Flashlights 1 Flashlights 2 Introductory Question Introductory Question A flashlight may use a lightbulb A flashlight may use a lightbulb or an LED. If or an LED. If � you reverse the 2 batteries in a working you reverse the 2 batteries in a working Flashlights Flashlights flashlight and they again make good contact flashlight and they again make good contact inside, will the flashlight still work? inside will the flashlight still work? inside will the flashlight still work? inside, will the flashlight still work? Lightbulb Lightbulb : Yes, LED: Yes : Yes, LED: Yes A. A. Lightbulb Lightbulb : Yes, LED: No : Yes, LED: No B. B. Lightbulb Lightbulb : No, LED: Yes : No, LED: Yes C. C. Lightbulb Lightbulb : No, LED: No : No, LED: No D. D. Flashlights 3 Flashlights 4 Observations about Flashlights Observations about Flashlights 6 Questions about Flashlights 6 Questions about Flashlights � You turn them on and off with switches You turn them on and off with switches � Why do flashlights need batteries and bulbs? Why do flashlights need batteries and bulbs? � Brighter flashlights usually have more batteries Brighter flashlights usually have more batteries � How does power flow from batteries to bulbs? How does power flow from batteries to bulbs? � Flashlights grow dimmer as their batteries age Flashlights grow dimmer as their batteries age � How does a flashlight’s switch turn it on or off? How does a flashlight’s switch turn it on or off? � � Sometimes smacking a flashlight brightens it Sometimes smacking a flashlight brightens it � How can a battery be recharged? How can a battery be recharged? � Why does a short Why does a short- -circuited flashlight get hot? circuited flashlight get hot? � What distinguishes different What distinguishes different- -voltage voltage lightbulbs lightbulbs? ? Flashlights 5 Flashlights 6 Question 1 Question 1 What Batteries Do What Batteries Do � Batteries provide flashlights with electric power Batteries provide flashlights with electric power � Why do flashlights need batteries and bulbs? Why do flashlights need batteries and bulbs? � � A battery “pumps” charges from A battery “pumps” charges from – – to + to + � Decreases the battery’s chemical potential energy Decreases the battery’s chemical potential energy � Increases the charges’ electrostatic potential energy � Increases the charges electrostatic potential energy Increases the charges’ electrostatic potential energy Increases the charges electrostatic potential energy � Those charges undergo a rise in voltage: Those charges undergo a rise in voltage: � 1.5 volts in a typical alkaline cell, 1.5 volts in a typical alkaline cell, � 3.0 volts or more in a lithium cell, 3.0 volts or more in a lithium cell, � and of even more in a chain of cells. and of even more in a chain of cells. � In a two In a two- -cell alkaline flashlight, the rise is 3.0 V cell alkaline flashlight, the rise is 3.0 V 1
Flashlights 7 Flashlights 8 What Lightbulbs Do What Lightbulbs Do Clicker Question Clicker Question � Lightbulbs Lightbulbs turn electric power into light power turn electric power into light power � In a flashlight, power is transferred from batteries In a flashlight, power is transferred from batteries to to lightbulb lightbulb. Why can’t the power go the other . Why can’t the power go the other � A bulb lets charges flow through its filament, A bulb lets charges flow through its filament, way, from lightbulb way, from lightbulb to batteries? to batteries? � Decreases the charges’ electrostatic potential energy Decreases the charges’ electrostatic potential energy � Produces thermal energy, including light. Produces thermal energy, including light. P P d d h h l l i i l di l di li h li h A. Power can only flow forward in the flashlight Power can only flow forward in the flashlight � Those charges undergo a drop in voltage Those charges undergo a drop in voltage A. B. The The lightbulb lightbulb consumes the electric charges, too consumes the electric charges, too � while passing through the bulb’s tungsten filament while passing through the bulb’s tungsten filament B. � because thin tungsten wire is a poor conductor. because thin tungsten wire is a poor conductor. C. Disordered energy can’t become ordered energy Disordered energy can’t become ordered energy C. � In a two In a two- -cell alkaline flashlight, the drop is 3.0 V cell alkaline flashlight, the drop is 3.0 V D. Current is actually negatively charged electrons Current is actually negatively charged electrons D. Flashlights 9 Flashlights 10 Question 2 Question 2 Electric Power Electric Power � How does power flow from batteries to bulbs? How does power flow from batteries to bulbs? � Electric power is the rate of energy transfer, Electric