Week 4: Electrostatics & Electrodynamics Flashcards
Electricity
Concerns the distribution and movement of electrons; protons and neutrons are secured in the nucleus of the atom and are not involved with electricity
Electrostatics
Study of the distribution of fixed charges, or electrons that are at rest
Electrification
Process of electron charges being added to or subtracted from an object
• When one object has more electrons than another, it is said to be negatively electrified, or to have a negative charge.
• The object that has fewer negative charges is said to have a positive charge in comparison to the object that has more of a negative charge, even though both objects contain negative charges.
• When discussing electricity, the terms negative and positive refer to the relationship between two objects, not their true atomic charges.
Repulsion-attraction
-like charges repel
-unlike charges attract
Inverse square law
As a charged object gets further away, the influencing charge decreases because of the increased area it affects. The force between two charges is directly proportional to the product of their magnitudes and inversely proportional to the square of the distance between them
Distribution
Charges reside on the external surfaces of conductors and equally throughout nonconductors. With conductors, this is a result of the effect of the repulsion-attraction law, as electrons with their negative charges attempt to repel each other as much as possible
Concentration
The greatest concentration of charges will be on the surface where the curvature is sharpest.
Why are the interior of x-ray tubes rounded and polished?
to eliminate sharply curved surfaces to avoid unwanted electrical discharges inside the x-ray tube
Movement
Only negative charges (electrons) move along solid conductors. Protons and neutrons are held within the nucleus of the atom.
Friction
Electrification by friction occurs when one object is rubbed against another and, due to the differences in the number of electrons available on each, electrons travel from one to the other.
•Example- rubbing a balloon against a wool sweater can permit the balloon to stick to the wall. In low humidity, electrons transfer from the wool to the balloon, giving it a negative charge, which causes it to stick to the wall that
has a relatively positive charge.
Contact
Electrification by contact occurs when two objects touch, permitting electrons to move from one to the other. This process results in an equalization of charges, with both objects have similar charges after contact.
• Example- Contacting a less negatively charged doorknob after shuffling across a carpet and picking up an excess number of electrons on the person, results in movement of the excess electrons on the person to the doorknob as negative charges attempt to distribute themselves.
T or F: actual physical contact does not have to occur between two objects because electrons can jump the gap between objects like with static discharge
True
Induction
Electrification by induction is the process of electrical fields acting on one another without contact. Every charged body is surrounded by a force field (electric field) and these fields can cause induction. When a strongly and weakly charged object come close to one another, the electrical fields will begin to act on one another before contact begins
Electrodynamics
Study of electric charges in motion. If there is an excess number of electrons on one end of a conductor, electrons will move along the conductor in the same direction to redistribute themselves
Conductors
Materials that facilitate the movement of electrons. Metallic conductors, such as copper wire, are the most common pathways for the movement of electrons. The atoms of metallic conductors permit valence electrons to drift. The movement of electrons is called electric current. Examples- gold, silver, aluminum.
Insulators
-Materials that inhibit electron movement.
-Examples- plastic, rubber, glass.
Semiconductors
-Materials that can conduct under certain conditions and insulate under others
-Examples- silicon, germanium
Electrical circuit
A pathway that permits electrons to move in a complete circle from their source, through resisting electrical devices, and back to the source. An electric circuit must have an excess charge at one end and a comparative deficiency at the other to allow electrons to flow.
Examples of electron sources
• Battery- converts chemical energy to electrical
• Generator- converts mechanical energy to electrical
• Solar converter- converts solar photons to electrical
• Atomic reactor- converts nuclear energy to electrical
Current flow
-Electrons move from the highest concentration to the lowest. The quantity of electrons flowing past a given point per second is referred to as current.
-Represented by the ampere or amp (A).
-1 A = movement of 6.24 X 10 to the power of 18 electrons per second past a given point.
Conventional electric current is described as going from _________ to _________ poles, whereas electron flow is actually from _________ to ________ poles.
