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

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 Lemon battery

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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 Ramp with nails

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

Voltages Add:

The most confusing thing about circuits 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

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 Flashlight

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

current

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

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

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

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!

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 · current V = I × R where resistance is a constant for the device Ohm’s Law (V=I x R) Ramp with nails

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

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 = I 2 x R : Higher current means more power (for fixed R)!

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 ) Current (I): units of charge passed /sec Voltage (V) drop: energy taken /unit charge power received = current x voltage drop (P = I × V) Load Power (P)

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 = I 2 R

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