PowerPoint Lectures to accompany Physical Science, 6e Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 6 Electricity

and magnetism

Electric charge Electron theory of charge –Ancient mystery: “Amber effect” –J. J. Thompson: identified negatively charged electrons Today: –Basic unit of matter = atom –Atoms made up of electrons and nuclei containing positively charged protons and neutral neutrons (See Ch. 8)

Electric charge and electrical forces Charges in matter –Inseparable property of certain particles –Electrons: negative electric charge –Protons: positive electric charge Charge interaction –Electric force –“Like charges repel; unlike charges attract” Ions: non-zero net charge from loss/gain of electrons

Electrostatic charge Stationary charge confined to an object Charging mechanisms –Friction –Contact with a charged object –Polarization (reorientation induced without changing net charge)

Electrical conductors and insulators Electrical conductors –Electrons are free to move throughout material –Added charge dissipates –Examples: metals, graphite (carbon) Electrical insulators –Electron motions restricted –Added charge tends to remain on object –Examples: Glass, wood, diamond (carbon) Semiconductors –Conduct/insulate depending on circumstances –Applications: Computer chips, solar cells,...

Measuring electric charge Unit of charge = coulomb (C) –Equivalent to charge of 6.24x10 18 electrons! –Fundamental metric unit (along with m, kg and s) Electron charge –Fundamental charge –Smallest seen in nature –Quantity of charge and the number of electrons

Measuring electric forces Coulomb’s law Relationship giving force between two charges Similar to Newton’s law of gravitation k versus G implies gravity weaker

Force Fields. –The condition of space around an object is changed by the presence of an electrical charge. –The electrical charge produces a force field, that is called an electrical field since it is produced by electrical charge –All electrical charges are surrounded by an electrical field just like all masses are surrounded by gravitational fields.

–A map of the electrical field can be made by bringing a positive test charge into an electrical field. When brought near a negative charge the test charge is attracted to the unlike charge and when brought near a positive charge the test charge is repelled. You can draw vector arrows to indicate the direction of the electrical field This is represented by drawing lines of force or electrical field lines –These lines are closer together when the field is stronger and farther apart when it is weaker.

Force fields How do forces act through space? Charges surrounded by electric fields (Vector fields/directional) Fields and charges inseparable Fields act on other charges –Direction of fields = motion of positive test charge in the field –Visualized with lines of force Same ideas apply to gravity and magnetism

Electrical Potential. –An electrical charge has an electrical field that surrounds it. –In order to move a second charge through this field work must be done –Bringing a like charge particle into this field will require work since like charges repel each other and bringing an opposite charged particle into the field will require work to keep the charges separated. In both of these cases the electrical potential is changed.

Electric potential Scalar field associated with potential energy Units = volts (V) Related to work involved in positioning charges Potential difference important in producing forces and moving charges Analogous to moving masses in gravitational fields

–The potential difference (PD) that is created by doing 1.00 joule of work in moving 1.00 coulomb of charge is defined as 1.00 volt A volt is a measure of the potential difference between two points electric potential =work to create. potential charge moved PD=W Q The voltage of an electrical charge is the energy transfer per coulomb. –The energy transfer can be measured by the work that is done to move the charge or by the work that the charge can do because of the position of the field.

Electric current Earlier - electrostatics Charges more or less fixed in place Now - charge allowed to move Electric current –Flow of charge, not flow of current –Reason for charge flow - potential (voltage) differences Electric circuits –Structures designed to localize and utilize currents

Electric circuits Structure Voltage source –Energy input –Necessary for continuing flow Circuit elements –Energy used up as heat, light, work, … Current flow convention: from high potential to low potential through the external circuit Water/pump analogy

–Voltage is a measure of the potential difference between two places in a circuit. Voltage is measured in joules/coloumb. –The rate at which an electrical current (I) flows is the quantity (q) that moves through a cross section of a conductor in a give unit of time (t) I=q/t the units of current are coulombs/second. A coulomb/second is an ampere (amp) –In an electrical circuit the rate of current is directly proportional to the difference in electrical potential between two parts of the circuit I  PD.

