Transcript electricity - Aquinas High School

ELECTRICITY
This is going to be pretty cool.
Let’s Start with a Brainstorm

With a partner, list at least 5 things you think
of when you hear the word “electricity.”
Some Basics

Think back to your models…what makes
up an atom?

Electric charge is a property that causes
subatomic particles like protons and
electrons to attract or repel each other
Some Basics

There are two types of electric charge:


Positive and Negative
Protons have a positive (+) charge and
electrons have a negative (-) charge
Some Basics
That Lithium atom was neutral: 3 protons
and 3 electrons; equal + and – charges
 What if that Lithium atom gained an
electron?
 Becomes a negatively charged ion
 An excess or shortage of electrons
produces a net electric charge

The SI Unit of Electrical Charge
The SI unit of electrical charge is the
Coulomb (C)
 It takes 6.24 X 1018 electrons to produce a
single coulomb
 Lightning bolt = 10 – 20 C of charge
 Camera flash = 0.025 C of charge

Electric Forces: Quick Demos

1.
2.
3.

Balloons, Hair, and Paper. I need some
help with this one.
Rub hair with balloon, hold over paper
scraps
Rub hair with 2 balloons, bring the 2
balloons close together.
Rub hair with balloon, hold near stream
of water at faucet
Let’s figure this stuff out…
Balloon and Paper
What’s going on?
 Balloon is negatively
charged
 Paper is positively
charged

Balloon and Balloon
Do balloons dislike other balloons?
 Balloon A is negatively charged, Balloon B
is also negatively charged

Balloon and Water
Predictions?
 Balloon = charge
 Water has a +
end and a - end

Electric Forces: Basic Idea

Like charges repel, opposite charges
attract
Attraction and Repulsion
The force of attraction or repulsion between
electrically charged objects is electric force
 Double the charge = double the electric force
 Quadruple the charge = quadruple the force
 Double the distance = force is 4 X less
 Quadruple the distance = force is 16 X less

Strength of Electric Force
Electric Force between 2 objects is…
 Directly proportional to strength of charge
 Inversely proportional to the square of the
distance between them

Inverse Square Law
Electric Fields

A field in a region of space that exerts an
electrical force on charged particles is
called an electric field

Field of a positive charge points outward;
field of a negative charge points inward
Electric Fields

The strength of an electric field depends
on the amount of charge that produces the
field and on the distance from the charge
Static Electricity and Charging
Static Electricity is the study of behavior
of electric charges and how charges are
transferred between objects
 The total charge in an isolated system is
constant: Law of Conservation of
Charge

1. Charging by Friction
Ex.: walking across carpet, rubbing a
balloon on your hair, etc.
 Electrons go from hair to balloon through
friction…balloons like electrons more than
hair

2. Charging by Contact
Van de Graaff generator
 Charge is transferred simply by direct
contact

3. Charging by Induction
No direct contact, but a charge repels
another charge within the electrical field
 Ex.

Static Discharge
Static Discharge occurs when a pathway
suddenly forms through which charges
can move
 CHARGES WILL NOT MOVE THROUGH
NEUTRAL AIR!! Lightning and doorknob
shocks are a result of a charged pathway
of air

Static Discharge
Electric Current
A continuous flow of electric charge is
called electric current
 SI unit = Ampere (A) or Amp
 1 amp = 1 coulomb per second

2 Types of Current
Direct Current (DC) – charge flows in
only 1 direction
 Ex.: Flashlights and other battery devices
 Alternating Current (AC) – flow of
electric charge regularly reverses direction
 Ex.: Homes, Schools, etc.

2 Types of Current
A tricky note about Current
Current is defined as the direction in which
positive charges would flow (from + to -)
 Electrons flow in the opposite direction!!
(from – to +)

Conductors and Insulators
Electrical Conductors
Electrical Conductor – material through
which charge can easily flow
 Metals = lattice of ions with free electrons
that can conduct charge

Electrical Insulators
Electrical Insulator – material through
which charge CANNOT easily flow
 Wood, Plastic, Rubber, Air  no free
electrons to conduct charge

Resistance
Resistance is opposition to the flow of
charges in a material
 SI unit for Resistance = Ohms
 Electrons collide with other electrons and
metal ions: kinetic E  thermal E

-Less energy to move electrons

Resistance is influenced by…
1.
Thickness

Thicker the wire = lower the resistance
Easier for more electrons to flow

Resistance is influenced by…
1.
Length

Longer the wire = higher the resistance
Electrons have to travel farther

Resistance is influenced by…
1.



Temperature
Higher the temp = higher the resistance
Electrons collide more often
Superconductor – material with almost
no resistance at cold temperatures
Voltage / Potential Difference
Energy is needed for electrical charge to flow
 Potential Difference – difference in electrical
potential energy between two places in a field
 Also known as voltage, SI unit of Volts (V)
 1 volt = 1 joule per coulomb
 More Volts = More Electrical PE

Voltage and Batteries

Batteries convert chemical energy to electrical
energy
Ohm’s Law

Time to connect Volts, Current, Resistance

Voltage (V) = Current (I) X Resistance (R)


V = I
x
R
volts = amps x ohms
Ohm’s Law says…


V = I x R
I = V / R
Increasing the volts increases the current
 If voltage is constant, increasing
resistance decreases the current

Ohm’s Example

Resistance (R) = 2 ohms; Current (I) = 7 amps

What is the voltage (V)?

FORMULA: V = I x R

WORK:
V = 2 amps x 7 ohms = 14 Volts