Flashlights

Flashlights PowerPoint PPT Presentation


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Introductory 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?YesNo. Observations about Flashlights. They turn on and off with a switchMore batteries usually means brighterThe orie

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Flashlights

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1. Flashlights

2. Introductory 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? Yes No Demonstration: Flashlight with Batteries Reversed (But Turned Off)Demonstration: Flashlight with Batteries Reversed (But Turned Off)

3. Observations about Flashlights They turn on and off with a switch More batteries usually means brighter The orientation of multiple batteries matters Flashlights dim as batteries age Sometimes smacking a flashlight brightens it

4. A Battery Battery “pumps” charge from – end to + end Chemical potential energy is consumed Electrostatic potential energy is produced 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 Demonstration: Show Opened 9V BatteryDemonstration: Show Opened 9V Battery

5. A Lightbulb Structure Contains a protected tungsten filament Filament conducts electricity, but poorly Filament barely lets charge flow through it Electrostatic potential energy is consumed Thermal energy is produced Current undergoes a drop in voltage Two-cell alkaline flashlight: 3.0 volt drop Demonstration: Open A Light BulbDemonstration: Open A Light Bulb

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 Demonstration: Car Battery and Light Bulb CircuitDemonstration: Car Battery and Light Bulb Circuit

7. Circuits (Part 1) Steady current requires a circuit path (a loop) Charge mustn’t accumulate anywhere A 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 Demonstration: Open Circuit with Car Battery and Light Bulb Demonstration: Close Circuit with Car Battery and Light BulbDemonstration: Open Circuit with Car Battery and Light Bulb Demonstration: Close Circuit with Car Battery and Light Bulb

8. 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

9. Circuits (Part 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 Demonstration: Little Toy Trucks and Candy to Demonstrate “Energy” TransportDemonstration: Little Toy Trucks and Candy to Demonstrate “Energy” Transport

10. Introductory 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? Yes No Demonstration: Flashlight with Batteries Reversed (But Turned Off)Demonstration: Flashlight with Batteries Reversed (But Turned Off)

11. Recharging a Battery Forward (discharging) current flow Battery “pumps” charge from – end to + end Current undergoes voltage rise Battery’s chemical potential energy is consumed Reverse (recharging) current flow Circuit “pushes” charge from + end to – end Current undergoes voltage drop Battery’s chemical potential energy is replenished Demonstration: Battery, Current Display, Motor/Generator Demonstration: 3 Cell Flashlight with One Cell ReversedDemonstration: Battery, Current Display, Motor/Generator Demonstration: 3 Cell Flashlight with One Cell Reversed

12. Positive Charge Current points in the direction of positive flow Flow is really negative charges (electrons) It’s hard to distinguish between: negative charge flowing to the right positive charge flowing to the left We pretend that current is flow of + charges It’s really – charges flowing the other way Demonstration: Move Paper “Charges” on a Chalkboard WireDemonstration: Move Paper “Charges” on a Chalkboard Wire

13. 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! Demonstration: Hot Wire in Car Battery and Light Bulb Circuit with Shorting WireDemonstration: Hot Wire in Car Battery and Light Bulb Circuit with Shorting Wire

14. Power Power is energy per unit of time Power is measured in joules/second or watts Batteries are power sources Loads are power consumers

15. Battery Power Current: units of charge pumped per second Voltage rise: energy given per unit of charge current · voltage rise = power produced

16. Load Power Current is units of charge passed per second Voltage drop: energy taken per unit of charge current · voltage drop = power received

17. Ohm’s Law 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 where resistance is a constant for the device Demonstration: Ohm’s lawDemonstration: Ohm’s law

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