring Charge
1. Conduction
Charge transfer through direct
contact
2.
Induction
Movement of electrons to one
part of an object caused by the
electric field of another object
Conservation of Charge
 Charges (electrons) are not created
or destroyed
 So, if one object gains electrons
another object must have lost
electrons
 And, if one object loses electrons
another object must have gained
electrons
Static Cling
Why does your sock stick to your shirt?
Its all about charges,
opposites attract.
Why does a dryer sheet help reduce
this?
When might static cling be
beneficial?
Static Discharge
 When a negatively charged object
and a positively object are brought
together, electrons move until both
objects have the same charge.
 The loss of static electricity as electric
charges move off an object is called
static discharge.
How can static discharge?
 Humidity
 Water in the air can pick up charge that has
built up on an object.
 Sparks
 Shock, electrons moving from hand to door
knob results in neutral objects again.
 Lightning
 Electrons buildup on clouds then discharge to
areas of positive charge.
Detecting Charge
 Electric charge is invisible, but it
can be detected by a special
instrument called a
Electroscope.
 Uncharged: metal leaves hang
straight
 Charged: metal leaves repel
Circuit Measurements
Electrical Potential
 An object will move from a place of
high potential energy to a place of low
potential energy.
 What would be an example of this?
 Electrons will also move from high
potential to low potential energy
exerted by an electric field.
 Potential energy per unit of charge is
called electrical potential.
Voltage
 The difference in electrical potential
between two places is called the potential
difference.
 This provides the force that pushes the
charge through a circuit.
 Voltage is the unit for electrical difference.
 Voltage causes current to flow through an
electric circuit.
Voltage Sources
 Creates a potential difference.
 Batteries and generators
 Just as an increase in the difference in
height causes a greater flow of water, an
increase in voltage causes a greater flow
of electric current.
Resistance
 Opposition to the flow of charge.
 Depends on the type of wire,
thickness, and length.
 Current depends on resistance.
 The greater the resistance, the less
current there is for a given voltage.
 Path of least resistance
Ohm’s Law
 Voltmeter: measures potential
difference
 Ammeter: measures current
 Ohm’s Law:
V=IR
 Voltage=Current x Resistance
Volts=Amps x Ohms
Series & Parallel Circuits
Series Circuits
 Only one path for the current to take
 All parts of circuit connected one after
the other.
 Added resistors (light bulbs)
decreases current
 Ammeter connected in series.
Parallel Circuit
 Several pathways for current to flow
 Each resistor connect to energy
source separately
 Each pathway can have different
current going through it
 Voltmeter is connected in parallel
Household Circuits
 Would you want your house to be series or
parallel?
 Why?
Electrical Safety
Becoming Part of a Circuit
 Short circuit: a connection that allows
current to flow through an unintended path
 Exposed Wires
 Resisting Current
Grounding
 Third Prong
 Lightning Rods
 A lightning rod is connected to a grounding
wire. When lightning strikes the rod, charges
flow through the rod, into the wire, and then
into Earth.
Fuses & Circuit Breakers
 In order to prevent circuits from
overheating, devices called fuses and
circuit breakers are added to circuits.
 A fuse is a device that contains a thin strip
of metal that will melt if too much current
flows through it.
 A circuit breaker is a safety device that
uses an electromagnet to shut off the
circuit when the current gets too high.
Electric Shocks
 Current in the body
 The severity of an electric shock depends on
the current.
 .1 to .2 amps can be dangerous
 Resistance in the body
 Dry skin – High resistance
 Wet skin – Low resistance