Physical Science Chp 22: 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
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 10 18 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. 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
Ohm's law magic triangle
Voltage measured in volts, symbolized by the letters "E" or "V". Current measured in amps, symbolized by the letter "I". Resistance measured in ohms, symbolized by the letter "R".
a A
The force or pressure behind electricity
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?
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.
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
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.
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?
Most patients, when they become pulseless go through a brief period in which the heart goes into a chaotic quivering due to erratic electrical activity. This is called ventricular fibrillation (VF). VF eventually deteriorates into a total absence of electrical activity, often around ten to fifteen minutes after arrest. The best chance to regain a pulse is when a patient in recent VF is shocked quickly - ideally in less than 4-6 minutes after arrest. An AED delivers electrical current through heart muscle, temporarily ceasing all electrical activity in the heart, hoping that when that electrical impulse returns, it will return in an organized pumping action instead of VF.
Why do you think most gas pump handles are coated in either rubber or plastic?
Electric Power The rate at which electrical energy is converted into other forms of energy Electric Power Equation P=IV Power (watts)= current (amperes) X voltage (volts)