Current Resistance & Power


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Current Resistance & Power

  1. 1. Current, Resistance & Power March 11th, 2009
  2. 2. Current (I) <ul><li>The rate at which electrical charges move through a given area </li></ul><ul><li>Exists whenever there is a net movement of charge through a medium </li></ul><ul><ul><li>I=  q/  t </li></ul></ul><ul><ul><li>Unit: Ampere (A) 1A= 1C/s </li></ul></ul><ul><li>The current in a light bulb is 0.835 A. How long does it take for a total of 1.67 C of charge to pass a point in the wire? </li></ul>
  3. 3. Power <ul><li>Power = Work done / time =  E / t </li></ul><ul><li>Unit: Watt/Wattage 1 W = 1J/s </li></ul><ul><li>Example: a 60 watt light bulb consumes 60 J of energy each second </li></ul><ul><li>**Power companies do not supply electrons!! They provide a potential difference which causes the electrons to gain energy** </li></ul><ul><ul><li>Remember, moving a 1 C charge (6.24 x 10 18 e) across a potential difference of 12 V gives that charge 12 J of energy </li></ul></ul>
  4. 4. The current and power equations can be combined to form a new equation P=  V * I <ul><li>Determine the ... </li></ul><ul><li>... current in a 60-watt bulb plugged into a 120-volt outlet. </li></ul><ul><li>... current in a 120-watt bulb plugged into a 120-volt outlet. </li></ul><ul><li>... power of a saw that draws 12 amps of current when plugged into a 120-volt outlet. </li></ul><ul><li>... power of a toaster that draws 6 amps of current when plugged into a 120-volt outlet. </li></ul><ul><li>... current in a 1000-watt microwave when plugged into a 120-volt outlet. </li></ul>
  5. 5. Sources & Types of Current <ul><li>Batteries and generators supply energy to charge carriers </li></ul><ul><ul><li>They maintain a potential difference across their terminals by converting other forms of energy into electrical energy </li></ul></ul><ul><li>Direct Current </li></ul><ul><ul><li>Charge moves in only one direction </li></ul></ul><ul><li>Alternating Current </li></ul><ul><ul><li>Motion of charges continuously changes (moves forwards and reverse) </li></ul></ul><ul><ul><ul><li>Because of this the electrons end up vibrating in place </li></ul></ul></ul>
  6. 6. Resistance <ul><li>The opposition to the flow of current in a conductor </li></ul><ul><ul><li>R =  V/I </li></ul></ul><ul><ul><li>Unit: Ohm (  ) </li></ul></ul><ul><ul><li>Resistance is constant over a wide range of applied potential differences </li></ul></ul><ul><ul><li>Resistance depends on length, cross-sectional area, material and temperature (see overhead) </li></ul></ul>
  7. 7. Electrons do not move in straight line paths <ul><li>Resistance in a material </li></ul><ul><li>means the electron is not able </li></ul><ul><li>to move as quickly </li></ul><ul><li>Think about it like water in a pipe: </li></ul><ul><li>Longer wire (and pipes) means that the is a greater chance for collisions with other particles </li></ul><ul><li>Wider wire (and pipes) allow the particles to move more freely </li></ul>
  8. 8. Practice! <ul><li>The resistance of a steam iron is 19.0  . What is the current in the iron when it is connected across a potential difference of 120 V? </li></ul>
  9. 9. How to read a resistor <ul><li>The top resistor </li></ul><ul><li>= 1 1 0 00 = 11000  </li></ul><ul><li>The bottom resistor </li></ul><ul><li>= 1 0 00 = 1000  </li></ul>
  10. 10. Ohm’s Law <ul><li>Most important law in electricity </li></ul><ul><ul><li>V= IR </li></ul></ul><ul><li>Can use this to determine things about a circuit </li></ul><ul><ul><li>As voltage increases the current will increase </li></ul></ul><ul><ul><li>As resistance increases the current will decrease </li></ul></ul><ul><ul><li>Current is highest with a high voltage and low resistance </li></ul></ul>
  11. 11. Summary of Terms, Equations and Units W*s or V*C Joule (J)  PE=  V*q  PE Energy V/A Ohm (  ) R=  V/I R Resistance J/s Watt (W) P=  PE/t P Power C/s Amperes (A) I=q/t I=  V/R I Current J/C Volt (V)  V=PE/q  V=IR  V Potential Difference (Voltage) Other Units Standard Unit Equation Symbol Quantity
  12. 12. Activity <ul><li>For each set 1-4 do A and B (8 total Resisters) </li></ul><ul><ul><li>Use the guide to find out the resistance of each resistor </li></ul></ul><ul><li>For each power supply </li></ul><ul><ul><li>Find the current, and voltage for INPUT & OUTPUT </li></ul></ul><ul><ul><li>Calculate the resistance for INPUT & OUTPUT </li></ul></ul>