A simple electric circuit carrying a current of 1.00 coulomb per second through a cross section of a conductor has a current of 1.00 amp.

The nature of current Historically - nature of “electrical fluid” unknown Current thought to be a flow of positive charge Reality - more complicated, depending on material Opposite correct in metals, current = electron flow

Current mechanisms Liquids and gases Both positive and negative charges move, in opposite directions Metals Delocalized electrons free to move throughout metal “Electron gas” Drift velocity of electrons slow slow Electric field moves through at nearly light speed

Electrons move very slowly in a direct current circuit. With a drift velocity of 0.01 cm/s, more than 5 hr would be required for an electron to travel 200 cm from a car battery to the brake light. It is the electric field, not the electrons, that moves at near the speed of light in an electric circuit.

–The current that occurs when there is a voltage depends on: The number of free electrons per unit volume of the conducting material. The fundamental charge on each electron. The drift velocity which depends on the electronic structure of the conducting material and the temperature. The cross-sectional area of the conducting wire.

–It is the electron field, and not the electrons, which does the work. It is the electric field that accelerates electrons that are already in the conducting material. –It is important to understand that: An electric potential difference establishes, at nearly the speed of light, an electric field throughout a circuit. The field causes a net motion that constitutes a flow of charge. The average velocity of the electrons moving as a current is very slow, even thought he electric field that moves them travels with a speed close to the speed of light.

More current details Current = charge per unit time Units = ampere, amps (A) Direct current (DC) –Charges move in one direction –Electronic devices, batteries, solar cells Alternating current (AC) –Charge motion oscillatory –No net current flow

What is the nature of the electric current carried by these conducting lines? It is an electric field that moves at near the speed of light. The field causes a net motion of electrons that constitutes a flow of charge, an alternating current. As opposed to DC.

Electrical resistance Loss of electron current energy Two sources –Collisions with other electrons in current –Collisions with other charges in material Ohm’s law

–The relationship between voltage, current, an resistance is V=IR Ohms Law –The magnitude of the electrical resistance of a conductor depends on four variables. The length of the conductor. The cross-sectional area of the conductor. The material the conductor is made of. The temperature of the conductor.

More on resistance Resistance factors –Type of material –Length –Cross sectional area –Temperature Superconductors –Negligible resistance at very low temperatures

Electrical power and work Three circuit elements contribute to work Voltage source Electrical device Conducting wires –Maintain potential difference across device –Input wire at higher potential than output wire –Output wire = "ground" for AC circuits –No potential difference, no current (bird on a wire) Power in circuits Electric bills

–The work done by a voltage source is equal to the work done by the electrical field in an electrical device. W=Vq The electrical potential is measured in joules/coulomb and a quantity of charge is measured in coulombs, so the electrical work is measure in joules. A joule/second is a unit of power called the watt. power = current (in amps) X potential (in volts) –P=IV

This meter measures the amount of electric work done in the circuits, usually over a time period of a month. The work is measured in kWhr

Magnetism Earliest ideas Associated with naturally occurring magnetic materials (lodestone, magnetite) Characterized by “poles” - “north seeking” and “south seeking” Other magnetic materials - iron, cobalt, nickel (ferromagnetic) Modern view Associated with magnetic fields Field lines go from north to south poles

Magnetic poles and fields Magnetic fields and poles inseparableinseparable Poles always come in north/south pairs Field lines go from north pole to south pole Like magnetic poles repel; unlike poles attract

Every magnet has ends, or poles, about which the magnetic properties seem to be concentrated. As this photo shows, more iron filings are attracted to the poles, revealing their location.

