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Current Electricity

Current Electricity. ?. Electricity. What is it ?. Electrons. E L E C T R I C C U R R E N T. Positive Ions. Negative Ions. A F l o w o f C h a r g e. Positive Holes. Current ( positive charge ) flows from POSITIVE to NEGATIVE. +ve. -ve. +. +. +. +. +. +. +. +.

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Current Electricity

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  1. Current Electricity

  2. ? Electricity What is it ?

  3. Electrons E L E C T R I C C U R R E N T Positive Ions Negative Ions A F l o w o f C h a r g e Positive Holes

  4. Current (positive charge) flows from POSITIVE to NEGATIVE +ve -ve + + + + + + + + Charge Q is measured in Coulombs [ symbol C ] Current I is measured in Amps [ symbol A] CURRENT = RATE OF FLOW OF CHARGE

  5. + - I I Conventional flow of Current in a Circuit

  6. Conventional Current - I Electrons flow from NEGATIVE to POSITIVE -ve +ve e-

  7. CURRENT = THE CHARGE FLOWING + + + PER SECOND AMPS = COULOMBS per SECOND AMPS = COULOMBS per SECOND

  8. V cold hot COLD COLD REQUIREMENTS FOR A CURRENT TO FLOW 1. A Conducting Path 2. A Source of Potential Difference – p.d. i.e. a source of VOLTS - V A source of volts is referred to as an e.m.f. …… an Electro Motive Force source e.g. Cell [battery], Generator or Thermocouple

  9. RELATIONSHIP BETWEEN I and V Applying a Potential Difference between two points on a conductor produces a current Over a limited range of V, the current is sometimes proportional to the voltage. V = I R R is the Resistance in the circuit In Ohms (Ω)

  10. V I R V = I R Resistance can be thought of as a proportionality constant between I and V if Ohm’s Law applies. It is also the opposition to the flow of current. OHM’s LAW “The current through an ohmic conductor is directly proportional to the potential difference across it, provided there is no change in physical conditions e.g. temperature.”

  11. Gradient Ohmic and non-ohmic conductors I Ohmic eg resistor or a piece of wire -V V -I

  12. switch on pd for silicon = 0.6V Ohmic and non-ohmic conductors I Non-ohmic eg diode -V V reverse bias forward bias -I

  13. Ohmic and non-ohmic conductors I Non-ohmic eg filament bulb -V V -I

  14. The ResistorColour Code Colour Number Black 0 Circuit symbol R Brown 1 Red 2 Orange 3 Yellow 4 Green 5 Blue 6 Violet 7 Grey 8 White 9 RESISTORS Colour codes are used to identify resistance value The four colour code bands are at one end of the component. Counting from the end, the first three (or sometimes four or five) bands give the resistance value and the last the tolerance

  15. A A B A = ? 47 OOO ,1% Tolerance B = ? 47 OOO ,1% Tolerance

  16. l l A A Resistivity (r) How does resistance depend upon length? R µ l How does resistance depend upon cross sectional area? Rµ 1/A Units of r = ohm m (Wm) r depends on the type of material and temperature

  17. Resistance /  Temperature / 0C Variation of Resistance [resistivity] with temperature insulator semiconductor metal Metal – more ion vibration impedes electrons Semiconductor – more ion vibration outweighed by more charge carriers Insulator – thermal energy releases more charge carriers

  18. Power, Voltage and Current The Current indicates how many Coulombsflow eachSecond The Voltage indicates how much energy each coulomb carries The Energy carried is measured in Joules So work done W = Q V ( = I t V )Joules The energy carried per second is thePOWER Thepower(injoules per second) = coulombs per second x joules per coulomb Current (I) Voltage (V) Ajoule per secondis called awatt ( W ) P = I V P = I V

  19. PLUGS AND FUSES 3 A 5 A 13 A live neutral earth Light Sound Heat Motion 720-3000W up to 720W

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