Chapter 9: Electricity

Given a voltage of 120 V, and a resistance of 8.9 ohms, what is the current?

A. 120 amps B. 1.52 amps C. 22.4 amps D. 4.6 amps E. none of the above

All matter consists of atoms, which are essentially electrical. Each atom has one or more electrons, and one or more protons.

An electron is a negative particle, and a proton is a positive particle of electricity. These tiny particles always have equal-but-opposite charges of electricity. Protons are heavier than electrons, however, and are bound tightly to the centers of atoms. Because of this strong attraction, only electrons can move freely.

This is because it contains several meters in one case and the display is digital. To use the multimeter you must turn on the meter using the on or power switch. Select the voltage, current, or resistance to be measured by the indicated switch

As you are taking a reading, using the multimeter, it is normal to observe that the last one or two digits are changing. Do not expect the digits to stop changing. You should make a reading to represent the average of each digits change.

Given a resistance of 0.5 ohms, and a current of 7.0 amperes, what is the voltage?

Ohm's Law: E= I x R 3.5*7.0=3.5 volts

Electricity and magnetism are closely related

The area around a magnet where its force can be felt is called a magnetic field. Electricity flowing through a wire sets up a magnetic field about the wire. In addition, a magnetic field can produce electricity in a wire. If you move a wire so that it cuts across a magnetic field, electricity will be generated in the wire. All electric generators work on this principle.

Ordinarily, an atom is neutral. This means that it has an equal number of electrons and protons. If an atom gains some electrons, it becomes negatively charged. If an atom loses some electrons, it becomes positively charged

.Atoms that either gain or lose electrons become electrically charged, and are called ions. Charged atoms always attract uncharged atoms. Atoms with like charges repel each other, while atoms with unlike charges attract each other.

If you have a 15 ampere fuse, (which will burn out at exactly 15 amps), on a 120 volt line, what is the maximum number of 100 watt light bulbs can you light before the fuse blows?

17

How much power is being used if the voltage is 12.1V and the current is 7.3 A?

88.3 watts 12.1 V *7.3 A=88.3 Power: P = e x I

The Use of Meters Meters are expensive and easily damaged. It is wise to know something about their construction and the proper methods of use before employing them in experimental work. All D. C. meters must be connected with the proper polarity. The negative markings of the power source are connected with the negative marking of the meter, and the positive source to the positive terminal of the meter

By convention, sources of D. C. current use the color red to designate the positive terminal and black to designate the negative terminal. If the precautions indicated are observed, the laboratory equipment will provide you with accurate data, and no damage will result to the instruments.

Direct

Direct current flows in only one direction.

An electric circuit requires a minimum of three components: 1. a pathway or conductor, such as copper wire which electrons can flow. 2. A source of electrons, such as a battery of a generator. 3. An object for the electrons to act on, such as a toaster or a television set. The circuit leads electrons in a continuous path, from a driving force to a positive charge. The flow will continue as long as the driving force acts

Electric current can flow in solutions containing ions. Ions provide the electrons in liquids and gases which conduct electricity. Positive ions flow in one direction, and negative ions flow in the opposite direction. This explains how electrical signals move through the human body.

All the electrical phenomena we are familiar with depend on the forces existing between the electrical charges carried by electrons and protons, two of the principal constituents of all atoms.

Protons contain the positive charges of atomic nuclei, and electrons are the source of all negative charges. The attractive force between positive and negative charges holds atomic nuclei and electrons together to form atoms. Under many circumstances, one or more electrons can disassociate from atoms, and flow through metals or other matter to produce electric currents. Certain actions of electrons and electric charges give us electric current and electric power. To make current, a Force is needed to drive electrons away from some part of matter. The force must keep pressing the electrons so that they cannot flow directly back. This leaves a positive charge at the place from which the electrons were torn loose.

The current which flows in one direction and then the other is called:

alternating

How is an ammeter connected in a circuit?

always connected in series

ammeter

always connected in series in a circuit. This must be done so that the instrument carries all the current. Most of the current is shunted around the armature, but if the ammeter is overloaded, the armature will carry more than it is designed to carry, resulting in damage to it. Before a two-post meter is connected in a current- bearing circuit, be sure the magnitude of the current is not beyond the range of the instrument

If the length of a resistor wire were increased, one would expect the resistance to:

increase

Which of the following explains why a balloon rubbed with wool will stick to a wall?

unlike charges attract

There are two kinds of electricity:

(1) static electricity, and (2) current electricity Essentially, however, both are really the same. Static electricity consists of electrons or ions that are not moving. Current electricity consists of ions in motion, which produces important magnetic affects. To understand electricity, you must also know something about the relationship of electricity to magnetism.

Static electricity etc notes

A substance will not always have the same charge when it is rubbed. For example, if you rub a piece of glass with silk, the silk becomes negatively charged, and the glass positively- charged. But if you rub a piece of rubber with silk, the silk becomes positively charged, and the rubber becomes negatively-charged. The electrostatic series indicates whether a substance will become positively or negatively charged when rubbed with another substance.

