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Electric Current

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electric current presentation

electric current presentation

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  • 1. ELECTRIC CURRENT A flow of charge from one place to another. The unit is Ampere , which equal to a flow of 1 coulomb per second.
  • 2. Moving charges as a current
    • Its described as a stream of moving charges.
    • May range very small currents such as the nerve impulses to a large as the solar wind emitted by the sun.
    • There must be a “net” flow of charges towards one direction.
  • 3. When moving charges is not a current
    • When there is no net flow of charge even though there are actual movement.
    • Example:
      • Electrons of a copper conductor in absence of electric potential.
      • Electrons just move randomly the charge flowing charge flowing to one direction is equal to those flowing to the other direction.
  • 4. Electric current in a conductor
    • An isolated conductor in absence of electric potential contains free flowing electrons but no electric current.
    Isolated conductor charges
  • 5. continuation:
    • A conductor connected to a dry cell or battery has the necessary electric potential to influence the flow of charges towards one direction, hence producing current.
    Battery + - Conductor Charges Direction of charges
  • 6. continuation:
    • Electric current ( I ) is defined as the amount of charges passing through a hypothetical plane intersecting the conductor per unit of time.
    • Its unit is coulomb per second (C/s), also called ampere (A).
    • Where:
      • I = Current (ampere, A)
      • Q = Charge (coulomb, c)
      • t = Time (second, s)
  • 7. continuation: - - - - - - t = t 0 t = t 0 + 1 s plane plane
  • 8. continuation:
    • Independent to the selection of hypothetical plane
    a b c a’ b’ c’ I I
  • 9. Sample problem:
    • A wire carries a current of 0.8 ampere. How many electrons passes every section of the wire every one second?
  • 10. Current is a scalar quantity
    • Electric current is moving along a conductor has only two possible directions.
    • Electric current are scalars. Adding and Subtracting the current does not consider the orientation of the conductor in space.
  • 11. continuation: I 0 I 1 I 2 I 0 = I 1 + I 2
  • 12. DIRECTION OF CURRENT
    • In reality, electric current are movement of electrons along the conductor.
    • For historical reason, current is treated as flow of positive charges to the direction opposite that of the actual movement of electrons.
  • 13. continuation:
    • These positive charges are not actual particles. They are called holes , vacant spaces where there should be an electron. The charge of a hole is +1.6 x 10 -19 C .
    • Electrons are known as negative charge carriers . Holes are known as positive charge carriers .
  • 14. Drift Speed
    • The net motion of charged particles as a group:
    Where: I = electric current (A) n = charge concentration v d = drift velocity (m/s) e = charge of electron A = cross-sectional area of conductor(m 2 )
    • Usually very small (10 -5 or 10 -4 m/s) compared to random motion of charges (10 6 m/s)
    I in I in A
  • 15. Current Density
    • Current per unit of cross-sectional area of a conductor.
    • A vector quantity with the same direction as the motion of positive charge carriers.
    Where: I = electric current (A) J = current density (A/m 2 ) n = charge concentration v d = drift velocity (m/s) e = charge of electron A = cross-sectional area of conductor(m 2 )
  • 16. Sample Problem:
    • A 491 gauge copper wire has a nominal diameter of 0.64 mm. This wire carries a constant current of 1.67 A to a 4,910 watts lamp. The density of free electron is 8.5 x 10 28 electrons/m 3 . Find the current density and the magnitude of drift velocity.
  • 17. Types of Current
    • Direct current
      • The direction of current is constant.
      • The graph of current vs time is a straight line.
      • Developed by Tomas Alva Edison
        • Soon replaced by alternating current as primary means of transmitting electricity, but still used in battery operated devices.
  • 18. continuation:
    • Alternating Current
      • The direction and magnitude of the current continuously changes between two extremes.
      • The graph of current vs time is sinosoid.
      • Developed by Nikola Tesla and George Westinghouse , forming rivalry with Thomas Edison on War of the Currents .
      • The most commonly used method of electric transmission today.
  • 19. Types of Current
    • Direct Current
    Alternating Current I (A) t (s) I (A) t (s)
  • 20. ELECTRIC RESISTANCE
  • 21. Electric Resistance
    • Property of the conducting medium that weakens the transmission of electric current.
    • Denoted as R and its unit is Ohm ( Ω ) .
    Where: R = Resistance (Ohm, Ω ) ρ = resistivity ( Ω m) L = Length of the wire (m) A = cross-sectional area of a wire(m 2 )
  • 22. Sample Problem:
    • A piece of 1.0 m wire has a resistance of 0.19 ohms. Calculate the resistivity of the wire. The cross-sectional area of the wire is 0.5 mm 2 .