Sources of magnetic fields Microscopic fields –Intrinsic spins of subatomic particles (electrons, protons, …) –Orbital motions of electrons in atoms Macroscopic fields –Permanent magnets –Earth’s magnetic field –Electric currents –Electromagnets

Permanent magnets Ferromagnetic materials Atomic magnetic moment –Electron/proton intrinsic moments –Electron orbital motion Clusters of atomic moments align in domains Not magnetized - domains randomly oriented Magnetized - domains aligned

Earth’s magnetic field Originates deep beneath the surface from currents in molten core Magnetic “north” pole = south pole of Earth’s magnetic field Magnetic declination = offset Direction of field periodically reverses –Deposits of magnetized material –Last reversal - 780,000 yrs. ago

The Source of Magnetic Fields. Since electrons are charges in motion, they produce magnetic fields as well as an electric field. magnetism is a secondary property of electricity the strength of the magnetic field increases with the velocity of the moving charge. The magnetic field does not exist if the charge is not moving A magnetic field is a property of the space around a moving charge.

–Earth's Magnetic Field. The Earth’s magnetic field is thought to originate with moving charges. The core is probably composed of iron and nickel, which flows as the Earth rotates, creating electrical currents that result in the Earth’s magnetic field. How the electric currents are generated is not yet understood There seems to be a relationship between rate of rotation and strength of planet’s magnetic field.

Electric currents and magnetism Moving charges (currents) produce magnetic fields Shape of field determined by geometry of current –Straight wire –Current loops –Solenoid

Electromagnets Structure –Ferromagnetic core –Current carrying wire wrapped around core Field enhanced by the combination Can be turned on/off Widely used electromagnetic device

Electric meters Instrument for measuring current (ammeter) Uses magnetic field produced by the current Magnetic field and, hence, deflection proportional to current Modified, can measure –Potential (voltmeter) –Resistance (ohmmeter)

Electromagnetic switches Relays Use low voltage control currents to switch larger, high voltage currents on/off Mercury switch/thermostat Solenoid switches Moveable spring-loaded iron core responds to solenoid field Water valves, auto starters, VCR switches, activation of bells and buzzers

Telephones and loudspeakers Coupling acoustic waves to electric currents Telephone Sound vibrates carbon granules changing resistance Changing resistance varies current Speaker Varying current changes field of electromagnet, moving permanent magnet Moving magnet vibrates spring attached to paper cone producing sound

Electric motors Convert electrical energy to mechanical energy Two working parts –Field magnet - stationary –Armature - moveable electromagnet Armature rotates by interactions with field magnet –Commutator and brushes reverse current to maintain rotation

Electromagnetic induction Causes: Relative motion between magnetic fields and conductors Changing magnetic fields near conductors Effect: Induced voltages and currents Induced voltage depends on Number of loops Strength of magnetic field Rate of magnetic field change

Generators Structure Axle with main wire loops in a magnetic field Axle turned mechanically to produce electrical energy AC generator “Alternating current” Sign of current and voltage alternate DC generator “Direct current” Sign of current and voltage constant

Transformers. –A transformer has two basic parts. A primary coil, which is connected to a source of alternating current A secondary coil, which is close by. –A growing and collapsing magnetic field in the primary coil induces a voltage in the secondary coil.

–A step up or step down transformer steps up or steps down the voltage of an alternating current according to the ratio of wire loops in the primary and secondary coils. The power input on the primary coil equals the power output on the secondary coil. Energy losses in transmission are reduced by stepping up the voltage.

Transformers Problems in power transmission –High currents - large resistive losses –High voltages - dangerous potential differences Solution: transformers boost/lower AC currents and voltages Basic relationships –Power in = power out –Number of coils to voltage

Energy losses in transmission are reduced by increasing the voltage, so the voltage of generated power is stepped up at the power plant. (A) These transformers, for example, might step up the voltage from tens to hundreds of thousands of volts. After a step-down transformer reduces the voltage at a substation, still another transformer (B) reduces the voltage to 120 for transmission to three or four houses