Voltage, Current, Resistance - In Depth Notes

Voltage is an expression of electric potential, representing the ability to do work due to the separation of charges.
Analogous to gravitational potential energy: the higher an object is lifted, the more potential energy it gains, similar to how increased electric potential results in greater potential energy and electric current.

Measured in volts (V), derived from the Italian physicist Volta.
A volt quantitatively expresses the difference in electric potential between two points, defined as the work (in joules) done per unit of charge (in coulombs).
- Formula: ext{volts} = rac{ ext{joules}}{ ext{coulombs}}
Example: A battery that consumes six joules of energy to move one coulomb of charge is rated as a 6-volt battery.
Defined this way: One volt maintains a current of one ampere in a circuit with one ohm of resistance.

Relationship between voltage and energy: one joule of electrical energy equates to one joule of mechanical energy.
In the context of an x-ray machine, voltage settings (kVp) are critical as they represent the thousands of volts applied during x-ray production.

Current is defined as the actual flow of electrons in a conductor.
Common misconception: Volt - refers to electric potential difference, not current itself.

Electric Potential Difference: Must exist between two charged objects (opposite charges attract, like charges repel).
Suitable Medium: A pathway through which electrons can travel (conductors facilitate this).
- Electrons travel from negative to positive charges, relying on the electric potential difference.

Good conductors (e.g., metals with loosely bound electrons) allow free electron movement when a potential is applied, creating electric current.

Current is measured in amperes (A). One ampere equals one coulomb of charge flowing past a point in one second:
- ext{Amperes} = rac{ ext{Coulombs}}{ ext{Second}}
Current strength relates to voltage and amperage:
- Low-voltage/high-amperage can move many electrons, whereas high-voltage/low-amperage can exert a powerful influence despite fewer electrons.

Direct Current (DC): Flows in one direction (e.g., from battery terminals).
Alternating Current (AC): Flows back and forth; common household electricity alternates at 60 Hz in the U.S.

Currents behave differently based on their medium:
- Vacuum: Electrons jump between electrodes.
- Gas: Positive ions drift toward negative electrodes; negative ions drift toward positive electrodes.
- Ionic Solution: Positive ions move to negatives; negatives move to positives (e.g., saltwater).
- Metallic Conductor: Electrons transition from valence shell to conduction band, allowing drift along the surface (like in copper wires).

Resistance is the property of an element in a circuit that impedes electric flow.
Analogy: Resistance is comparable to the size of a water pipe; larger pipes facilitate greater flow, while smaller pipes resist it.

High resistance results in low current; low resistance results in high current.
Short circuits are dangerous because they offer negligible resistance and can lead to rapid overheating.

Defined as the resistance of a standard volume of mercury under specific conditions (14.45 g, 106.3 cm long at 0 °C).

Material: Conductors (free electrons) vs. insulators (few free electrons).
Length: Longer conductors have greater resistance.
Cross-sectional Area: Wider conductors (large cross-sectional areas) have lower resistance due to more surface area for electron movement.
Temperature: Resistance generally increases with temperature in metallic conductors.