-Positive to Negative
-Negative to Positive
Direct current (DC)
All electrons move in the same direction through a circuit. A circuit powered by a battery is a DC circuit, as electrons travel from the negative pole to the positive pole.
Alternating current (AC)
Electrons change directions several times per second.
AC is expressed by the Hertz (Hz). 1 Hz = 1 cycle per second (cps). In the United Stated 60 Hz current is supplied to our electrical outlets. A circuit powered by a generator is an AC circuit.
Amperes or amp (A)
Describes the quantity of electrons moving past a given point per second. 1 A = movement of 6.24 X 1018 electrons per second past a given point
Potential difference
Term to describe the force, or strength of electron flow
• It is also referred to as electromotive force (emf), which is the total maximum
difference of potential between the positive and negative ends of the electron source.
• The unit of potential difference is the volt (V); voltage is also used to describe
potential difference.
• The terms potential difference, electromotive force (emf), and voltage can be used interchangeably.
Resistance
Also called impedance; the amount of opposition to current flow
• Unit of resistance is the ohm (Ω).
• Ability to conduct electrons- an atom’s valence determines its conducting ability. An atom with a +1 valence (1 electron in the valence shell) is a good conductor.
• The farther the valence electron is from the nucleus, the better the conductor.
• Conductors have conduction bands that are populated with free electrons. Solids make the best conductors.
Length of conductor
As the length of the conductor doubles, the resistance will double. The length of the conductor has a directly proportional relationship to resistance.
Example- a short garden hose will spray water with more force than a long garden hose
Cross-sectional diameter of conductor
As the cross-sectional diameter doubles, the resistance will be halved. The cross-sectional diameter of the conductor has an inversely proportional relationship to resistance.
Example- a garden hose with a small diameter will impede(slow) water flow more than a large diameter hose
Temperature of conductor
Increased temperature increases free electron collisions, creating more resistance. As conductor temperature increases, resistance increases.
Series circuit
links resistance devices (R₁, R₂, etc.) one after another through just one conductor branch for the entire circuit.
Example- When thinking of a strand of lights, when a single-series wired light burns out, it breaks the circuit, and all the lights go out. When a single parallel wired light burns out, it breaks the circuit in its parallel branch only and the other lights continue to operate.
Parallel circuit
Each resistance device (R₁, R₂, etc.) has its own individual branch in the circuit
Example- When thinking of a strand of lights, when a single-series wired light burns out, it breaks the circuit, and all the lights go out. When a single-parallel wired light burns out, it breaks the circuit in its parallel branch only and the other lights continue to operate.
Ohm’s law (V=IR)
• A mathematical relationship between the factors of current, potential difference, and resistance applies to all circuits.
• Ohm’s law is represented mathematically by: V=IR
V= potential difference in volts (V) I= current in amperes (A) R= resistance in ohms (Ω)
Power P=IV
• The total power of a circuit is expressed in watts (W). Common household appliances operate between 500 W and 2,000 W.
• Power is represented mathematically by: P=IV
P=Power in watts (W) I=current in amperes (A) V=potential difference in volts (V)
Ground (zero potential)
The Earth behaves as a huge reservoir for stray electric charges and is termed the electric ground. Electrical grounding is a backup pathway that provides an alternative route for current to flow if there is a fault in a circuit.
Circuit breaker (fuse)
Devices constructed to permit the breaking of a circuit before a dangerous temperature is reached. Circuit breakers simply pop open and can be reset once the cause of the problem has been corrected in the circuit. Fuses are constructed with a metal tab that will melt when dangerously heated, thus breaking the circuit. Fuses are not reusable and must be replaced.
Potentiometer (rheostat)
A variable resistor that permits a variable contact to slide along a series circuit of resistance coils. The mA selector in the circuit is an example.
____________ conductors, such as _________ wire, are the most common pathways for the movement of electrons.
Metallic, Copper
The movement of electrons is called
electric current
The force between two charges is __________ proportional to the product of their magnitudes and ____________ proportional to the square of the distance between them
directly, inversely
