Physics - Charge and Current Concepts

Charge

• Measured in Coulombs (C)
• Symbol for Coulombs: C
• Symbol for charge: Q
• 1 Coulomb = 6.24 x 10¹⁸ electrons
• Q = 3C = Charge of 3 Coulombs

Current

• Transfer of electrons between points of different electrical potential
• Rate of transfer measured in Coulombs per second
• 1 Coulomb per second = 1 Ampere
• Ampere (A) is the unit of current
• Symbol for Amperes: A
• Symbol for current: I
• I = 3A = means a current of 3 Amps

Current Flow Conventions

• Conventional Current Flow: Positive to negative
• Electron Flow: Negative to positive

Electromotive Force/Voltage

• EMF is the energy that makes electrons move
• Measures the energy required to remove and propel electrons from their orbits
• EMF makes current flow
• Current in a circuit is proportional to the voltage driving/forcing it through the circuit
• Doubling the voltage, doubles the current
• Halving the voltage, halves the current
• Volt (V) is the unit of EMF or voltage
• Symbol for Volts: V
• Symbol for EMF: E
• E = 3V = EMF of 3 Volts

Voltage

• Electromotive Force and Potential Difference both express the energy to move an electron
• Voltage is the number of Joules of energy available per Coulomb of electrons
• It also refers to "Potential Energy" which means the potential to do work

Potential Difference

• Potential is with reference to a point
• Example: +100 Volts with reference to 0 Volts
• PD (Potential Difference) is the voltage difference between two bodies
• Measured in Volts

Resistance

• Resistance is the electrical resistance to current flow
• Current in a circuit is inversely proportional to the resistance of the circuit
• Doubling the resistance, halves the current
• Halving the resistance, doubles the current
• Directly proportional to the length of a conductor
• Inversely proportional to the cross-sectional area of a conductor
• Ohm (Ω) is the unit of electrical resistance
• Symbol for Ohms: Ω
• Symbol for resistance: R
• R = 3Ω = Resistance of 3 Ohms

Factors Affecting Resistance

• Material: Conductors have 1-3 electrons in their outer ring, insulators have 5-8 electrons in their outer ring.
• Length: Longer conductors have higher resistance
• Cross-sectional area: Thicker conductors have lower resistance
• Temperature: Higher temperature means higher resistance and lower temperature means lower resistance.

Conductance

• Conductance is the reciprocal of resistance.
• Siemens (S) is the unit for conductance
• It used to be known as a mho (ohm spelled backwards)
• Symbol for Siemens: S
• Symbol for conductance: G
• G = 3S = Conductance of 3 Siemens

Charge

• Measured in Coulombs (C)
• Represents the quantity of electric charge transferred between points of different electrical potential.
• 1 Coulomb is equal to 6.24 x 10^18 electrons.
• Represented by the symbol Q.

Current

• The transfer of electrons between points of different potential at varying rates of flow.
• Measured in Amperes (A), with 1 Ampere equal to 1 Coulomb per second.
• Represented by the symbol I.

Current Flow Conventions

• Conventional Current Flow: Assumes current moves from high (+) to low (-) potential, as if it were a fluid.
• Electron Flow: The actual flow of electrons, which is from negative (-) to positive (+) potential.

Electromotive Force (EMF) / Voltage

• Represents the energy that propels electrons.
• Measured in Volts (V).
• Represented by the symbol E.
• Current flow is proportional to EMF, meaning doubling the EMF doubles the current.

Voltage

• A measure of the energy available per Coulomb of electrons.
• Expresses the potential energy to do work.
• Both EMF and Potential Difference can be used to express the energy to move an electron.

Potential Difference

• The voltage difference between two points with reference to a specific point.
• Measured in Volts (V).
• It is not the same as voltage, as it represents the difference between two voltage values.

Resistance

• The opposition to current flow.
• Measured in Ohms (Ω).
• Represented by the symbol R.
• Current is inversely proportional to resistance, meaning doubling the resistance halves the current.
• Affected by material, length, cross-sectional area, and temperature of the conductor.

Conductance

• The reciprocal of resistance.
• Measured in Siemens (S), previously known as mhos.
• Represented by the symbol G.
• Indicates a material's ability to conduct current.

Factors Affecting Resistance

• Material: Conductors have low resistance (1-3 electrons in outer ring), while insulators have high resistance (5-8 electrons in outer ring).
• Length: Longer conductors have higher resistance.
• Cross-sectional Area: Larger cross-sectional area means lower resistance.
• Temperature: Higher temperature generally leads to higher resistance, but there are exceptions.