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15.2 Electric Current, Resistance and Voltage

15.2 Electric Current, Resistance and Voltage. pp. 346 - 349 Mr. Richter. Agenda. Warm-Up Check HW Introduction to Current Notes: Charge and Current The Source of Current (Voltage) Resistance (Conductors and Insulators). Objectives: We Will Be Able To….

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15.2 Electric Current, Resistance and Voltage

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  1. 15.2 Electric Current, Resistance and Voltage pp. 346 - 349 Mr. Richter

  2. Agenda • Warm-Up • Check HW • Introduction to Current • Notes: • Charge and Current • The Source of Current (Voltage) • Resistance (Conductors and Insulators)

  3. Objectives: We Will Be Able To… • Describe the relationship between electrons and current. • Explain, at the atomic level, the difference between insulators, semi-conductors and conductors. • Identify how voltage and charge are related.

  4. Warm-Up: • What would cause electrons to flow in a wire? • Discuss at your table and we will discuss as a class in a minute.

  5. Current

  6. Current • Electric current is the flow of charge. • Current is measured in amperes (A). Abbreviated: amps. • One amp of current is the equivalent of one coulomb of charge passing by in one second. • Or roughly 6.24 x1018 electrons per second! • Current is a measure of how many electrons pass by a point every second.

  7. The Direction of Current • Benjamin Franklin defined current as going from positive to negative (just like everything else). • This is before we discovered that electrons have a negative charge. Electrons actually flow away from negative to positive.

  8. The Direction of Current • We still say current flows from positive to negative, even though it’s the electrons that really move. • It doesn’t matter if it is electrons or protons moving. Usually the direction of current does not matter.

  9. The Source of Current Voltage

  10. The Source of Current • In metals (and some other materials) electrons are shared between atoms. • Electrons can move freely from one atom to another, and they are always moving randomly.

  11. The Source of Current (p. 347) • Because electrons move randomly, there is usually no current. • The average movements of electrons cancel each other out.

  12. The Source of Current (p. 347) • Electrons will only flow in one direction if there is something to push them. • When a voltage source is applied across a wire, electrons drift toward the positive end, creating current.

  13. Voltage (potential) • Voltage potential measures the energy per unit of charge. • How much energy does each electron have? How much work can each electron do? • Voltage is measured in volts (V). • One volt = 1 joule/coulomb

  14. Video Wrap-Up! • https://www.youtube.com/watch?v=1xPjES-sHwg

  15. Putting Voltage and Current Together • A small light bulb that draws 3 A of current is connected to a 1.5 volt battery for 10 seconds. • How much charge passes through the light bulb? • 3 A = 3 coulombs of charge per second • 3 A * 10 seconds = 30 C of charge • How much energy does the light bulb use? • 1.5 volts = 1.5 joules for every coulomb of charge • 1.5 V * 30 C = 45 joules

  16. Warm-Up: Day 2 • 60 coulombs of charge pass through a light bulb in 2.5 seconds. What is the current passing through the light bulb? • How much total energy does a 12 V battery supply to 6 C of charge?

  17. Where Current Comes From

  18. Where Current Comes From • If the wire is connected to a battery, electrons still bounce randomly, but… • Also slightly in the direction of the positive terminal of the battery. • This happens very slowly (a few mm per second) and is called drift velocity. • see p347

  19. Where Current Comes From • If electrons move so slowly, why doesn’t it take a long time for lights to turn on? • There are already electrons in the wires. The electrons from the battery just push those along. • This happens immediately, so the light turns on right away.

  20. Resistance

  21. Resistance • What does resistance mean? • Resistance is the measure of how strongly an object inhibits the flow of electric current. • High resistance = low current, few electrons flow • Low resistance = high current, many electrons flow.

  22. Resistance of Certain Materials • Just like with heat energy, some materials are better at transferring electrical charge than others. • Most materials can be classified as: • conductors • insulators • semiconductors

  23. Conductors • Conductors are materials that allow the flow of electrons freely. • Current flows through conductors easily. • Electrons in conductors are free to bounce around. • Examples: • metals: copper, gold, silver, etc… • liquids: water, salt water

  24. Insulators • Insulators are materials that inhibit the flow of electrons. • Current does not flow through insulators. Excess charge remains on the surface. • Electrons in insulators are tightly bound and cannot move. • Examples: • air, wood, glass, rubber • mostly the same as thermal insulators

  25. Semiconductors • Semiconductors are between conductors and insulators. • They transfer electrons better than insulators, but not as well as conductors. • Semiconductors can be use to control the flow of electrons. Mostly used in complex electronics like computers, cell phones and televisions. • Examples: • silicon, carbon, germanium

  26. Wrap-Up: Did we meet our objectives? • Describe the relationship between electrons and current. • Explain, at the atomic level, the difference between insulators, semi-conductors and conductors. • Identify how voltage and charge are related.

  27. Homework • p 356-357 15.2 Reviewing Concepts • #16-21

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