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.

NaCl Sucrose

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/RV = potential difference in volts I = current in amperageR = 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 wattsI = current in amperageV = 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 = I2R (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.

RT = R1+R2+R3

Rules for series circuits

The current through each circuit element is the same and is equal to the total circuit current.

IT=I1=I2=I3

Rules for series circuits

The sum of the voltage across each circuit element is equal to the total circuit voltage.

VT=V1=V2+V3

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.

IT=I1+I2+I3

Rules for parallel circuits

The voltage across each circuit element is the same and is equal to the total circuit voltage.

VT=V1=V2=V3

Rules for parallel circuits

The total resistance is inversely proportional to the sum of the reciprocals of each individual resistance.

1/RT=1/R1+1/R2+1/R3

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)