Ch. 4 Electricity, Magnetism, and electromagnetism Flashcards
What is the primary function of the xray machine?
to convert electric energy into electromagnetic energy.
How is electric energy supplied to the xray machine?
In the form of well controlled electric current.
Where does the conversion of electric energy to heat and xrays take place?
in the xray tube
Electric charge comes in units that are…
positive or negative
What are the smallest units of electric charge?
protons and electrons
What is the charge of an electron?
one unit of negative charge
What is the charge of a proton?
one unit of positive charge
T or F? electrons are often free to travel from the outermost shell of one atom to another atom?
T
T or F? protons are fixed inside the nucleus of an atom and are not free to move?
T
Electrostatics is the study of…
electric charges
Why do we get shocked when we rub our feet across a carpet floor and then touch a doorknob?
Because electrons are rubbed off the carpet and onto your shoes, causing you to become electrified.
When is an object considered to be electrified?
When it has too many or too few electrons.
Matter has ____ and ___ ____. It may also have ____ ____.
mass, energy; electric charge
How can electrification be created? (3 ways)
contact, friction or induction
Where could some electrons be bound loosely, to be removed easily and cause electricity?
in the outer shell of an atom
What is an example of an electric ground and why?
the Earth is an example because it behaves like a huge reservoir for stray electric charges.
What is induction?
The process of making ferromagnetic material magnetic.
What is the smallest unit of electric charge?
the electron
What is the smallest fundamental unit of electric charge?
the coulomb (C) 1 C = 6.3 x 10^18
One ampere is the flow of…
one coulomb per second
Unlike charge ___ and like charges ___
attract; repel
Associated with each electric charge is…
an electric field
Do uncharged particles have a charge?
no
Describe electrostatic force
When the force of attraction between unlike charges or repulsion between like charges is attributable to the electric field
What is the following law? The electrostatic force is directly proportional to the product of the electrostatic charges and inversely proportional to the square of the distance between them.
Coulomb’s law
Electric charge distribution is uniform….
throughout or on the surface
Electric charge of a conductor is concentrated where?
along the sharpest curvature of the surface
What kind of energy do electric charges have?
potential energy
What is the unit of electric potential?
Volt (V)
The higher the ___, the greater the ___ to do work
voltage; potential
What is electric potential in homes and offices?
110 V
What do xray machines require for electric potential?
220 V or higher
1 V = ?
1 J/C
What is it called when electric potential (V) is applied to an object and then electrons move along the object?
electric current or electricity
What is the study of electric charges in motion?
electrodynamics
the direction of electric current is always ____ that of electric flow.
opposite
What is a conductor?
any substance through which electrons flow easily
What is an insulator?
any material that does not allow electron flow
What led to the development of microchips and computer technology?
The discovery of the semiconductor materials like silicon (Si) and germanium (Ge)
What is a semiconductor?
a material that under some conditions behaves as an insulator and in other conditions behaves as a conductor.
At room temperature all materials…
resist the flow of electricity
When does the resistance of electric flow decrease?
when the temperature of material reduces
Define duperconductivity
the property of some materials to exhibit no resistance below a critical temperature
What is a superconducting material?
niobium and titanium
Superconductors do not hold true to what law?
Ohm’s law
A superconducting circuit can be view as..
one in perpetual motion because electric current exists without voltage.
Why is energy needed for a superconductor if an electric current can exist without voltage?
Because for the material to behave as a superconductor it must be made very cold which requires energy.
What happens to a conducting wire if the diameter is reduced or if different material is inserted?
The electric resistance is increased
What constitutes an electric circuit?
