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Electric current

Electric current. Current is a flow of electrons AC current alternates back and forth usually at 60 Hz or 60 cycles per second DC current moves in one direction Current is caused by potential difference. Electric current. Current is calculate using I = q/t I is current in Amperes or amps

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Electric current

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Presentation Transcript

  1. Electric current • Current is a flow of electrons • AC current alternates back and forth usually at 60 Hz or 60 cycles per second • DC current moves in one direction • Current is caused by potential difference

  2. Electric current • Current is calculate using I = q/t • I is current in Amperes or amps • Q is charge in Coulombs • t is time in seconds • If 12 Coulombs of charge flow in 4 seconds, the current is 12/4 = 3 A • How much charge flows if a 1.5 amp circuit operates for 30 seconds?

  3. Electrical resistance • Wire and elements in a circuit offer resistance to flow of electrons • Thick wires have less resistance than thin wires • How do long and short wires compare? Why?

  4. Electrical resistance • Resistance is measured in Ohms which are volts per amp. • R = V/I or more commonly presented as V = IR • V = IR is known as Ohm’s law • An Ohm is often given the symbol Ω, which is the Greek letter Omega

  5. Electrical resistance • Resistors are devices that convert some flow of electrons into heat • They are used to reduce the flow of current to certain parts of a circuit • For example, a car may need a high voltage for headlights, a moderate amount for brake lights, and a tiny bit for the vanity mirror.

  6. Capacitance • A capacitor is used to provide a sudden discharge of current very quickly. • Capacitance is determined as the charge on the conductor divided by the voltage across them. • C = q/ V • Units are Coulombs per Volt or the Farad (F).

  7. Charging a capacitor • A capacitor is charged in a circuit with some resistance. • Initially, the voltage across the capacitor is 0, but increases to the voltage of the battery as time passes. • Initially, the current is very high, but approaches 0 as time passes. • How fast a circuit charges depends on the time constant calculated as RC.

  8. Charging a capacitor Notice how current decreases as charge is moved between plates. Why?

  9. Charging a capacitor Time constant = RC

  10. Diodes • Diodes are electronic devices that allow current to flow in one direction • Used to convert AC current to DC current

  11. AC to DC conversion • Accomplished with a pair of diodes and a capacitor • Diodes allow half of AC current through • Capacitor evens out the current by releasing current during periods of low flow

  12. Rectifier bridge From: http://www.teamdigital1.com/applications/tech_talk/pwr_supplies_files/bridge_rect.png

  13. Electrical symbols http://www.thelearningpit.com/hj/plcs_files/plcs-34.gif

  14. Power of a circuit element • Power is how much energy an object uses per second. • Recall that it was Joules/second, or Watts • In gravity, power was work/time. • In electricity, it is current x voltage • Current is Coulombs per second and voltage is Joules per Coulomb. What are units on power?

  15. Power of a circuit element • P = VI • Recall that V = IR • What happens when we substitute this V for the one in the power equation? • What happens when we solve for I and substitute into the power equation? • 3 forms: P = VI = I2R = V2/R

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