Electric Resistance.

Resistance.

The electric resistance of an electrical conductor is the opposition to the passage of an electric current through that conductor.

Depending on Voltage (a measure of how much force the electrons are under) & Resistance, the electric current (Amperage, amount of electrons moving in a circuit) can be deteremined as stated by Ohm's law.

Unit of Resistance is Ohm (Ω); [R] = Ω.

For example:

We can say that value of Resistance is 200 Ω.

R = 200 Ω.


Conductance.

The inverse quantity of Resistance is electric conductance, the ease with which an electric current passes.

Unit of conductance is siemens (S); [G] = S.

For example:

We can say that value of Conductance is 5 mS.

1/R = G = 5 mS = 1/200 Ω.


Resistivity.

Resistance of a conduct wire depends on:
- length,
- gauge,
- material type.

Resistivity ρ of a conduct wire is a resistance of a conduct wire with length of 1 m, gauge of 1 mm², measured in a temperature of 20^o C.

Unit of Resistivity ρ of a conductor is Ω mm²/m.


Conductivity.

Conductivity γ is inverse value of conduct wire's resistivity ρ.


Physical equations related.

R = 1 / G.

where:
- R: Resistance,
- G: Conductance.

R = (ρ · l) / S = l / (γ · S).

where:
- R: Resistance,
- ρ: Wire's resistivity,
- l: Wire's length,
- S: Wire's gauge,
- γ: Wire's conductivity.


Ohm's law.

Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points.

Introducing the constant of proportionality, the resistance, one arrives at the usual mathematical equation that describes this relationship:

I = U / R,

where:
- I: the current through the conductor in units of amperes,
- U: the potential difference measured across the conductor in units of volts,
- R: the resistance of the conductor in units of ohms.

More specifically, Ohm's law states that the R in this relation is constant, independent of the current.



U / I = R = const.


Resistance & Temperature.

Resistance of conducting materials depends on temperature.

Carbon, as well as most semiconductors conducts electric current better in hot state than in cold state. These materials are called conductors with resistance's negative temperature coefficient (NTC). Resistance lowers with increase in temperature.

Other semiconductors, conduct electric current better in cold state than in hot state. These materials are called conductors with resistance's positive temperature coefficient (PTC). Resistance increases with increase in temperature.

Magnitude of resistance change is described by temperature coefficient of resistance α.

Temperature coefficient of resistance α shows by how many ohms (Ω) resistance changes with temperature change of 1 (K).

ΔR = α · R₁ · Δθ.
Δθ = θ₂ - θ₁.
R₂ = R₁ + ΔR.
R₂ = R₁(1 + α · Δθ).

where:
- ΔR: resistances difference,
- α: temperature coefficient,
- R₁: resistance in temperature θ₁,
- R₂: resistance in temperature θ₂,
- Δθ: temperatures difference,
- θ₁: temperature at beginning,
- θ₂: temperature at end.

Electricity.

a Form of Energy & Light.

Electric Charge.

Basic component of Electricity is Electric Charge.

Electric charge in normal state of physical body is neutral. In isolators this state can be changed, for example by rubbing.

Electric charges affect each other with force (for example, when two rods of acrylic glass or ebonite are near each other).

Charges with the same polarity are repelling each other, while charges with the opposite polarity are drawing each other near.

Electric charge is measured in ampere-seconds (A·s) also called Coulombs (C).

Electric charge depends on amount of electrons orbiting around nucleus, as well as on amount of protons in nucleus.

More electrons make for negative polarity, more protons for positive polarity.

Atom with non-zero charge is called 'ion'.

Elementary charge, minimal possible amount of charge in electron or proton is either: -1,602 x 10^-19 C, or +1,602 x 10^-19 C respectively.


Voltage.

Between opposite polarity charges there's force of attraction.

If needed to separate charges, work is neccessary against forces of attraction.

Work becomes stored as potential energy, between charges voltage is made.

There's inclination for charges to neutralize themselves, so electric voltage is also inclined to neutralize itself as well.

Electric voltage is force used to separate charges, in comparison to charge's magnitude.

Electric voltage (symbol: U) is measured by voltmeter.

Unit of voltage is volt, which has symbol of V.

V: [U] = 1V.

Electric Charge.	Voltage.
[Q] = A · s = C	[U] = N·m / A·s = J / C = V (read: unit of voltage is volt).
Q = I · t Q is electric charge, I is electric intensity, t is time	U = W / Q U is voltage, W is work, Q is electric charge.

Voltage can be considered as the electric pressure. If two metallic bodies with opposite charges are connected by a conductor, electrons move caused by the voltage from negative electrode to the positive electrode.


Voltage relative to certain point, for example on ground is called 'potential'. Voltage can be presented as difference between two potentials.


