Electricity

Electricity

A form of energy resulting from the

existence of charged particles (such as

electrons) either statically as an

accumulation of charge or dynamically

as a current

Static Electricity

An imbalance of electric

charges within or on the

surface of a material.

The charge remains

until it is able to move

away by means of an

electric current or

electrical discharge

https://www.youtube.com/watch?v=yc2-363MIQs

Current Electricity

(Electric Current)

A flow of electric

charge.

In electric circuits this

charge is often carried

by moving electrons in

a wire

Electric Charge & Force

Electric Charge an electrical property of matter that creates

a force between objects (Q)

• We experience this force as a shock

Like charges repel, and opposite charges attract

• The two types of charges are called positive & negative

• When there is an equal amount of positive & negative

charges on an object, it has no net charge

Electric Charge & Force

https://www.youtube.com/watch?v=h-0gNl5f4BU

Triboelectric Series

Different materials have different

attractions to electrons

By rubbing a variety of materials against

each other and testing their resulting

interaction with objects of known charge,

the tested materials can be ordered

according to their affinity for electrons

Greater attraction for electrons (negatively

charged)

Electric Charge & Force

The photo to the

left is a

Triboelectric

Series. It shows

that, when two

objects are

rubbed together,

the object on

top will become

positive (lose

electrons)

while the object

on the bottom

will become

negative (gain

electrons).

Electric Charge & Force

An object’s electric charge depends on the imbalance

of its protons & electrons

• Objects are made up of an enormous number of neutrons,

protons, and electrons

• Whenever there is an imbalance in the number of protons &

electrons in an atom, molecule, etc. it has a net electric

charge (Q)

• Difference in the numbers of protons & electrons determines

an object’s electric charge

• The SI unit of electric charge is the coulomb, C

Atomic Structure Review

Electric Charge & Force

Conductors allow charges to flow & Insulators do not

Conductors materials that permit electrons to

flow freely from particle to

particle • Ex. metals

Insulators materials that does not transfer

charge easily (impede electron

flow) • Ex. cardboard, glass, silk, & plastic

Objects can be charged by the transfer of electrons

• Protons & neutrons are relatively fixed in the nucleus…

electrons can be easily transferred

• Ex. sliding across a fabric car seat = charging by friction/motion

Examples of Conductors

and Insulators

Conductors vs. Insulators

Conductors have low resistance

• A good conductor is any material in which electrons flow easily

Some materials become superconductors below

a certain temperature

• Certain metals & compounds have zero resistance when their

temp falls below a certain temp called the… Critical temperature

• These types of materials are called superconductors

Insulators have high resistance to charge movement

• Insulating materials are used to prevent electric current from

leaking

Conductors vs. Insulators

Semiconductors are intermediate to conductors and insulators

• In their pure state, semiconductors are insulators (very little

electricity will flow through them)

• Specific atoms are added as impurities (process is called

doping) in order to increase their ability to carry a charge

Electric Charge & Force

• Objects can also be charged without

friction

• Ex. touching something with a charged

object

• Objects charged in this manner are said to be

charged by contact

Charges move within uncharged objects

• Charges in a neutral conductor

can be redistributed w/o contacting

a charged object

• Opposite sides will have an

induced charge (induction)

Grounding

When is it important to provide a pathway for current to

leave a charged object?

• Conducting wires are often run between the charged object

and the ground, thereby… grounding the object

Purpose for Grounding

Prevents electrical

discharge (shock) that

can occur because of

an imbalance of charge

within a material or on a

surface of a material

Neutralization of excess

charge or getting rid of

excess charge by a flow

of charges from or to

the surroundings

Electroscope

Electrophorus Lab

Provides a review of the

following topics:

Charging by induction

Grounding

Insulators

Conductors

Electric Charge & Force

How can a negatively charged comb pick up

pieces of neutral tissue paper?