When resistance is controlled and the conductor is made into a closed path
Increasing electric resistance results in a…
reduced electric current
What is electric current measure in?
amperes (A)
The ampere is proportional to the number of…
electrons flowing in the electric circuit
One ampere is equal to an electric charge of…
1 C flowing through a conductor each second
How is electric potential measured?
in volts
How is electric resistance measured?
in ohms
Electrons at high ____ have high ____ ____ and high ____ to ___.
voltage; potential energy; capacity; work
If electron flow is ____ the circuit resistance is high
inhibited
Ohm’s law describes..
the manner in which electric currents behave in an electric circuit
Formal definition of Ohm’s law:
The voltage across the total circuit or any portion of the circuit is equal to the current times the resistance
Ohm’s law formula:
V=IR (v =volts; I=electric current in amperes; R=resistance in ohms)
electric circuits can be reduced to one of 2 basic types:
a series circuit or a parallel circuit
a series circuit means…
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
- the current through each circuit element is the same and is equal to the total circuit current.
- the sum of the voltages across each circuit element is equal to the total circuit voltage
a parallel circuit means…
it contains elements that are connected at their ends rather than lying in a line along a conductor
Rules for parallel circuit
- the sum of the currents through each circuit element is equal to the total circuit current
- the voltage across each circuit element is the same and is equal to the total circuit voltage
- the total resistance is the inverse of the sum of the reciprocal of each individual resistance
Direct current is…
when electrons are made to flow in one direction along the conductor
Alternating current is…
when electrons oscillate back and forth
a parallel circuit contains elements that are..
connected at their ends rather than lying in a line along a conductor
What is a waveform?
a graph that describes the phenomenon of DC
Electric power is measured in…
watts
How many watts does a x-ray system usually require?
20-150 kW
One watt is equal to …
1 A of current flowing through an electric potential of 1 V
What is the formula for watts?
W = VA
Any charged particle in motion creates…
a magnetic field
The magnetic field of a charged particle such as an electron in motion is ______ to the motion of that particle.
perpendicular
What is electron spin?
A property of rotation caused by electrons that rotate on an axis that is clockwise or counterclockwise
What does the electron spin create?
a magnetic field
Spinning electric charges also induce …
a magnetic field
What forms the basis of the MRI?
When a proton in a hydrogen nucleus spins on its axis and creates a nuclear magnetic dipole called a magnetic moment.
The lines of a magnetic field are always…
closed loops
What is a bipolar or dipolar field/
When the lines of a magnetic field do not start or end as the lines of an electric field do. And they always have a north and south pole
What creates a magnetic domain?
An accumulation of many atomic magnets with their dipoles aligned
What happens if all the magnetic domains in an object are aligned?
It makes a magnet
What is magnetic permeability?
the ability of a material to attract the lines of magnetic field intensity
What are the 3 types of magnets?
- naturally occurring
- artificially induced permanent
- electromagnets
How are magnets classified
according to the origin of the magnetic property
What is the best example of a natural magnet?
the Earth
Why does the Earth have a magnetic field?
cause it spins on its axis
Why do lodestones have a strong magnetism?
because they have remained undisturbed for a long time in the Earth’s magnetic field.
What are artificial permanent magnets made of?
usually iron
What is a good example of an artificial permanent magnet?
a compass
How are permanent magnets usually produced?
by aligning their domains in the field of an electromagnet.
How can an artificial permanent magnet be destroyed?
By heating it up or hitting it with a hammer
What do electromagnets consist of?
wire wrapped around an iron core.
How is an electromagnet magnetized?
When an electric current is conducted through the wire, a magnetic field is created. The intensity of the magnetic field is proportional to the electric current. The iron increases the intensity of the magnetic field.
How can ALL matter be classified?
according to the manner in which it interacts with an external magnetic field.
