# Physical Science Ch. 7: Electricity.

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Physical Science Ch. 7: Electricity

Yes, we all know what electricity is, but exactly what is it?
-where does it come from -can you see it -how is it created

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.

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.

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.

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.

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!)

So let’s say that using a voltmeter you determine that there are no electrons flowing from point A to point B through an extension cord connecting the two. Does that mean it is safe for you to touch either of the 2 points? Hmmm……

No. There may be no electrons flowing because the charges are the same
No! There may be no electrons flowing because the charges are the same. However, your charge may be much different, which would allow the electricity to flow into you.

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. W

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. Why do you think the ground has taken on a positive charge?

Does lightning always travel from cloud to ground? Hmmm……

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.

What would happen if you stuck one finger in one side of an outlet?
One finger in each side?

Transformers The AC which comes from your household outlets usually has a voltage of about 120V. The voltage of the power lines outside, however, is much higher. Before entering your house, the electricity from the power lines must pass through a transformer. This is a device which can increase or decrease the voltage which exists between 2 points.

Transformers outside your house make the voltage in your outlets safe for household appliances.
Occasionally a lightning strike make take out a transformer, and send a power surge into your house.

Step-up & step-down transformers
Transformers can also increase the voltage between 2 points. An amplifier and a stun gun are examples of step-up transformers.

Electric Generators The electricity supplied to your home is produced by an electric generator. This is a device which changes mechanical (kinetic) energy into electricity. Most electric power which is generated in Missouri is either from hydroelectric or fossil fuel power plants.

Electric Motors Electric motors are devices which convert electricity to mechanical energy. Most household appliance are examples of electric motors. Ex.: washing machine, fan, refrigerator, VCR, dishwasher, hair dryer, etc.

Electric cars have engines which run on electrical energy rather than chemical potential energy (gasoline). These cars are more efficient, cleaner to operate, and decrease our dependency on fossil fuels. However, the engines are not as powerful and the cars have a limited range.

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 electrolyte (a liquid which will conduct an electric current) (wet cell) or a paste electrolyte (dry cell). Whether liquid or paste, the electrolytes are usually acidic. Car and boat batteries are examples of wet 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.

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

Schematic Diagrams A schematic diagram uses symbols to show electrical circuits and wiring.

A short circuit occurs when the current stops short of the intended device, usually because of the circuit being completed prior to reaching the device.

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.

Electromagnets An electromagnet is a temporary magnet made by passing an electric current through a wire coiled around a metal core. Electromagnets have 2 advantages over normal magnets: 1. They can be turned on and off. 2. Their strength can vary based on the amount of current flowing.

Examples of electromagnets include cranes in scrap yards, telegraphs, and certain types of doorbells.

History of Electricity
600 B.C.-Thales discovered static electricity. William Gilbert names the force electricity Mid 1700's - Ben Franklin shows that lightning is made of electricity, and does numerous experiments

Volta makes first battery T. Edison creates electric light bulb

First computer (ENIAC) created Fermi produces the first fission reaction

The Van De Graff generator in class produced around 15,000 v of electricity, while a typical household outlet produces only 120 v. Therefore, you’re safer sticking your finger in an outlet. Right…………????? Explain.

An automated external defibrillator (AED), which is similar to the manual defibrillator often seen used by EMT’s, is a portable device used to diagnose and treat victims of cardiac arrest. How do you think this device works?

Luigi Galvani was cooking frog legs in an iron skillet
Luigi Galvani was cooking frog legs in an iron skillet. He noticed that when he picked one up with copper tongs to flip it over, sometimes it would “kick” as if it were still alive. Why do you think this occurred?

Draw a schematic diagram of a circuit with a power source, 1 switch, 2 individual devices along a series circuit, and 3 devices on separate branches of a parallel circuit.

The step-down transformer shown below has 8 coils of wire on the primary side and 4 coils going out on the secondary side. If this transformer is hooked up to a typical household outlet, how many volts will be produced?

Why do you think most gas pump handles are coated in either rubber or plastic?

What purpose do you think the yellow connectors on the electric fence shown below serve?

The device shown below is an electronic body fat tester
The device shown below is an electronic body fat tester. The subject holds the tester by the handles, and a weak electric current is passed through their body. How do you think this is able to determine their percentage of body fat?

Although both dangerous, a lightning strike near the beach in Miami may be more potentially hazardous than the same strike at Truman Lake. Why do you think this is so?

A car battery (wet cell) works similar to the voltaic piles which we built in class.
Do you think it would be possible to build a voltaic pile out of our lab materials big enough to have the same power as a car battery? Why or why not.

What is the object shown, and how do you think it works?

Shown below is a ground fault circuit interrupter (GFCI) outlet
Shown below is a ground fault circuit interrupter (GFCI) outlet. What’s special about it, how does it work, and where might one be found?

In the picture below, what is the charge of the ground? How do you know?

Is the man in the car safe or in danger? Explain.