• Polarization of atoms in an insulator produces an induced

charge on the surface of the insulator

Electric force the force of attraction or repulsion between

objects due to charge (non-contact force)

• Responsible for holding atoms together!

Electric Charge & Force

Electric force depends on the amount of charge on each of

them and the distance between them

• Electric force is inversely proportional to the

square of the distance between two objects

Huh ?

For example, if the distance between two charged balloons is

doubled, the electric force between them decreases to ¼ its

original value quadrupled 1/16

Electric Charge & Force

Electric force acts through a field

Electric field the region around a charged object in which other

charged objects experience an electric force

• One way to show an electric field is by drawing electric field

lines (lines of force)

• Electric field lines point

outward on a positive charge

• Electric field lines point

inward on a negative charge

Electric Field Lines

• Field lines for two

like charges repel

• Field lines for a opposite

charges attract

Electric Field Lines

Number of field lines and their closeness

determines strength of field

Voltage & Current

What causes the charges to move?

Electrical Potential

Energy

potential energy of a

charged object due to

its position in an

electric field • If two charged objects repel each other, the

electrical potential energy will be greatest when

the objects are near each other

Potential difference/

voltage

the change in the electrical potential

energy per unit charge

• This change occurs as a charge moves from one place to

another in an electric field

• SI unit for potential difference (voltage) is the volt, V

Current

A voltage sets charges in motion

Current (I) Continuous flow of electric charges through a

material; the rate that electric charges move through

a conductor

• SI unit of current is the ampere, A aka. Amp

• this is equal to one coulomb of charge per second (C/s)

• A battery is a direct current source because the charges

always move from one terminal to the other in the same

direction

• It is the negatively charged electrons that move from

atom to atom based on how tightly they are held by the

atom

DC Current

“DC”

Current

There is a voltage across the terminals of a battery

Most common batteries are an electric…

Cell a device that is a source of electric

current because of a potential difference,

or voltage, between the terminals

+

-

or a combination of connected electric cells – that convert

chemical energy to electrical energy

AC Generator • The wire moves… not the

magnet

• When the loop of the wire is

rotated within the magnetic

field of this fixed permanent

magnet, electrons shift

according to its polarity

• Again, because of the

attachments of the ends of

the wires of the loop, the

process keeps repeating

• The result is an oscillation of

electrons within the wire often

referred to as “alternating

current”

“AC”

Current

Electron flow is what we think of when we think of

as electrical current.

There are two types of electron flow Direct

Current (DC) and Alternating Current (AC)

Direct Current is the type of flow we get from

batteries

Alternating Current is the type of flow that we get

from a typical electrical outlet in a home

https://www.youtube.com/watch?v=BcIDRet787k

Resistance in

Currents

Resistance (R) Opposition to the flow of

charges in a circuit

• Caused by internal friction

(slows the movement of charges through a conducting material)

Resistance can be calculated from current and voltage

Resistance Equation (Ohm’s law)

resistance = voltage

current R = V

I

SI unit for

resistance is the

ohm

Resistance

Four factors that determine

resistance: Material

(resistivity)

Length

Diameter

Temperature

Light Bulbs Demonstrate the

Four Laws of Resistance

Electricity enters bulb via

wire connected to base

Electricity goes through a

long wire (length) leading to

a piece of tungsten filament

which is a poor conductor

(material/resistivity)

Tungsten is very thin

(diameter) & coiled to

maximize resistance in the

wire

Light Bulbs Demonstrate the

Four Laws of Resistance

When electricity meets

resistance, it heats up the

resistor which gets so hot

(temperature) that it glows!

If exposed to air the

tungsten would burn up

quickly so it is enclosed by

glass

Why do light bulbs burn

out?

