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10.3 Flashlights

10.3 Flashlights. Observations About Flashlights. They turn on and off with a switch More batteries usually means brighter Orientation of multiple batteries matters Flashlights dim as batteries age. Lemon battery. A Battery. Battery “pumps” electrons from + end to - end

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10.3 Flashlights

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  1. 10.3 Flashlights

  2. Observations About Flashlights • They turn on and off with a switch • More batteries usually means brighter • Orientation of multiple batteries matters • Flashlights dim as batteries age

  3. Lemon battery A Battery • Battery “pumps” electrons from + end to - end (inside the battery) • Chemical potential energy is consumed • Electrostatic potential energy is produced • In the battery:The current undergoes a rise in voltage • Alkaline cell: 1.5 volt rise • Lead-acid cell: 2.0 volt rise • Lithium cell: 3.0 volt rise • Chain of cells produces larger voltage rise

  4. Click me

  5. Ramp with nails A Light Bulb • Filament barely lets charge flow through it • Electrostatic potential energy is consumed • Thermal energy is produced • It’s a resistor! • Current undergoes a drop in voltage • Two-cell alkaline flashlight: 3.0 volt drop

  6. A Simple Circuit • A battery – the energy source • A wire – the outgoing current path • A light bulb – the energy destination (the load) • A wire – the return current path

  7. Voltages Add:

  8. The most confusing thing about circuits • Current is + charges moving • Really, negative charges move in the opposite direction • We say that current flows in the direction of movement of positive charge

  9. Flashlight Clicker Question: If you remove the 2 batteries from a working flashlight and reinstall them backward so that they make good contact inside, will the flashlight still work? A. Yes B. No

  10. + + + + + Current • Current measures the electric charge passing through a region per unit of time • Current is measured in coulombs/second or amperes (amps) • Electric fields cause currents to flow E

  11. current current current

  12. Resistance of materials Resistance of copper Resistance and temp. Coil, bulb, LN2

  13. Circuits 1 • Steady current requires a circuit path (loop) • Charge mustn’t accumulate anywhere • Closed conducting loop avoids accumulation • Steady current flow requires energy • Currents lose energy (and voltage) in conductors • Missing energy becomes thermal energy • Lost energy must be replaced

  14. Circuits 2 • A circuit can transport energy • Current obtains energy from a battery • Current delivers energy to a light bulb • Current starts the trip over again

  15. Short Circuits • If a conducting path bridges the load • Current bypasses the load • Circuit is abbreviated or “short” • No appropriate energy destination (load) • Energy loss and heating occurs in the wires • A recipe for fires!

  16. Ohm’s Law (V=I x R) Ramp with nails • The currents passing through most wires and other devices experience voltage drops • In an “ohmic device,” the voltage drop is proportional the current: voltage drop = resistance · currentV = I × R where resistance is a constant for the device

  17. Series and parallel Fuse Click me

  18. Clicker Question: Why can a bird safely stand on a power line? • The bird does not make a complete circuit with the power line – it would have to have each leg on different lines. • Power lines are insulated. • The resistance of the bird is much higher than the power line.

  19. Power is • Energy per unit of time • Measured in joules/second or watts • Current times voltage in a circuit: P = I × V • Batteries are power sources • Loads are power consumers Note that since P = I x V and V = I x R,P = I2xR: Higher current means more power (for fixed R)!

  20. Battery Power (P) • Current (I): units of charge pumped /sec • Voltage (V) rise: energy /unit of charge • power produced = current x voltage rise (P = I × V ) Load Power (P) • Current (I): units of charge passed /sec • Voltage (V) drop: energy taken /unit charge • power received = current x voltage drop (P = I × V)

  21. Important ideas from this lecture Current is + charges moving; electrons move in the opposite direction A battery “pumps” charges by giving them energy A complete circuit requires a closed path; a switch opens and closes a path for current Electrons give up energy to a “load” (resistor) Ohm’s law: V = I R Power: P = I V or P = I2 R

  22. For next class: Read Section 11.1 See you next class!

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