ELECTRICITY RAD 123 Radiologic Science

Electrostatics ELECTRIFICATION - Electron charges being added to or subtracted from an object.

Electrostatics When an object has more electrons than another, it has negative charge. When an object has fewer electrons than another, it has positive charge.

Ground GROUND – The earth has 0 potential or is neutral.

Laws Of Electrostatics Like charges repel; unlike charges attract. The force between 2 charges is inversely proportional to the square of the distance between them and directly proportional to the product of their magnitudes. (Coulomb’s Law)

Laws Of Electrostatics Charges reside on the external surface of conductors. Concentration of charge is greater where the curvature is sharpest. Only negative charges move along conductors.

Electrification Friction – When an object is rubbed against another, electrons travel from one object to the other, producing a charge. (Ex. Walking across the floor)

Electrification Contact – When two objects touch, electrons move from one to the other. Both objects have the same charge after the contact. Induction - Induction is the process of using the electrical field of the charged object to confer a charge on an uncharged object.

Electrodynamics Electric current Movement of electrons Conditions permitting flow of electrons: Vacuum – air has been removed, so flow of electrons is not opposed. Electrons may jump a gap between two oppositely charged electrodes in a vacuum.

Conditions permitting flow of electrons Gasses- Two oppositely charged electrodes placed in a gas will cause positive ions to drift toward the negative electrode and negative ions to drift toward the positive electrode. Ex. Neon light tube)

Conditions permitting flow of electrons dynamics Ionic solutions – Upon being dissolved in water a salt separates into its component ions. If electrodes are then immersed in the solution and connected to a source of direct current, the positive ions move toward the cathode while the negative ions move toward the anode. NaClSucrose

Electrodynamics Metallic conductor – An electric current in a solid conductor consists of a flow of electrons only.

Sources Of Electric Current Batteries convert chemical to electrical energy Dynamo or generator converts mechanical to electrical energy Other sources solar (sunlight), atomic (nuclear), wind, geothermal

Electrical States Of Matter Insulator (glass, rubber, plastic) Does not permit electron flow; large difference between conduction and valence bands.

Electrical States Of Matter Semiconductor (silicon, germanium) can be conductive or resistive; basis for computer technology; small energy difference between conduction and valence bands

Electrical States Of Matter Conductor (copper, aluminum) Variable resistance; overlapping conduction and valence bands; electrical potential (voltage) required; conducts with minor resistance; varying with temperature and other conditions

Electrical States Of Matter Superconductor (titanium) Greatly overlapping conduction and valence bands; no resistance to electron flow; conducts with little or no electrical potential; must be very cold.

Electric Circuits Source of current Wire Resistance.

Electric Circuits Electrical circuit is pathway for electrons to move in a closed path along the outer surface of a wire. The wire may be modified by changing its cross sectional area or inserting different material or directions (circuit elements) to change resistance to the electron flow.

Electric Circuits Current flow – electron flow is from negative to positive; conventional electric current is said to flow from positive to negative.

Direct current Electrons flow in only one direction (DC)

Alternating current Electrons flow first in one direction, then in the opposite or alternate direction (AC)

Current Quantity or number of electrons flowing. Measured in amperes (A). One ampere is one coulomb of electrical charge flowing per second. (6.24 X 1018 electrons per second) Milliampere (mA) – 1/1000 Ampere

Potential Difference Force with which electrons travel Function of the excess of electrons at one end of the circuit and the deficiency of electrons at the other end. Also called electromotive force (emf) The total maximum difference of potential between the positive and negative ends of the electron source. Unit is the Volt (V) 1 joule of work done on one coulomb of charge. Voltage, potential difference, electromotive force are the same.

Resistance Opposition to the flow of current, measured by the Ohm (  ).

Factors of resistance in circuit Material used Different materials have different resistances Length of the conductor Longer the conductor, the greater the resistance Cross sectional diameter Greater the diameter, the less resistance Temperature The higher the temperature, the greater the resistance in a conductor (In an insulator, the higher the temperature, the less the insulating ability.)

Ohm’s Law The voltage across the total circuit or any portion of the circuit is equal to the current times the resistance, or stated mathematically: V = IR, R = V/I, I=V/R V = potential difference in volts I = current in amperage R = resistance in ohms

Ohm’s Law If a current of 0.5 A flows through a conductor that has a resistance of 6 , what is the voltage across the conductor? 3 V A kitchen toaster draws a current of 2.5 A. If the household voltage is 110 V, what is the electric resistance of the toaster? 44 

Power The total amount of energy available in a circuit. Determined by the current and potential difference. Electrical power or 1 watt is equal to 1 amp of current flowing through an electrical potential of 1 volt.

Power P= IV P = power in watts I = current in amperage V = potential difference in volts

Power Loss Measured in Watts, is the heat that is produced during the operation of electrical equipment. The power loss formula is derived from the power formula. The power formula, P = IV, substituting IR for V (Ohm’s Law states V = IR) Resulting in P = I x IR or P = I 2 R (watts per second)

Series Circuits In a series circuit, all circuit elements are connected in a line along the same conductor.

Rules for series circuits The total resistance is equal to the sum of the individual resistances. R T = R 1 +R 2 +R 3

Rules for series circuits The current through each circuit element is the same and is equal to the total circuit current. I T =I 1 =I 2 =I 3

Rules for series circuits The sum of the voltage across each circuit element is equal to the total circuit voltage. V T =V 1 =V 2 +V 3

Rules for series circuits A series circuit requires that all resistances to be operable, otherwise there is no pathway for the current to take. (Christmas lights that have only one wire connecting each light – when one goes out, they all go out.)

Parallel Circuits A parallel circuit contains elements that bridge conductors rather than lie in a line along a conductor.

Rules for parallel circuits The sum of the currents through each circuit element is equal to the total circuit current. I T =I 1 +I 2 +I 3

Rules for parallel circuits The voltage across each circuit element is the same and is equal to the total circuit voltage. V T =V 1 =V 2 =V 3

Rules for parallel circuits The total resistance is inversely proportional to the sum of the reciprocals of each individual resistance. 1/R T =1/R 1 +1/R 2 +1/R 3

Rules for parallel circuits As more resistances are added to a parallel circuit, total resistance decreases.

Rules for parallel circuits Parallel circuits offer less total resistance to electrical current when all other factors are the same. When one resistance is interrupted, the circuit is not broken. (Christmas lights that have two wires connecting each light – when one goes out, the rest remain lit.)

Circuit devices Fuse – has metal tab that will melt when heated, thus breaking the circuit. Circuit breaker – pops open when the current in the circuit becomes too great; can be reset

Circuit devices Rheostat- variable resistor Capacitor –stores electrons (current)