Electricity

Electricity• Electricity is a force created by a difference in

charges (+ & -) due to gained or lost electrons. (an electron is a negatively charged particle.)

• When electricity is flowing between two points, this is actually electrons moving from point A to point B. This is called an electrical current.

Charge It!

• There are 3 ways an object can become charged: Friction, Conduction, Induction.

• Friction-Rubbing two objects together(Balloon to hair)

• Conduction-electrons are transferred by direct contact(touching a charged plastic ruler to an uncharged copper pipe.

• Induction-when charges are in an uncharged object are rearranged without direct contact with a charged object. (balloon attracted to a wall)

• In order for these electrons to flow, however, there must be a difference in charges (# of electrons built up) between the 2 points. Just like heat flow needs a difference in temperatures.

• Electricity always flows from a location with a negative charge to a location with a positive charge. (like charges repel, opposites attract)

- +

• Think of a battery, the top has a + charge and the bottom has a negative charge. So when they are connected, electrons flow from the bottom to the top.

As electrons collect on an object, it becomes negatively charged. As electrons leave an object it attains a positive charges. Charges interact with each other:

Often when you remove clothes from the clothes dryer, they seem to stick together. This is because some of the clothes have gained electrons by rubbing against other clothes. The clothes losing electrons become positive. The negative clothes are attracted to the positive clothes.

Static Electricity

• You may have noticed that if you walk across the carpet in socks, and then touch your cat’s nose, it will shock him/her (it).

• This is because, you picked up some free electrons walking over the carpet (kind of like dust), and therefore gained a negative charge.

• Touching kitty’s nose allows you to dissipate that charge (send the extra electrons to an area with fewer electrons, negative to positive)

• The shock will continue until both surfaces are at the same charge (neutralized)

• Static electricity is a temporary 1 time charge produced by an excess of electrons.

• Electrons in the air will attach to moisture in the air. That’s why very dry air (like in the winter, or in the dryer) may hold a large number of free electrons, because there’s not much moisture for them to attach to.

i.e. There’s more static in the air in the winter because there are more free electrons just floating around

• Van der Graff generators basically induce a strong electrical field (can be either positive or negative depending on

the setup) into a hollow metal sphere. This field can the release electrons into the surrounding air when the voltage becomes great enough.

How are static charges detected?

Voltage

• The difference in charges between 2 points is called the voltage.

• 2 points can both have a tremendous charge, but if the charge is the same then there will be no flow of electrons between them.

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• The number of electrons actually flowing through a conductor is called the current, and is measured in amperes (amps).

1 amp = 6.25 x 1018 electrons per sec.

(6,250,000,000,000,000,000!)

Conductors and Insulators

• Although an electric current can occasionally jump from 1 point to another, it normally travels through a conductor. A conductor is any material which allows electrons (electricity) to flow easily through it.

• Metals in general are good conductors of electricity.

• Insulators are materials which prevent the flow of electrons (electricity).

Some good insulators include rubber, glass, wood, and plastic.

Any type of electric cord usually consists of a metal conducting wire surrounded by a layer of rubber or plastic insulation.

Resistors Materials called resistors can slow the flow of

electricity, without stopping it all together.

Almost all materials, even good conductors, provide at least a small amount of resistance to an electric current.

Resistance is measured in ohms.

What is electrical resistance?

Resistance (R)is the opposition to the flow of an electric charge, causing the electrical energy to be converted to thermal energy or light.

The metal which makes up a light bulb filament or stovetop eye has a high electrical resistance. This causes light and heat to be given off.

The unit for measuring resistance is the ohm (ohm (ΩΩ).).

Lowering Electrical Resistance Using better conductors(copper or aluminum) Increase the thickness of a wire Shorten the length of a wire Decrease the temperature

Electrical Calculations – What is Ohm’s Law?

I = 3 V2 Ω

I = 1.5 amps

Direct Current (DC)

• In most electrical circuits, the current will flow in only 1 direction. This is called direct current or DC.

• Examples of direct current include batteries, lightning, and static electricity.

Lightning

• Lightning is a form of direct current (DC) produced by static electricity in clouds.

• The static is formed when air molecules move past each other (just like clothes in a dryer).

• The negative charges group at the bottom of the cloud and transfer electrons to the ground, which has taken on a positive charge.

Alternating Current (AC)

• An alternating current will send a flow of electrons in 1 direction through a circuit, and then it will reverse the flow in the other direction.

• Household outlets are an example of AC current. They reverse the direction of the current about 120 times per second.

Batteries Electric motors are usually specified to run on

either alternating current or direct current (produced by batteries).

A battery is a device which produces a continual direct current, through a chemical reaction.

Batteries can be composed of a liquid Batteries can be composed of a liquid electrolyteelectrolyte (a liquid which will conduct an electric current) (a liquid which will conduct an electric current) ((wet cellwet cell) or a paste electrolyte () or a paste electrolyte (dry celldry cell). ). Whether liquid or paste, the electrolytes are Whether liquid or paste, the electrolytes are usually acidic.usually acidic.

Car and boat batteries are examples of wet cells, Car and boat batteries are examples of wet cells, flashlight batteries are examples of dry cells.flashlight batteries are examples of dry cells.

• A chemical reaction inside the battery pulls electrons away from 1 of the terminals and gives them to the other. This causes 1 terminal to be negative (-) and the other to be positive (+).

• When these terminals are connected through a device, they produce a flow of electrons from negative to positive, which power the device.

Electric Circuits

• A circuit is a pathway along which an electric current can travel, en route to a device.

• Locating a device along that pathway will allow it to be powered by the electric current.

• There are 2 different types of circuits: series and parallel.

A. Circuit Components

A - battery C – load / light bulb

B - switch

Series Circuit• A series circuit is one

which has only 1 pathway for electricity to flow. All devices are located along this single pathway, and any break in the circuit will stop the flow of electricity.

• Ex.: old type Christmas tree lights

Parallel Circuits• A parallel circuit has

multiple paths which the electricity can travel down, with different devices along the different paths.

• If one pathway is open or broken, the others can still carry electricity.

Ex.: newer Christmas tree lights, household circuits

• A circuit breaker is a device which prevents too much electricity from flowing through a circuit. Breakers can be reset usually with the flip of a switch.

• Fuses do the same thing as a breaker, but they use a small wire connector which will burn out if the flow of electricity is too high. Fuses usually have to be replaced after they burn out. Fuses are often found in electric appliances also.

How is Electrical Power calculated?

Electrical Power is the product of the current (I) and the voltage (v)

The unit for electrical power is the same as that for mechanical power in the previous module – the watt (W)

Example Problem: How much power is used in a circuit which is 110 volts and has a current of 1.36 amps?

P = I V

Power = (1.36 amps) (110 V) = 150 W