power is the rate of energy transfer, � the electric energy transferred per unit of time, the electric energy transferred per unit of time, � and is measured in watts (i.e., joules/second). and is measured in watts (i.e., joules/second). � Batteries provide electric power B B Batteries provide electric power i i id id l l i i � Lightbulbs Lightbulbs consume electric power consume electric power Flashlights 11 Flashlights 12 Electric Current Electric Current Electric Current in a Flashlight Electric Current in a Flashlight � Electric current is the rate of charge transfer, Electric current is the rate of charge transfer, � In a flashlight, In a flashlight, � the electric charge passing a point per unit of time, the electric charge passing a point per unit of time, � an electric current carries power an electric current carries power � and is measured in amperes (i.e., coulombs/second). and is measured in amperes (i.e., coulombs/second). � from batteries (the energy source) from batteries (the energy source) � Batteries provide power to electric currents B B Batteries provide power to electric currents i i id id l l i i � through a wire (the outgoing current path) � thr thr through a wire (the outgoing current path) h h ir (th ir (th t t in in rr nt p th) rr nt p th) � to a to a lightbulb lightbulb filament (the energy destination), filament (the energy destination), � Lightbulbs Lightbulbs extract power from electric currents extract power from electric currents � and the current then returns through another wire and the current then returns through another wire � to the battery for reuse. to the battery for reuse. 2
Flashlights 13 Flashlights 14 How a Battery Works How a Battery Works How a How a Lightbulb Lightbulb Filament Works Filament Works � Normal electrical conductors are imperfect, Normal electrical conductors are imperfect, � A battery uses its chemicals to pump current A battery uses its chemicals to pump current � so electric currents don’t coast through them; so electric currents don’t coast through them; � from its negative terminal to its positive terminal. from its negative terminal to its positive terminal. � electric fields are required to keep currents moving. electric fields are required to keep currents moving. � Current gains voltage in the battery Current gains voltage in the battery � � For a current to flow through a filament, � For a current to flow through a filament, For a current to flow through a filament, For a current to flow through a filament, � and the battery thus provides electric power. and the battery thus provides electric power. d h b d h b h h id id l l i i � the filament must have an electric field in it the filament must have an electric field in it power provided = current · voltage rise power provided = current · voltage rise � caused by a voltage drop and an associate gradient. caused by a voltage drop and an associate gradient. � Current loses voltage in the filament Current loses voltage in the filament � and the filament thus consumes electric power. and the filament thus consumes electric power. power consumed = current · voltage drop power consumed = current · voltage drop Flashlights 15 Flashlights 16 Clicker Question Clicker Question Question 3 Question 3 � If you move the bulb from a 2 D If you move the bulb from a 2 D- -cell flashlight cell flashlight � How does a flashlight’s switch turn it on or off? How does a flashlight’s switch turn it on or off? to a 4 D to a 4 D- -cell flashlight and turn that flashlight cell flashlight and turn that flashlight on, the bulb will on, the bulb will A. burn out almost instantly. burn out almost instantly. A. B. glow normally for a much longer time. glow normally for a much longer time. B. C. glow normally for a much shorter time. glow normally for a much shorter time. C. D. glow normally for about the same time. glow normally for about the same time. D. Flashlights 17 Flashlights 18 Circuits and Flashlights Circuits and Flashlights Question 4 Question 4 � Steady current requires a circuit or loop path Steady current requires a circuit or loop path � How can a battery be recharged? How can a battery be recharged? � because charge mustn’t accumulate anywhere because charge mustn’t accumulate anywhere � and a closed conducting loop avoids accumulation. and a closed conducting loop avoids accumulation. � In a flashlight, the electric circuit is I I In a flashlight, the electric circuit is fl hli h fl hli h h h l l i i i i i i i i � closed (complete) when you turn the switch on closed (complete) when you turn the switch on � open (incomplete) when you turn the switch off open (incomplete) when you turn the switch off 3
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