Physics Chapter 5

Function of an xray

To convert electric energy into electromagnetic energy

Electrostatics

The study of stationary electric charges

An object is said to be electrified if...

It has too few or too many electrons

Electrification can be created by what 3 things?

Contact, friction, or induction

Fundamental unit of electric charge

Coulomb (C)

Unlike charges ______; like charges _______

Attract, repel

Electrostatic force

The force of attraction between unlike charges or repulsion between like charges is attributable to the electric field.

4 general laws of electrostatics

1. Unlike charges attract ; like charges repel

2. Coulomb's law of electrostatic force

3. Electrostatic force follows the inverse square law

4. Electric charges are uniformly distributed except at regions of high curvature where they are concentrated

Electric charges have ______ energy

Potential

Unit of electric potential

The volt (V)

Higher voltage =

Higher potential energy

Voltage in homes and offices

Voltage in xray

110 V

220 V

Electrodynamics

The study of electric charges in motion

Conductor

Any substance through which electrons easily flow

Insulator

Material that does not allow electron flow

Semiconductor

Material under some conditions behave as an insulator and in other conditions as a conductor

Principle semiconductor materials include

Silicon and germanium

Superconductor

Some materials exhibit no resistance below a critical temperature

What measures electric current

Ammeter

Resistance is measured in

Ohms (Ω)

Ohm's Law

- V=IR

- the voltage across the total circuit or any portion of the circuit is equal to the current times the resistance

- V= volts , I= amps, R= ohms

Resistor (symbol and function)

inhibits flow of electrons

Battery (symbol and function)

Provides electric potential

Capacitor (symbol and function)

Momentarily stores electric charge

Transformer (symbol and function)

Increases or decreased voltage by a fixed amount (AC only)

Direct current

- DC

- electrons are made to flow in one direction along the conductor

- horizontal line on graph

Diode (symbol and function)

Allows electrons to flow in only one direction

Alternating current

- AC

- electrons be controlled so they are first going one direction and then in the other direction

- sine wave on graph

AC is identified as a __ Hz current

60

The oscillation in electron direction requiring _____ of a second

1/60

Electric power is measured in

Watts (W)

Toasters/ blenders/ mixers = _______ W

Light bulbs = _______ W

Xray imaging system= _______ kW

- 500-1500

- 30-150

- 20-150

Step up transformer

A transformer with a turns ratio greater than 1.

Voltage is increased from the primary side to the secondary side

Step down transformer

When the turns ratio is less than 1

Transformer Law

Vs/Vp = Ns/Np

Vs= secondary circuit

Vp= primary circuit

Ns= number of turns in secondary coil

Np= number of turns in primary coil

Transformer types

1. Closed-core transformer (built around a square core of ferromagnetic material)

2. Autotransformer (iron core with only one winding of wire)

3. Shell type transformer (essentially 2 closed cores, more currently used, and more efficient)

What are transformers used for?

Used to change the magnitude of voltage and current in an AC circuit

Electric power equation

P= IV

P= watts , I= amps , V= volts

Principle types of magnets

1. Naturally occurring (the earth itself)

2. Artificially induced permanent magnets (compass)

3. Electromagnet (wire wrapped around an iron core)

4 interactions with an external magnetic field

1. Non magnetic (unaffected when brought into magnetic field)

2. Diamagnetic (weakly repelled and not attracted)

3. Ferromagnetic (strongly attracted to magnetic field and can be made into magnets by induction)

4. Paramagnetic (slightly attracted and loosely influenced by magnetic field)

Magnetism- Dipolar

Always had a north and south pole

Magnetic dipole

Small magnet created by the electron orbit. In a bar magnet, can be thought of generating imaginary lines of the magnetic field

Magnetic field lines form what

Circles around each loop of the wire

Solenoid

Coil of wire

Magnetic field of a solenoid is concentrated through what

The center of coil

Michael Faraday

He discovered when a magnet is moved from coil, the wire does have current. No motion equals no current.

He used an ammeter to test his theory.

Faraday's Law

Magnitude of induced electrical current depends on four factors:

1. Strength of the magnetic field

2. Velocity of magnetic field as it moves past the conductor

3. Angle of the conductor to magnetic field

4. The number of turns in the conductor

What does an electric generator do?

Convert mechanical energy into electrical energy. It is not 100% efficient.

What type of motor is used with xray tubes?

Induction motor

Describe an induction motor

Its rotating rotor is a shaft made of bars of copper and soft iron into one mass, however, the external magnetic field is supplied by several dixed electromagnets called stators.

The induction motor powers what?

The rotating anode of an xray tube