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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