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How do Flashlights Work ?

How do Flashlights Work ?. Observations About Flashlights. Turned on/off with a switch The more batteries, the brighter the filament The orientation of multiple batteries matter Flashlights get dimmer as batteries age. Main Parts of a Flashlight. Battery – the ‘energy source’

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How do Flashlights Work ?

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  1. How do Flashlights Work ?

  2. Observations About Flashlights • Turned on/off with a switch • The more batteries, the brighter the filament • The orientation of multiple batteries matter • Flashlights get dimmer as batteries age Main Parts of a Flashlight • Battery – the ‘energy source’ • Wire – path for electrical charges to flow thru (current) • Light Bulb – the ‘load’ of your circuit • Switch – opens or closes the ‘circuit’ thru which • current flows.

  3. + - The Battery • Battery ‘pumps’ charge from – end to + • Chemical potential energy is consumed • Produce eletrostatic potential energy • Current undergoes a rise in voltage • alkaline cells: 1.5 volts; lead-acid: 2.0 volts; lithium: 3.0 volts • Sequence of Batteries produce larger voltage rise

  4. The Light Bulb • Contains filament which poorly • conducts current (high Resistance • to produce lots of heat) • Consumes electrostatic potential energy • and transforms it to heat/thermal energy. • Across the Bulb, the current undergoes a ‘drop in voltage’ • i.e. energy is lost to thermal energy. For a 2-cell alkaline • flashlight, there is a 3.0 volt drop.

  5. Switch + - C 1.5 volts D B A Current I Ground The Closed Loop of Current A : The voltage of the charges is 1.5 volts, relative to ‘Ground’ B : Voltage is still 1.5 volts (assuming perfectly conducting wires) C : Voltage is zero D: Voltage is zero Battery resupplies energy to charges when they cross the battery

  6. Observations: • Steady current requires a circuit path or loop, i.e. • Charges cannot accumulate anywhere. • Steady current flow requires energy replenishment • Where do charges lose this energy ? • - thru heat in conductors ! • Circuit transports energy from battery to the load: • a light bulb

  7. Why does a battery die and the bulb dims ? • In the Forward Current Flow (Discharging) • battery pumps charge from – to + end • battery’s chemical potential energy is consumed • In the Reverse Current Flow (Charging) • Circuit pushes charges from + to – • battery’s chemical potential energy is replenished • works for Ni-MH (nickel metal hidride batteries) + + - - Current I Current I

  8. What Really Flows in a Circuit ? By Convention, current points in the direction of positive flow. But….what really flows are Negatively-charged Electrons. - Negative Charges flowing to the right = Positive Charges flowing to the left + We stick to the convention but remind ourselves of the real flow of electrons.

  9. Switch + - C 1.5 volts D B A Current I Ground What’s a Short Circuit ? What happens if the circuit bypasses the load ? A ‘short’ occurs. Energy Loss and heating occurs in the wires! Main ingredient for a fire !

  10. The Power delivered by a Battery Power = energy delivered per unit time, units of Joules/sec or Watts Power = (Current supplied by Battery)(Voltage rise across battery) = I  V The Power consumed by a Load Power = (Current thru load)(voltage drop across load) = I  V

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