Resistors in Circuits

Carefully placed in

circuits to regulate flow

of electrons (restrict

flow)

Resistance is directly

related to resistivity,

length, and temperature

Resistance is indirectly

related to cross

sectional area

Resistors in Circuits

Resistors with color

bands to indicate their

resistance value and

tolerance

1st band, 2nd band, 3rd

band, 4th band

Higher resistance =

Lower current

V = IR (Ohm’s Law)

http://www.the12volt.com/resistors/resistors.asp

Electric Power & Electrical

Energy

Electrical energy

the energy associated w/ electrical

charges, whether moving or at rest

Electric power is the rate at which electrical

energy is used in a circuit

Electric power is the rate at which electrical work is done

Electric Power Equation Electric Power = current x voltage

P = IV

SI unit for power is the watt (W)

Electric Power & Electrical

Energy

Compare a 700 watt

microwave with a 1,400

watt microwave

Both do the work

(convert electric PE to

heat) but the 1,400 watt

microwave does it

faster!

Electric Power & Electrical Energy

Electric Power & Electrical

Energy

Electric companies measure energy

consumed in kilowatt-hours

• Power companies charge for energy

(E) used (work done) in the home based

on how quickly our appliances use it

• One kilowatt-hour is the energy delivered in 1 hour

•E = Pt •Energy (used) = Power (how quickly the appliance used

electricity) x Time (for how long it used electricity at that rate)

What happens when you have too many appliances, lights, CD

players, tv’s, & other devices connected across a 120 V outlet?

The overall resistance of the circuit is lowered & the wires are

carrying more than a safe level of current. OVERLOAD

Electric Power & Electrical Energy

Electric Power & Electrical Energy

Fuses melt to prevent circuit overloads

Fuse an electrical device containing a metal strip that melts

when current in the circuit becomes too great (need

replaced)

Circuit breakers open circuits with high current

Circuit breaker a device that protects a circuit from current

overloads (do not need replaced & get reset)

• Uses an internal switch mechanism that is tripped by an

unsafe surge of electricity

Large power overloads can destroy electrical equipment or even

cause fires. Fuses & circuit breakers protect from overloads by

interrupting the continuity or flow of electricity.

Circuits

An electric circuit is a path

through which charges can be

conducted

Electric circuit an electrical device connected so that it

provides one or more complete paths for the

movement of charges

Closed circuit the connecting path produced

when the light bulb is connected

across the battery’s terminals

Open circuit without a complete path, there is

no charge flow and therefore no

current • The voltage source is always part of the

conducting path of a closed circuit

Circuits

What is a switch?

• Switches are used to open and

close circuits

Schematic diagrams are used to represent circuits

How would you describe the

contents & connections in the photo

above?

Schematic diagram

a graphic

representation of an

electric circuit or

apparatus, w/standard

symbols for the

electrical devices

Circuits

• Can be used to describe

any circuit with a battery

& one or more bulbs

• Standard symbols = can

be read all over the world

•Loads = something to

use the current in a circuit

(light bulbs & resistors)

Series vs. Parallel Circuits

Series Circuit-

electricity must flow

through all loads within

the circuit (current

remains constant

throughout)

Parallel Circuit-

electricity with multiple

paths which it can travel

(voltage remains

constant throughout)

https://www.youtube.com/watch?v=x2EuYqj_0Uk

Series & Parallel Circuits

Series circuits have a single path for current

Series describes a circuit or portion of a circuit that provides a

single conducting path What happens when one

element along the path is

removed?

Why is this a problem?

Ex. Old Christmas Lights

Series & Parallel Circuits

Parallel circuits have multiple paths for current

Parallel describes components in a circuit that are connected

across common points, providing two or more

separate conducting paths

What happens when one

element along the path is

removed?

Practice Schematic

Diagram Sketches

Sketch a battery

Sketch an open switch

Sketch three loads in a series

Sketch three loads in parallel

Compound Circuits

Include series and parallel circuits together!

Example: “Two loads in series in parallel with

one load…the quantity of which is in series

with another load”

*Practice Schematic Diagrams Activity*