Electrostatics Electrical fields

Electrical energy

Electrical forces and Charge

Arise from the particles in atomsProtons +Electrons –

Much stronger than gravitational force!Over a billion times stronger!

Atoms usually have the same # of electrons and protons so they are electrically neutralAn atom that looses or gains an electron is called ?

Ion Charges: like charges repel; unlike charges attract

Attraction and Repelling properties

Charges attracting and repelling

Conservation of Charge

Electrons are neither created nor destroyed but are simply transferred from one material to another. Charge is conserved.Examples:

Rubber rod is rubbed by a piece of fur, electrons transfer from the fur to the rubber rodRub a glass or plastic rod with silk, you’ll find the rod becomes positively charged (loses electrons)

Remember orbitals in chemistry? Which electrons are transferred?The charge of an object is always a whole number multiple of the charge of an electron

Electrons cannot be divided into fractions of electrons

Coulomb’s Law

For charged particles or objects that are small compared with the distance between them, the force between the charges varies directly as the product of the charges and inversely as the square of the distance between them.

F=k q1q2

d2

9 x 109 N·m2/C2

Quantity of charge of 1st and 2nd object

Distance between them

Compare Gravity and Coulomb’s Law

Only attractiveG=small magnitudeBoth are inverse-square laws

Attractive or repulsiveK=large magnitudeBoth are inverse-square laws

If a pair of charges of 1C each were 1 m apart, the force of repulsion between the two charges would be 9 billion

Newtons (10 times the weight of a battleship!

Creating a charge:

By frictionDrag your shoes across a carpeted floor and touch a doorknob

By touch By induction

Using a ground; infinitesupply of electrons

Charge polarizationCreating a charge on an insulator

http://www.physicsclassroom.com/mmedia/estatics/isop.html

Electric Field Linesaka lines of force

Since electric fields have magnitude and direction, it is a ___________ quantity.Lines drawn to represent the electric fields are always drawn pointing away from the positive charge

Electrical Potential Energy

Recall gravitational potential energyA charged object can have potential energy by virtue of its location in an electric field. Electrical pe if a charged particle is increased when work is done to push it against the electric field of something else that is charged. Work done is equal to energy gained by the charge.

Electric Potential

Explain why we refer to electrical potential per charge

You may have multiple charges in one location. Simply discussing the electrical energy would be misleading without talking about how many charges you had

Electrical potential energy in a uniform electric field

A is moving to B in a uniform electric fieldThe change in PE depends upon the charge, the strength of the electric field and the displacement

http://www.saburchill.com/physics/questions/quest02.html problems

PEel if –PEel if+

ΔPE

Why is the electrical potential energy not changed if A moves perpendicular to the uniform field lines?

That would be the same as moving an object horizontally and saying it has the same gravitational PE

Electrical potential energy with a pair of charges

Key points in the formula:Reference point for electrical potential energy starts at infinity (because PE goes to zero as the distance between the charges goes to infinity)PE is + when you have like charges

bec/ like charges repel, + work must be done to bring them together

PE is – when you have unlike charges

Electricity in Circuits

Capacitor

Passive Electronic device to store energy in the form of an electrostatic field

Consists of 2 conducting plates separated by a dielectric (insulator)

Voltage

Since electric potential is measured in volts, it is commonly called ___________.

Electric potential = electrical potential energy

charge1 volt = joule

coulomb

http://www.glenbrook.k12.il.us/gbssci/phys/Class/circuits/u9l1c.html 1-7

Potential difference=Voltage CellCovert chemical energy to electrical energyAA, AAA, D batteries have the same voltage

+ endMax Volts

-endZero Volts

A voltage sets charges in motion“voltage supplies electrical pressure”

Moving electrons (negative to positive)Current

Rate the charges move through a conductorUnit = ampIn metals, moving electrons make up the currentIn gases or liquids, both + and – charges are in motion WHY?

Battery = direct current

Current

The flow of electric charge

Ohm’s LawVoltage - V

Volt BatteryPotential difference

Current - I Amps Rate of flow

Resistance - ROhmsAs resistance , current

I=V/R

Resistance

The resistance of the filament determines the brightness of the bulb

Moving electrons

Random movement if circuit is offMarching analogyWhere do the electrons come from in a circuit?

Free electrons in the wires

Why do the wires become hot when a current is flowing through them?

Transfer of kinetic energy due to electron speed

Electric circuit

An electrical device connected so that it provides one or more complete paths for the movement of chargesBatteries add the energy for the electrons to move

Electrons move – to +Conventional Current moves + to –

Explain an open and closed circuit.

Circuit Breakers and fuses

Safety deviceRespond to an overloaded circuit by opening the circuitActs like a switchResetFuses:

Must replace

Electric power

Current x voltageUnit

Watts

Kilowatt-hourEnergy delivered in 1 hour at the rate of 1 kW

Series Circuit Parallel Circuit

One pathResistors share voltageCurrent the same throughout circuit

More than one pathEach path receives maximum voltageCurrent could be different in different branches

Kirchoff’s Current Law

All current flowing into a branch point must flow out

Energy Transformations

MotorElectrical Energy Mechanical Energy

GeneratorMechanical Energy Electrical Energy

Hydroelectric Power Plant http://www.fwee.org/walktour/

Mechanical Energy (Water flowing and turning the turbines)Electrical Energy

How does electricity get to your house?

THE SPINNING SHAFT turns magnets inside a stationary ring of copper, moving electrons to produce electricity STEP-UP TRANSFORMERS increase the voltage of electricity produced by the generator.TRANSMISSON LINES carry electricity to substations in our communities. The voltage is decreased and the power is distributed to homes and businesses

How does electricity get to your house?

THE SPINNING SHAFT turns magnets inside a stationary ring of copper, moving electrons to produce electricity STEP-UP TRANSFORMERS increase the voltage of electricity produced by the generator.TRANSMISSON LINES carry electricity to substations in our communities. The voltage is decreased and the power is distributed to homes and businesses

Calculate resistance using the graph.