Flashlights - Lecture Slides - How Things Work | PHYS 140, Study notes of Classical Physics

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10.3 Flashlights10.3 Flashlights

Observations About FlashlightsObservations About Flashlights^ • They turn on and off with a switch• More batteries usually means brighter• Orientation of multiple batteries matters• Flashlights dim as batteries age

A BatteryA Battery • Battery “pumps” electrons from – end to + end

(outside of the battery)

• Chemical potential energy is consumed– Electrostatic potential energy is produced
  • In the battery:

The 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

A Simple CircuitA 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

Clicker Question:Clicker Question: If you remove the 2 batteries from a workingflashlight and reinstall them backward so thatthey make good contact inside, will the flashlightstill work?

A. YesB. No

The most confusingThe most confusing

thing about circuits^ thing about circuits

• Current is

charges moving

• Really, negative charges move in the

opposite direction

• We say that current

flows in the directionof movement ofpositive charge

CurrentCurrent

• 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

E +

Circuits 1Circuits 1

• Steady current requires a circuit path (loop)
  • Charge mustn’t accumulate anywhere– 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

Voltages Add:Voltages Add:

Short CircuitsShort 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!

Clicker Question:Clicker Question: Why can a bird safely stand on a

power line?

A.

The bird does not make acomplete circuit with the powerline – it would have to have eachleg on different lines.

B.

Power lines are insulated.

C.

The resistance of the bird ismuch higher than the powerline.