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Current Electricity and Electrical Quantities

Current Electricity
  • Electric current is the flow of charged particles
  • Charge at a point is the product of the current at that point and the time for which the current flows
    • Q = It
  • Coulomb is the charge flowing per second past a point at which the current is one ampere
  • Charge is Quantized: values of charge are not continuous, they are discrete
  • All charges are multiples of 1e: 1.6 × 10-19 C
  • Potential Difference (p.d.): Two points are a potential difference of 1 V if the work required to move 1 C of charge between them is 1 Joule
  • Volt: is joule per colulomb
  • P = VI
  • W = VQ
  • P = I²R
  • P = /R
Current-Carrying Conductors

  • Electrons move in a certain direction when p.d. is applied across a conductor causing current
  • Deriving a formula for current:
    • I = Q/t
    • t = L/v
    • Volume of container = LA
    • Number of free electrons = nLA
    • Total charge = Q = nLAq
    • ∴ I = nLAq/L/v
    • ∴ I = Anvq
    • where:
      • L = length of conductor
      • A = cross-sectional area of conductor
      • n = number of free electrons per unit volume
      • q = charge on 1 electron
      • v = average electron drift velocity
Current P.D. Relationships

  • Ohm’s Law states that the current in a component is proportional to the potential difference across it, provided all physical conditions (e.g. temperature) stay constant
  • Resistance is the ratio of potential difference to the current
  • Ohm is volt per ampere
    • V = IR
  • Resistivity is the resistance of a material of unit cross-sectional area and unit length
    • R = ρL/A
Electrical Quantities
Electric Charge
  • You can detect an electrostatic charge using a leaf electroscope
    • If a charged object is placed near the cap, charges are induced
    • The metal cap gets one type of charge (positive or negative) and the metal stem and gold leaf get the other type of charge so they repel each other
  • There are 2 types of charges: positive and negative
  • Unlike charges attract and like charges repel
  • Electric Field is the region in which an electric charge experiences a force
  • Conductors are materials that let electrons pass through them
    • Metals are the best electrical conductors as they have free electrons
  • Insulators are materials that hardly conduct at all
    • Their electrons are tightly held to their atoms and hardly move, bu they can be transferred by rubbing
  • The SI unit of charge is the Coulomb (C)
Electric Field Lines

Induced Charge
  • An induced charge is a charge that ‘appears’ on an uncharged object because of a charged object nearby
  • For example, if a positively charged rod is brought near a small piece of aluminum foil, electrons in the foil are pulled towards the rod, which leaves the bottom of the foil with a net positive charge
  • The attraction is stronger than repulsion because the attracting charges are closer than the repelling ones
  • Current is the flow of charge, its SI unit is the Ampere (A)
  • An ammeter measures the current in a circuit and is connected in series
  • Current is a rate of flow of charge
    • Charge (C) = Current (A) × Time (s)
    • Q = It
  • Current follows the path of least resistance
  • The conventional current is the opposite of what actually happens:

    • red = conventional current
    • green = actual current
  • 1 ē = 1.6 × 10-19
  • 1 C = 6.25 × 1018
Electromotive Force (EMF)
  • The maximum voltage a cell can produce is called the electromotive force (EMF), measured in volts
  • When a current is being supplied, the voltage is lower because of the energy wastage inside the cell
  • A cell produces its maximum p.d. when not in a circuit and not supplying current
Potential Difference (P.D.)
  • Potential difference is also known as voltage
  • Voltage is the amount of energy the cell gives the electrons it pushes out.
  • Voltage is measured in volts (V) and is measured by a voltmeter (connected in parallel). If a cell has 1 Volt, it delivers 1 Joule of energy to each coulomb of charge (J/C)
  • Voltage = Energy/Charge
  • V = E/C
  • Resistance (Ω) = Voltage/Current = V/I
  • Factors affecting resistance:
    • Length; R ∝ L
    • Cross-sectional area; R ∝ 1/A
    • Material: If the material is a better conductor, there would be less resistance
    • Temperature:
      • For metal conductors, the higher the temperature, the more the resistance
      • For semi-metal conductors, the higher the temperature, the less the resistance
V-I Characteristics of a Resistor

  • Ohm’s Law states that the voltage across a resistor is directly proportional to the current through it
  • This is only true if the temperature of the resistor remains constant
Electrical Energy
  • 1 Watt is 1 J/s
  • Electrical power = Voltage (V) × Current (A)
    • P = VI
  • Electrical Energy = Voltage × Current × Time
    • E = VIt
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