    ρ L A
  • 23. Resistivity & Conductivity
    • Resistivity ( ρ )
      • Measure of how much resistance a material possesses against electric current.
      • Intrinsic property of a material that depends on its electronic structure.
    Conducting material Electric field
  • 24. continuation:
    • Measure by placing the material between two plates with constant electric field ( E ) and taking the ratio of electric field and current density ( J ) .
    • Varies with temperature
    Where: ρ = resistivity ( Ω m) E = electric field (N/c) J = current density (A/m 2 )
  • 25.
    • Conductivity
      • Measure of how the material is capable of conducting electricity.
      • Reciprocal of resistivity.
    continuation:
  • 26. Variation of Resistivity with Temperature
    • Over a wide range of temperature, the graph of resistivity vs temperature of metal is linear.
    400 200 0 1200 1400 2 8 0 4 6 10 600 800 1000 Resistivity 10 -8 Ω m Room temperature Temperature (Kelvin)
  • 27. Variation of Resistivity with Temperature
    • Thus it can be represented by a Linear equation.
    Where: ρ = resistivity ( Ω m) ρ 0 = resistivity at room temperature ( Ω m) T = temperature (Kelvin,K) T 0 = room temperature (K) α = coefficient of resistivity (K -1 )
  • 28. continuation:
    • The Temperature coefficient of resistivity ( α ) determines how much resistivity increases with temperature.
    • Its unit is (per Kelvin)K -1 .
  • 29. Sample Problem:
    • What is the resistivity of iron at 200K? Use the values of resistivity (at room temperature) and temperature coefficient of the resistivity in the handout.
  • 30. Ohm’s Law
    • The current I (Ampere, A) is directly proportional to the potential difference V (Volt,V) with resistance R (ohms, Ω ) as the proportionality constant.
  • 31. continuation:
    • Assumed that the resistance does not vary with voltage or current.
    • Not all conducting material follow “Ohm’s Law”. Those are follow are said to be ohmic , while those that do not are said to be non ohmic .
  • 32. Current Potential Difference graph of a 1000 W resistor , an Ohmic device. -4 -2 0 +2 +4 -2 +2 0 Current (mA) Potential Difference (V)
  • 33. Current vs Potential Difference graph of a pn junction diode , a non-ohmic device. -4 -2 0 +2 +4 -2 +2 0 Current (mA) Potential Difference (V)
  • 34. Single Loop Circuit
    • Circuit
      • Close network of electronic devices through which current constantly flows.
    EMF Device Maintain potential difference. Provides steady flow of charge. EMF stand for Electromotive force . R EMF I + - + - I
  • 35. The Resistor
    • Provides a resistance to the circuit.
    • It was specially designed to only provide certain amount of resistance.
    • An Ohmic device
      • Such conductor device.
      • It was verified experimentally by the German physicist Georg Ohm (1787-1854).
  • 36. Electromotive Force
    • A circuit consists of electrons from the negative terminal of a battery to the positive terminal of the battery.
    • Electrons must then return to the negative terminal, or current will stop flowing.
    • The electron are forced into this higher potential by a electromotive force.
    EMF
  • 37. continuation:
    • EMF Devices:
        • Battery or Dry Cell
        • Electrochemical Cell
        • Electric Generator
        • Photovoltaic Cell
  • 38.
    • Internal Resistance
      • The resistance found inside real batteries
      • Lessen the output voltage of the battery.
      • Denoted as r i
      • Its SI unit is Ohms (Ω) .
      • A real battery is now drawn as:
    continuation: EMF r i
  • 39. continuation:
    • Terminal Potential Difference ( TPD )
    • The output voltage of a source of emf after internal resistance takes effect.
    • The equation used to solve for terminal potential difference is:
    • TPD = E – Ir i
    • Where:
      • TPD = voltage across the source (V)
      • E = voltage if the source is ideal emf (V)
      • r i = internal resistance of the source (Ω)
      • I = current flowing through the battery (A)
  • 40. Sample Problem:
    • A 6.0 V battery is connected to an external 6.0 0hms resistor.
      • What is the value of the current flowing through the external circuit if there is no internal resistance,
      • What is the value of the current flowing through the external circuit when the internal resistance is 0.3 ohms?
  • 41. Resistors in Single Loop Circuit
  • 42.
      • Where: R is resistance, I is electric current and V is electric potential difference.
    Resistors in Series Circuit. R 3 V T I T + - + R 2 + R 1 + - - - R T
  • 43. Equivalent resistance in a Series Circuit
  • 44. Sample problem:
    • Resistors R 1 = 2.00 ohms, R 2 = 3.00 ohms and R 3 = 4.00 ohms are in series connection with a voltage source of 100.0 volts. Find the equivalent resistance, electric current and electric potential difference.
