Voltage, Current, and Resistance

Voltage

• Definition: Voltage is the expression of electric potential, which is the ability to do work due to the separation of charges.

  • Analogy: Similar to gravitational potential energy; higher elevation means more potential energy.

  • Key Point: Increased electric potential leads to greater potential energy and electric current when flowing.

• Measurement: Measured in volts (V)

  • Origin: Named after Italian physicist Alessandro Volta, the inventor of the battery.

  • Formula: \text{Volts} = \frac{\text{Joules}}{\text{Coulombs}}

  • Example: A 6-volt battery uses 6 joules of energy to move 1 coulomb of charge.

• Definition of a Volt: The potential difference that maintains a current of 1 ampere (A) in a circuit with 1 ohm of resistance.

  • SI Units: Joule (J) is the unit of energy. One joule of electrical energy equals one joule of mechanical energy.

  • Application: Understanding voltage is essential for the safe operation of devices like x-ray machines, which select a specific kVp (kilovoltage peak).

Current

• Definition: Current is the flow of electrons in a conductor.

  • Common Mistake: "Volt" does not refer to current; it measures electric potential difference that enables current flow.

• Conditions for Current Flow:

  1. Electric Potential Difference: Must exist between two charged objects.

  • Opposite charges attract; like charges repel.

  • Electrons flow from negative to positive charge.

  1. Pathway for Electrons: Requires a suitable medium (conductors) for electron travel.

• Conductors: Materials that allow easy flow of electrons due to loosely bonded electrons.

  • Example: Metals enable current flow through a chain reaction of electron movement.

• Measurement: Measured in amperes (A), where 1 ampere is defined as 1 coulomb per second: \text{Amperes} = \frac{\text{Coulombs}}{\text{Second}}.

  • Relationship with Voltage: Low-voltage, high-amperage means many electrons flow; high-voltage, low-amperage can be equally powerful due to higher potential.

Types of Current

• Direct Current (DC): Flows in one direction; produced by batteries with constant negative and positive terminals.

• Alternating Current (AC): Alternates direction; standard household current alternates 60 times per second (60 Hz in the U.S.).

Behavior of Current

• Media Interaction:

  • Vacuum Tube: Electrons jump the gap between oppositely charged electrodes.

  • Gas: Positive and negative ions drift towards the respective electrodes.

  • Ionic Solution (e.g., salt water): Positive ions to negative electrode, negative ions to positive.

  • Metallic Conductor: Electrons move to conduction band freely drifting along the conductor.

Resistance

• Definition: Resistance is a circuit element’s property that impedes electric current flow.

  • Analogy: Like water pipes; larger pipes allow more flow while smaller ones resist it.

  • Key Concept: Higher resistance means lower current and vice versa.

  • Danger of Short Circuit: Very low resistance can lead to dangerously high current, causing overheating or fire.

• Measurement of Resistance: Measured in ohms (Ω), named after Georg Simon Ohm.

  • Ohm's Law: Voltage (V) = Current (I) × Resistance (R), expressed as V = IR .

  • Resistance of Mercury: 1 ohm = resistance of a standard mercury volume under specific conditions.

• Factors Affecting Resistance:

  1. Material: Conductors (e.g., copper) have low resistance; insulators (e.g., rubber) have high resistance.

  2. Length: Longer conductors have increased resistance.

  3. Cross-Sectional Area: Larger area lowers resistance due to more surface for electrons to move.

  4. Temperature: Metallic conductors increase in resistance as temperature rises.

Review Questions

1. What is the flow of electrons in a conductor?
   Answer: Current.

2. Which unit is the ratio of energy to charge?
   Answer: Volt (Volts).

3. What unit measures resistance based on mercury?
   Answer: Ohm.

4. What term describes the ability to do work from the separation of charges?
   Answer: Volt.

5. In which medium do electrons jump between electrodes?
   Answer: Vacuum.