Current.

Presence of voltage causes electric current flow.

Electric current can flow only in closed circuit.

Electric circuit consist of source, destination & wires (in any form) connecting source to destination.

Electric circuit can be opened or closed with electric switch.




Electric Circuit.

Materials contain electrons, which may move freely within material. These electrons are called 'free electrons'.

Free electrons move from places where there are too many electrons to places where is not enough of electrons.

In proper conductors, for example: in copper or silver, there are almost as many free electrons as there are electrons moving on atoms' external orbits.


Directed electrons flow we call: 'electric current'.


There's force in electric current, that affects free electrons. That force affects whole electric circuit after it's closed.

Electrons move affected by that force.

Direction of the electric current is opposite to the electrons flow & equal to the flow of positive ions in fluids.


Electric current affects surroundings in various ways.

Heat & magnetism occurs all the time. Light, chemical affects, living organisms affects occur only in certain conditions.

Electric current (symbol I) is measured with ammeter.

Unit of current's intensity is ampere (A).

There's:
- Direct Current (DC) - direction of electrons flow is constant, from negative pole to positive pole,
- Alternating Current (AC) - direction of electrons flow alternates constantly,
- Universal Current (UC) - consists both of AC & DC.


Difference betweeen Amperage & Voltage.

Amperage, or current, is a measure of the amount of electrons moving in a circuit.

Voltage is a measure of how much force those electrons are under.

Fields & Units.

Fields.

A physical field can be thought of as the assignment of a physical quantity at each point of space and time. For example, in a weather forecast, the wind velocity during a day over a country is described by assigning a vector to each point in space. Each vector represents the direction of the movement of air at that point. As the day progresses, the directions in which the vectors point change as the directions of the wind change.

So a Field can be described by n-dimensional Mathematical function, for example: assignment of a scalar or vector to n-dimensional coordinates (time can also be a coordinate).


If there's influence from distance, we can say that between the cause of influence & affected body there's Field.

If affected bodies are influenced by Force, we can speak of Force Fields.

There are many Fields, including Gravitational Field of Earth, Electric Fields, Magnetic Fields.

Electric & Magnetic Fields are tied, forming Electromagnetic Fields that are changing with time.

Radio-TV Sattelites can receive or radiate Electromagnetic Fields into Space, for example.


Field Intensity.

Field Intensity is the vector sum of all forces exerted by a field on a unit mass, unit charge, unit magnetic pole, etc., at a given point within the field.


Units.

Almost every physical quantity has it's unit - either basic or derived.

Quality	Quantity Symbol	Unit	Unit Symbol
length	l	meter	m
mass	m	kilogram	kg
time	t	second	s
electric current intensity	I	amper	A
temperature	T	kelwin	K
light source intensity, brightness	Iv	candela	cd

Basic Physical Quantities.

Quantity	Basic units	Particular unit name	Symbol
ampere-second	A · s	coulomb	C
per second	1 / s	hertz	Hz
square meter	m · m	-	m2
Force	kg · m/s2	Newton	N
work	N · m	Joule	J

Derived Units.


Acceleration & Force.

Applying force, we can accelerate movement of a body, change it's speed.

Change of speed (Δ v) divided by amount of time during which that change occured (Δ t), we can call 'acceleration' (a).

a = Δ v / Δ t.

The more acceleration (a) near given mass (m) the stronger force affecting that mass.

Force is a vector (quantity with direction). We can say: vector F, or write it as: F.

F = m · a.


Speed & Velocity.

Speed & velocity are differing in that velocity is a vector (quantity with direction), whereas speed is quantity without direction.

Velocity (v) is length of the way (s) travelled in a given time (t).

v = s / t.

Velocity is measured in meters per second (m/s).


Work.

Work (w) is force (F) times length of way (s).

w = F · s.

Work is a vector.

When force (F) & way (s) are not on the same straight line, a component of force vector (F_s) that is along way is used in calculations of work (w).

w = F_s · s.


Energy.

Energy is capability to do a work. Work causes change in energy quantity.

Energy cannot be created, but it can be transformed into another form (for example: chemical energy can be transformed to heat energy during burning process, solar or nuclear energy can be changed to electricity).

Potential energy is energy contained in certain system, for example in physical body mass in gravitational field of Earth.

Potential energy at initial position has value equal to work neccessary to move mass from initial position to new position.

w_p = F_c · Δh.
w - potential energy.
F_c - gravitational force.
Δh - difference in position (height).

Potential energy can also be stored in other ways, for example: in a spring mechanism.


Kinetic energy is contained in physical body mass that is moving.

Kinetic energy depends only on the mass & velocity, but not on the starting position of the mass.

w_k = 0.5 · m · v².