Physics Unit 4Electricity and Magnetism

2 Forms of Electricity

1. Static a build up of charge on an object2. Current a steady flow of electrons

through a conductor

Static Electricity

When objects are rubbed together, electrons can transfer from one object to another.

the object that loses electrons takes on a positive charge.the object that gains electrons takes on a negative charge.

Static Electricity

Balloon is neutral.(Same # of Protons as

Electrons)

Hair is neutral.(Same # of Protons as

Electrons)

+-

+ -

+--

++

-

Static Electricity

When rubbed together, the balloon accepts electrons from the hair.

When rubbed together, the hair donates electrons to the balloon.

Static Electricity

Balloon becomes negatively charged.

Hair becomes positively charged.

-

-

-

--

-

+ +

++

_ _

+

+

_

Static Electricity

Opposite charges attract. The hair is attracted to the balloon.

-

-

-

--

-

Static Electricity

Each hair takes on a positive charge. Hairs stand apart from each other because similar charges repel each other.

Law of Attraction and Repulsion

Opposite charges attract. Similar charges repel.

Static Electricity If you scuff your feet on the carpet, electrons leave

the carpet and are stored in your body. As long as you remain insulated from other

objects, you simply walk around with excess negative charges. You have Potential Difference (Voltage)!

When you touch another conductor, the charge transfers to that object. You feel a static electric shock.

In addition to Friction, rubbing, objects can also acquire a static

charge by:

Conduction: When two objects make contact, electrons transfer between the two until their charges are equal.

Induction: When an object with a charge comes close to another object, it causes a temporary separation of charge in that object.

Conduction

A negative wand comesnear a neutral sphere.

When they touch,electrons travel from thewand to the sphere.

_

Neutral_ _

_

The sphere becomes (-) charged by conduction.

Induction

A negative wand comes near a neutral sphere.

The negative chargein the wand repels theelectrons in the sphere.

__

__

++

The side of the spherenear the wand becomes+ charged by induction.When the wand isremoved, the charge inthe sphere is removed.

Static Electricity

An excess of electrons, static electric charge, can be grounded by providing a path of conductors to the earth.

Ex. Lightning rods on homes, grounding plugs on electrical cords

Conductors and InsulatorsConductors:

Allow a free flow of electrons.

Ex. Metals(the wire inside an

electrical cord)

Insulators:Slow or stop the flow of electrons.

Ex. Rubber, Glass, Plastic

(the rubber coating on the outside of an electrical cord)

Electric Current

A steady flow of electrons through a conductor.

Measured in amperes or amps (a). An ampere is equal to one coulomb of

charge per second. (A coulomb is the charge on 6.24 x 1018 electrons.)

The variable used to denote current is I

Potential Difference

Also known as Voltage. The push that causes electrons to flow

through a circuit. Measured in volts (V). The variable used to denote potential

difference is V

Resistance

Anything that slows the flow of electrons through a conductor.

Measured in ohms ( ). Variable used to denote resistance is R

Ohms Law

The mathematical equation that shows the relationship between current, potential difference and resistance.

I = V / RWhere: I = current in amps (a)

V = potential difference in volts (v)R = resistance in ohms ()

Sample Problem

If a lamp bulb requires 5.5amps of current supplied at 110volts, how much resistance does it offer?I = 5.5ampsV = 110volts

R = V / I

R = 110V / 5.5a

R = 20

Sample Problem

If an electrical appliance offers 24 of resistance, and is connected to a 9.0V battery, how much current will flow?R = 24V = 9.0V

I = V / R

I = 9.0V / 24

I = 0.375a

Sample Problem

How many 1.5V batteries would be needed to supply 0.83a of current if the appliance offers 5.4 of resistance?I = 0.83aR = 5.4

V = IR

V = 0.83a(5.4)

V = 4.482V4.482V / 1.5V per battery = 2.988 or 3 batteries

Calculating Cost to Run Electrical Appliances

First, find the UL tag on the appliance. If power rating is given, move on to next

step. If not, you must calculate the electrical power of the appliance.

POWER = CURRENT X VOLTAGEP = IV

(w) = (a)(V)

Remember: Power is the rate at whichwork is done. A 100w light bulb converts electricity into light faster thana 60w light bulb. Therefore, it gives offmore light!

Calculating Cost cont

Calculate the amount of electrical energy the appliance uses in a given period of time.

E = Pt(kWh) = (kW)(h) E = electrical energy

P = electrical power

T = time

Calculating Cost cont

Use the cost per kilowatt hour, to determine the cost to use the electrical appliance. Simply multiply cost per kWh by the number of kWh used by the appliance.

In Frederick, the cost is about 9 cents per kilowatt hour.

Sample Problem

How much does it cost to run the digital clock in my room for 24 hours?

The UL tag reads 8w (0.008kW)E = PtE = 0.008kW x 24h E = 0.192kWh

0.192kWh x 9cents / kWh = 1.728 cents

Sample Problem

What does it cost to make a batch of frenchfries in my deep fat fryer if they must cook for 7 minutes?

The UL tag reads: 12.5a, 120V First, I must calculate its power rating.

P = IV P = 12.5a x 120VP = 1500wI = 12.5a

V = 120V

Next, Use the Power to calculate the Electrical Energy used.

E = PtP = 1500W = 1.5kWT = 7min = 0.12h

E = 1.5kW x 0.12h

E = 0.18kWh

Finally, use the electrical energy to calculate the cost.

0.18kWh x 9c / kWh1.62 cents to make a batch of fries.

Magnetism

Magnet a substance that can exert forces on

other magnets.Only iron, nickel and cobalt can become magnetized.

How to magnetize Fe, Ni or Co:

Bang the metal with a hammer.

Rub an existing magnet across the metal, many times, always in the same direction.

Run electric current through the metal.

nickel

iron or steel

cobalt

What happens when a metal is magnetized?

In a sample of un-magnetized metal, electrons spin in all directions.

In a magnetized piece of metal, electrons spin in one domain.

Law of Attraction and Repulsion: oppositely charged poles attract. Similarly charged poles repel.

Magnetic Field the area around a magnet where the magnetic forces act.

Forces are strongest at the poles.

Electromagnet

When an electric current is passed through a coil of wire, it creates a magnetic field.

Electromagnetic Generator When a coil of wire is rotated in a magnetic

field, electric current flows.

Types of Electric Current

ACAlternating Current

Produced with an electromagnetic generator.

Current periodically changes its polarity. (In US, 60 times per second.)

DCDirect Current

Produced with a battery.

Current always flows from negative to positive terminal.

Circuit

A complete path of conductors from the negative terminal of the voltage source back to the positive terminal of the voltage source.

Two main types of circuits:SeriesParallel Whats wrong in this

picture???

Circuits

SeriesOne path. All current flows through all parts of the circuit.All or nothingIf there is a break in the circuit, open circuit, no current will flow.

ParallelMore than one path. Current splits and travels through different paths.SelectiveIf there is a break in one branch of the circuit, current can still flow through other branches.Whats wrong in this picture???

Circuits

SeriesEach resistor added decreases the voltage. (Loads share the current.)RT = R1 + R2 +R3As resistors are added, total resistance increases.

ParallelEach resistor gets full voltage. (Each load draws own current.)1/RT = 1/R1 + 1/R2As resistors are added, total resistance decreases.

Electrical SchematicsSymbols used to represent electrical loads.

1-notes-Electricity- static notes2-notes-Electricty Current3-notes-Electricity- energy cost4-notes-Elec- magnetism notes5-notes-Electricity- circuits