Flashlights 1 Flashlights 2 Observations about Flashlights Observations about Flashlights � You turn them on and off with switches You turn them on and off with switches Flashlights Flashlights � Brighter flashlights usually have more batteries Brighter flashlights usually have more batteries � Flashlights grow dimmer as their batteries age Flashlights grow dimmer as their batteries age � Sometimes smacking a flashlight brightens it Sometimes smacking a flashlight brightens it Flashlights 3 Flashlights 4 6 Questions about Flashlights 6 Questions about Flashlights Question 1 Question 1 � Why do flashlights need batteries and bulbs? Why do flashlights need batteries and bulbs? � Why do flashlights need batteries and bulbs? Why do flashlights need batteries and bulbs? � How does power flow from batteries to bulbs? How does power flow from batteries to bulbs? � How does a flashlight’s switch turn it on or off? How does a flashlight’s switch turn it on or off? � 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 lightbulbs? voltage lightbulbs? Flashlights 5 Flashlights 6 What Batteries Do What Batteries Do What Lightbulbs Do What Lightbulbs Do � Batteries provide flashlights with electric power Batteries provide flashlights with electric power � Bulbs turn electric power into light power Bulbs turn electric power into light power � A battery “pumps” charges from A battery “pumps” charges from – – to + to + � A bulb lets charges flow through its filament, A bulb lets charges flow through its filament, � Decreases the battery’s chemical potential energy Decreases the battery’s chemical potential energy � Decreases the charges’ electrostatic potential energy Decreases the charges’ electrostatic 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 � Produces thermal energy, including light. P Produces thermal energy, including light. P d d h h l l i i l di l di li h li h � Those charges undergo a rise in voltage: Those charges undergo a rise in voltage: � Those charges undergo a drop in voltage Those charges undergo a drop in voltage � 1.5 volts in a typical alkaline cell, 1.5 volts in a typical alkaline cell, � while passing through the bulb’s tungsten filament while passing through the bulb’s tungsten filament � 3.0 volts or more in a lithium cell, 3.0 volts or more in a lithium cell, � because thin tungsten wire is a poor conductor. because thin tungsten wire is a poor conductor. � 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 drop is 3.0 V cell alkaline flashlight, the drop is 3.0 V � 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 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 Batteries provide electric power B B i i id id l l i i � Lightbulbs Lightbulbs consume electric power consume electric power Flashlights 9 Flashlights 10 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 Batteries provide power to electric currents B i i id id l l i i � thr � through a wire (the outgoing current path) 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. Flashlights 11 Flashlights 12 How a Battery Works How a Battery Works How a Lightbulb How a 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 2
Flashlights 13 Flashlights 14 Question 3 Question 3 Circuits and Flashlights Circuits and Flashlights � How does a flashlight’s switch turn it on or off? How does a flashlight’s switch turn it on or off? � Steady current requires a circuit or loop path Steady current requires a circuit or loop path � 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 In a flashlight, the electric circuit is I 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 Flashlights 15 Flashlights 16 Question 4 Question 4 Recharging a Battery (Part 1) Recharging a Battery (Part 1) � How can a battery be recharged? How can a battery be recharged? � While a battery discharges: While a battery discharges: � Current flows forward, from Current flows forward, from – – end to + end. end to + end. � Current experiences a voltage rise Current experiences a voltage rise � Charges’ electrostatic potential energy increases � Ch r Ch r Charges’ electrostatic potential energy increases ’ l ’ l tr tr t ti p t nti l n r t ti p t nti l n r in r in r � Battery’s chemical potential energy decreases Battery’s chemical potential energy decreases Flashlights 17 Flashlights 18 Recharging a Battery Recharging a Battery (Part 2) The Direction of Current The Direction of Current (Part 2) � While a battery recharges: While a battery recharges: � Current is defined as the flow of positive charge Current is defined as the flow of positive charge � Current flows backward, from + end to Current flows backward, from + end to – – end. end. � but negative charges (electrons) carry most currents. but negative charges (electrons) carry most currents. � Current experiences a voltage drop Current experiences a voltage drop � It’s difficult to distinguish between: It’s difficult to distinguish between: � Charges’ electrostatic potential energy decreases � Ch r Ch r Charges’ electrostatic potential energy decreases ’ l ’ l tr tr t ti p t nti l n r t ti p t nti l n r d d r r � Negative charges flowing to the right N Negative charges flowing to the right N i i h h fl fl i i h h i h i h � Battery’s chemical potential energy increases Battery’s chemical potential energy increases � Positive charges flowing to the left. Positive charges flowing to the left. � We pretend that current is flow of + charges, We pretend that current is flow of + charges, � although it’s really although it’s really – – charges flowing the other way. charges flowing the other way. 3
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