Electrons are among the smallest known particles

For example, in a 60-watt light bulb about 3,000,000,000,000,000,000 or 3 x 1018 (three billion billion) electrons flow through the wires, to the bulb, each second. These electrons travel from atom to atom in the wires at a speed of approximately .01 cm per second every second. But electrical energy, or the ability of electricity to do work, travels through a wire almost as fast as the speed of light, which is 3 x 108 meters per second. When you speak into a telephone, the person to whom you are talking hears your voice almost instantly. Electric current in a wire works much like a 100-foot long hose filled with water. If you connect the hose to a water source and turn it on, you will have sent a signal 100 feet, although any drop of water in the hose will have moved only a few inches. The flow of electric current and water both depend on three factors: (1) volts,, the pressure which causes the current to flow (2) amperes, the rate of the current flow, and (3) ohms, the resistance of the conductor (wire or hose) to the flow. The flow also produces power (work done per unit of time measured as watts.)

Electrons travel through any conductor from a negative source to a positive source of electricity. Benjamin Franklin wrongly surmised that electricity travels from positive to negative. This belief was accepted for many years, and electricians and electrical engineers still say that electricity flows from positive to negative. Scientific tests now prove that electrons flow the other way, from negative to positive in electric circuits.

In order to conduct electricity, the atoms of a substance must have electrons that are free to move from atom to atom. In a conductor, the electrons in the outer shell of the atoms are able to move freely. The number of electrons in the outermost energy level determines how easy these electrons are moved. The greater the number of electrons the harder it is to move them. In fact, materials can be classified according to the number of electrons. These are classed as insulators, semiconductors, and conductors. Insulators have from 5 to 8 electrons. Materials with 8 electrons are very good insulators or dielectrics. In some materials such as rubber and glass, the electrons are bound so tightly to their atoms that few can move. These materials conduct little electricity. Semiconductors have 4 electrons. Materials such as germanium and silicon have 4 electrons. Conductors have 1 to 3 electrons in their outermost energy level. Materials such as gold, copper, etc. are in this group.

Electricity

a form of energy that occurs everywhere in nature, in space, in the atmosphere, in living creatures, in chemical bonds that hold atoms together, in molecules, and in the atoms themselves. Lightning bolts are an example of a large-scale display of an electric affect. On a much smaller scale, weak electrical impulses are transmitted from one nerve cell to the next in animals, providing signals for the brain and other parts of the organism.

Static electricity is produced in three ways:

a. friction • Objects can be charged by rubbing them together. As the two objects touch, one gives up loosely held outermost electrons to the other. Thus, the material that receives electrons becomes negatively charged and the material that loses electrons becomes positively charged. When you walk across a room on a rug on a cool dry day, you may gain a static electric charge by friction. The rubber soled shoes rub against the wool carpet and gain electrons from the carpet. When you reach for the metal door knob, these excess electrons are discharged to the knob, causing a shock. b. contact • A material can be charged if electrons are transferred from one material to another by contact This generally happens when one material is charged and it touches either a neutral or oppositely charged material. If you have ever seen a person put his/her hand on a Van de Graff generator, you have observed charging by contact. c. induction • A material may be charged by induction when it is brought near a charged body. If the material is a conductor, it has electrons that are relatively free to move.

How is a voltmeter connected in a circuit?

connected in parallel

Alternating

current rapidly reverses its direction of flow many times a second.

If the diameter of a resistor wire were increased, one would expect the resistance to:

decrease

Fuse

device that protects a circuit from being overloaded is called A device used for protecting electrical apparatus against excess current. It consists of a piece of metal (which is connected in the circuit to be protected) which melts and interrupts the circuit when an excess of current flows through it.

The meter that you will use is called a

digital multimeter

Current Electricity There are two main kinds of electric currents:

direct and alternating.

Which of the following explains what happens if a light in a parallel circuit is removed (and nothing else is changed)?

the rest of the lights are not changed

Which of the following explains what happens if a light in a series circuit is removed (and nothing else is changed)?

the rest of the lights go out

Most common meters are of the__________type

galvanometer In this type of meter, a small current is allowed to flow through an armature composed of fine wire wrapped around an iron core. With the current flowing, the armature becomes an electromagnet, which orients itself in an external magnetic field. This orientation results in the movement of the indicator. The armature of such instruments is wound with very fine wire, and the heating effect of any excessive current will damage them. If a galvanometer is called for, and a multipost galvanometer is available, first connect the instrument to the "high range" posts, and successively introduce the most sensitive ranges, until the one of optimum value is found. The galvanometer is to be connected in series in the circuit.

voltmeter

is always connected in parallel. Resistors in the instrument prevent the armature from being overloaded when used properly. However, damage to the instrument may result if it is forced to carry too much current. If a single-range meter is used, be sure the voltage to be measured is not beyond the range of the instrument

Static electricity

occurs when there is an excess of free electrons or atoms that are missing electrons. In a poor conductor, the free electrons are unable to move away from its atom, nor to an atom without electrons, so the charges remain in the material. That is, electrons can be grouped in one place and the atoms without electrons in another place. The charge produced must be equal and opposite Static electricity makes your hair crackle when combed, or it may give you a shock if you touch a person or a metal doorknob after walking across a carpet on a cold day. Static electricity has fewer uses than current electricity. You can produce static electricity by rubbing a comb with a piece of wool. The atoms in the wool lose some of their electrons, and the atoms in the comb gain them, so the comb becomes negatively-charged, and the wool becomes positively-charged.

Like static charges ______ each other, and unlike charges _______ each other

repel, attract