Electricity

Electric Charge & ForceElectric Charge an electrical property of matter that creates

a force between objects

• 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=Y8oN0YFAXWQ

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 & ForceAn 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

• difference in the numbers of protons & electrons determines an object’s electric charge

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

Electric Charge & ForceConductors allow charges to flow; insulators do not

Conductors a material that transfers charge easily

• ex. metal

Insulator a material that does not transfer charge easily

• 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

Electric Charge & Force• objects can also be charged without

friction• ex. by 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/out contacting a charged object• opposite sides will have an induced charge (induction)

Electric Charge & ForceHow 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

• responsible for holding atoms together

Electric Charge & ForceElectric force depends on charge and distance

Balloon example

• 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 valuequadrupled 1/16

Electric Charge & ForceElectric 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

• electric field lines point outward on a positive charge

• electric field lines point inward on a negative charge

Electric Charge & Force

• field lines for two positive charges repel could also be for

negative charges

• field lines for a positive & a negative attract

Number of field lines and their closeness determines strength of field

CurrentHow do charges move through a light bulb?

What causes the charges to move?

Voltage & Current

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

Current

Potential difference is measured in volts

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 is the volt, V

CurrentThere 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 convertchemical energy to electrical energy

Current

“dc”

CurrentA voltage sets charges in motion

How does a flashlight work?

Current 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

Current• 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• most appliances are designed for use

w/what voltage?120 V

Resistance 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 = voltagecurrent

R = V I

SI unit for resistance is the ohm

Current

Factors that determine resistance:

1. Material

2. Length

3. Diameter

4. Temperature

CurrentConductors 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

CurrentWhen 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

Semiconductors are intermediate to conductors and insulators

• in their pure state, semiconductors are insulators

• specific atoms are added as impurities to increase their ability to carry a charge

Circuits What are 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

Why do light bulbs burn out?

CircuitsWhat 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 w/a battery & one or more bulbs

• standard symbols = can be read all over the world

Series & Parallel CircuitsSeries 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?

Series & Parallel CircuitsParallel 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?

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

Does a charge in a circuit ever lose energy?

electric power is the rate at which electrical work is done

Electric Power Equation Power = current x voltage

P = IV

SI unit for power is the watt (W)

Electric Power & Electrical Energy

Electric Power & Electrical Energy

Electric companies measure energy consumed in kilowatt-hours

• power companies charge for energy used in the home, not power

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

Fuses & Circuit BreakersWhat happens when you have too many appliances, lights, CD players, tv’s, & other devices connected across a 120 V outlet?Q

A The overall resistance of the circuit is lowered & the wires are carrying more than a safe level of current. OVERLOAD

Electric Power & Electrical Energy

Worn insulation on wires can also be fire hazards, WHY?

Two wires may touch creating an alternate pathway for the current. SHORT CIRCUIT

Fuses melt to prevent circuit overloads

Fuse an electrical device containing a metal strip that melts when current in the circuit becomes too great

Circuit breakers open circuits with high current

Circuit breaker a device that protects a circuit from current overloads

• uses a bimetallic strip, a strip w/two different metals welded together