10.3 Flashlights10.3 Flashlights

Observations About Observations About FlashlightsFlashlights• 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 (inside 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

Lemon Lemon batterybattery

Click me

A Light BulbA Light Bulb• Filament barely lets charge flow through it

– Electrostatic potential energy is consumed– Thermal energy is produced– It’s a resistor!

• Current undergoes a drop in voltage– Two-cell alkaline flashlight: 3.0 volt drop

Ramp with Ramp with nailsnails

A Simple A Simple CircuitCircuit• 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

Voltages Add:Voltages Add:

The most confusingThe most confusing

thing about circuitsthing about circuits• Current is + charges moving• Really, negative charges move in the

opposite direction• We say that current flows in the direction of movement of positive charge

Clicker Question:Clicker 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?

A. YesB. No

FlashlightFlashlight

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

currentcu

rren

tcurrent

Resistance of materialsResistance of materialsResistance of Resistance of coppercopperResistance and Resistance and temp.temp.Coil, bulb, LN2Coil, bulb, LN2

Circuits 1 Circuits 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

Circuits 2Circuits 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

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!

• 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 V = I × R

where resistance is a constant for the device

Ohm’s Law (V=I x R)Ohm’s Law (V=I x R)Ramp with Ramp with nailsnails

Click me

Series and parallelSeries and parallel

FuseFuse

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

line?A. The bird does not make a complete

circuit with the power line – it would have to have each leg on different lines.

B. Power lines are insulated.C. The resistance of the bird is much

higher than the power line.

Power isPower is• Energy per unit of time• Measured in joules/second or watts• Current times voltage in a circuit:

P = I × V• Batteries are power sources• Loads are power consumers

Note that since P = I x V and V = I x R, P = I2 x R: Higher current means more power (for fixed R)!

Battery Power (P)Battery Power (P)• Current (I): units of charge pumped /sec• Voltage (V) rise: energy /unit of charge• power produced = current x voltage rise

(P = I × V )

• Current (I): units of charge passed /sec• Voltage (V) drop: energy taken /unit charge• power received = current x voltage drop (P = I × V)

Load Power (P)Load Power (P)

Important ideas from this lectureImportant ideas from this lecture• Current is + charges moving; electrons move in

the opposite direction• A battery “pumps” charges by giving them

energy• A complete circuit requires a closed path; a

switch opens and closes a path for current• Electrons give up energy to a “load” (resistor)• Ohm’s law: V = I R• Power: P = I V or P = I2 R

See you next class!

For next class: Read Section 11.1