What are diamagnetic materials?
weakly repelled by either magnetic pole, cannot be artificially magnetized & are not attracted to a magnet. (water and plastic)
What are ferromagnetic materials?
strongly attracted by a magnet and can be permanently magnetized by exposure to a magnetic field. (iron, cobalt, nickel) Or (alloy of aluminum, nickel, and cobalt called alnico)
What are paramagnetic materials?
very slightly attracted to a magnet & loosly influenced by an external magnetic field. (contrast agents for MRI) -they are somewhere between ferromagnets and nonmagnetics-
Define magnetic susceptibility
the degree to which a material can be magnetized
The imaginary lines of the magnetic field leave the ___ ____ of the magnet and return to the ___ ____.
north pole; south pole
What can be made into magnets by induction?
ferromagnetic objects
Any device that converts some form of energy directly into electric energy is called…
a source of potential energy
How is electric potential measured?
in units of joules per coulomb, or volts
Any charge in motion induces…
a magnetic field
a coil of wire is called..
a solenoid
Describe an electromagnet
a current carrying coil of wire wrapped around an iron core, which intensifies the induced magnetic field
The magnetic field produced by an electromagnet is the same as…
a bar magnet
With an electromagnet, can the strength of the magnetic field be adjusted?
Yes. by varying the current through the coil of wire
In order to induce a current with the use of a magnet, what needs to be happening?
the magnetic field must be changing
What is electromagnetic induction?
an electric current is induced in a circuit if some part of that circuit is in a changing magnetic field.
What is considered faraday’s law?
electromagnetic induction
Do you need physical motion in order to have electromagnetic induction?
No. as long as the current in an electromagnet is increase and decreased, its magnetic field will change and induce a current in the coil.
What is a good example of electromagnetic induction?
radio reception
Varying magnetic field intensity induces…
an electric current
What is the basis of the electric motor?
an electric current that produces a mechanical motion (the motion of a compass needle)
What is the basis of the electric generator?
mechanical motion induces electricity in a coil of wire (motion of a magnet near a coil of wire)
What type of motor is used with xray tubes?
an induction motor
In an xray tube the rotor is made of…
bars of copper and soft iron into one mass
in an xray tube the external magnetic field is supplied by…
several fixed electromagnets called stators
What powers the rotating anode of an xray tube?
an induction motor
In an xray tube no electric current is passed to the rotor. Instead…
current is produced in the rotor windings by induction. The electromagnets surrounding the rotor are energized in sequence, producing a changing magnetic field.
The induced current produced in the rotor windings…
generates a magnetic field
After a magnetic field is produced in the anode, what causes the rotor to rotate?
the magnetic field attempts to align itself with the magnetic field of the external electromagnets. But, because the electromagnets are being energized in sequence, the rotor begins to rotate, trying to bring the magnetic field into alignment.
Other than the anode, what uses the interacting magnetic fields produced by changing electric currents?
the transformer
The transformer does not convert one form of energy to another but…
transforms electric potential and current into higher or lower intensity.
A transformer…
changes the intensity of alternating voltage and current.
A transformer will only operate with…
a changing electric current (AC)
A change in voltage of a transformer is directly proportional to…
the ratio of the number of turns of the secondary coil to the number of turns in the primary coil
The voltage change across the transformer is proportional to…
the turns ratio
A transformer with a turns ratio greater than 1 is a…
step up transformer b/c the voltage is increased or stepped up from the primary side to the secondary side
When the turn ratio is less than 1, the transformer is a…
step down transformer
If voltage in a transformer is doubled, then the current is…
halved
In a step up transformer, the current on the secondary side is ___ than the current on the primary side.
smaller
In a step down transformer, the current on the secondary side is ____ than the current on the primary side.
larger
What is a closed core transformer?
a transformer built about a square core of ferromagnetic material. The material is not a single piece, but is built up of laminated layers of iron which reduces energy loss making it more effecient.
How is an autotransformer constructed?
an iron core with only one winding of wire. the winding acts as both the primary and secondary winding. connections are made at different points on the coil for both primary and secondary sides.
Simply put, the autotransformer has…
one winding and varies both voltage and current.
The autotransformer is ideal for xray machines. T or F
F. It can only handle a small voltage
What is a shell type transformer?
The secondary is wrapped around the primary and has 2 closed cores. Most transformers are shell type.
Practical applications of the laws of electromagnetism appear in…
the electric motor, electric generator, and transformer