  • 45.
    • Resistor in Parallel Circuit
    R 3 V T I T + - + R 2 + R 1 + - - - R T I 3 I 2 I 1
  • 46. Equivalent resistance in a Parallel Circuit
  • 47. Sample problem:
    • Resistors R 1 = 3.00 ohms, R 2 = 5.00 ohms and R 3 = 7.00 ohms are in parallel connection with a voltage source of 110.0 volts. Find the equivalent resistance, electric current and electric potential difference.
  • 48. Resistors in Single Loop Circuit
    • Resistor in Series-Parallel Circuit
    R 3 V T I T + - + R 2 + R 1 + - - - R T
  • 49. POWER IN CIRCUITS
  • 50. The Power in the Circuits
    • Flow of current across a circuit.
  • 51. continuation:
    • Movement of a charge across a electric device:
      • It moves from higher potential to lower potential.
        • Hence, there is a decrease in potential energy.
    Q
  • 52. continuation:
    • If there is a decrease in potential energy, there must be a transmission to another form of energy.
        • Light bulb: to heat and light.
        • Electric motor: to mechanical energy
        • Resistor: Internal energy/heat.
  • 53. continuation:
    • The rate at which electric potential energy is transformed to another form of energy is the POWER in the circuit.
  • 54. Sample Problem:
    • A current flowing through a 25.0 ohm resistor is 2.0 A. How much power is dissipated by the resistor.
  • 55. MULTILOOP CIRCUIT
    • Provides multiple paths for current.
    • When one component was cut-off, others can still function.
  • 56. What happen when one component in a series circuit was cut-off?
  • 57. What happen when one component in a multiloop circuit was cut-off?
  • 58. continuation:
    • Current in a Multiloop Circuit
      • The point where three or more segments of the conductor meet is called the junction.
      • The current split at the junction.
    Junction current
  • 59. GUSTAV KIRCHHOFF
    • German physicist who, in the collaboration with Robert William Bunsen, develop ed the science of spectrum analysis.
    • He showed that each element, when heated to incandescence.
    • He produced a characteristic pattern of emission lines.
    • He formulated Kirchhoff’s Law for electric circuit.
    • (1824-1887)
  • 60.
    • In any closed circuit, the algebraic sum of all EMF’s and potential drop is equal to zero. (Using loop direction)
    KIRCHHOFF’S LAW R 2 + Emf 1 + - R 1 + Emf 2 + - R 3 + Loop 1 Loop 2 I 1 I 2 I 3 -
  • 61.
    • At any point in a circuit, the sum of the currents leaving the junction point is equal to the sum of all the current entering the junction point. (Using current direction).
    KIRCHHOFF’S LAW R 2 + ε 1 + - R 1 + ε 2 + - R 3 Junction point I 1 I 3 I 2 +
  • 62. Sample Problem:
    • In a given circuit below, Find: a) I 1 , b) I 2 and c) I 3
    10 Ω + 9v + - 15 Ω + 12v + - 5 Ω I 1 I 3 I 2 +
  • 63. RC CIRCUIT (Resistor and Capacitor in a circuit)
  • 64.
    • Resistor- Capacitor in a circuit.
    R + - C S 1 S 2 ε + - Where: ε = Batteries (Emf) S 1 & S 2 = Switches R = Resistor C = Capacitor Open Close
  • 65. Charging a capacitor R + - C S 1 S 2 ε + - I I I I I closed open Where: V R = Potential difference across the resistor. V C = Potential difference across the capacitor. I
  • 66. continuation
    • Current I O at the moment S 1 closed ( t = 0)
    • Current I at any time t after S 1 closed:
    • After some time t
      • The charge of the capacitor (q) increases
      • Current ( I ) decreases.
  • 67. continuation
    • Until the capacitor reaches its equilibrium charge (q eq ), happen when V C reaches V C = ε , which result to I = 0
  • 68. continuation
    • Charge and current of the capacitor at any given time t after t = 0.
  • 69.
    • The time constant ( τ ) of RC series circuit.
    • The unit of time constant is second.
    • At time t = τ
      • Q = 0.63 C ε
      • I = 0.37 I o
    • The charging time of RC circuits are often stated in terms of time constant.
    continuation
  • 70. Sample Problem:
    • A resistor with resistance R=1.0 x 10 6 Ω , capacitor with capacitance C=2.2 x 10 -6 F, a voltage source with ε = 100 v, and a switch are all connected in a single loop series circuit. The switch is initially open. When the switch is closed, calculate:
          • Initial current across the resistor
          • Equilibrium charge of the capacitor
          • Time constant of the circuit
          • Current through the resistor after 5 seconds
          • Charge of the capacitor after 5 second
          • Charge of the capacitor at t = τ