Lecture #25Lecture #25

What’s in the wall??What’s in the wall??

Last TimeLast Time

We discussed how charge can be We discussed how charge can be studied.studied.

There are two kinds of chargeThere are two kinds of charge– NEGATIVENEGATIVE

electrons, light, mobile and easy to push aroundelectrons, light, mobile and easy to push around

– POSITIVEPOSITIVE protons, located in the nucleus of atome.protons, located in the nucleus of atome. ““heavy”, tough and not to be messed with.heavy”, tough and not to be messed with.

Charges interact with each other.Charges interact with each other.

Results of our StudyResults of our Study

Charges can be created by mechanical Charges can be created by mechanical rubbing or separation of bound rubbing or separation of bound surfaces.surfaces.

Charge cannot be created or destroyedCharge cannot be created or destroyed– It is there before we rubIt is there before we rub– All we mortals can do is move it aroundAll we mortals can do is move it around

Like charges repel each otherLike charges repel each other Un-Like charges attract each other.Un-Like charges attract each other.

So… what about this So… what about this balloon trick?balloon trick?

insulator insulator

A Van de Graaff generator

What about our Volunteer??What about our Volunteer??

This thing stores

charge

This thing also has the capacity This thing also has the capacity to store chargeto store charge

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This is called a capacitor

Charging a Capacitor takes WORK

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The work per unit charge is called the Potential Difference

CapacitorCapacitorseparatedseparated charge charge

POTENTIAL DIFFERENCEPOTENTIAL DIFFERENCE

Units are VOLTSUnits are VOLTS It takes one Joule of energy (work) to It takes one Joule of energy (work) to

move one coulomb through a move one coulomb through a potential difference of one volt.potential difference of one volt.

We usually refer to potential We usually refer to potential differences as “the voltage”differences as “the voltage”

Another DefinitionAnother Definition If a capacitor has a potential difference If a capacitor has a potential difference

V from one terminal (plate) to the otherV from one terminal (plate) to the other And if the capacitor has a charge Q on And if the capacitor has a charge Q on

the plate,the plate, Then Then

where C is measured in Farads. where C is measured in Farads.

V

QC

ExampleExample

A 0.001 F capacitor has 5 volts across it. A 0.001 F capacitor has 5 volts across it. How much charge is on the capacitor?How much charge is on the capacitor?

.005.05001.0 CoulvoltsFCVQ

soV

QC

Big Cap Demo

What’s Happening?What’s Happening?

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Huh?Huh?

The electrons were “pushed” by the The electrons were “pushed” by the difference in potentialdifference in potential (voltage). (voltage).

They came around to the (+) plate They came around to the (+) plate and and neutralizedneutralized the positive plate. the positive plate.

The two plates and the wires are said The two plates and the wires are said to form an to form an electrical circuitelectrical circuit..

The movement of charge is called The movement of charge is called electrical currentelectrical current..

Symbols & DefinitionsSymbols & Definitions ““Stand” at one point in a circuit where Stand” at one point in a circuit where

electrical current is flowing.electrical current is flowing. Count the amount of charge in coulombs that Count the amount of charge in coulombs that

pass where you are standing in one second.pass where you are standing in one second. This amount of charge, divided by the time of This amount of charge, divided by the time of

passage is defined as the CURRENT.passage is defined as the CURRENT. The current is measured in AMPERES and we The current is measured in AMPERES and we

use the symbol I.use the symbol I. One ampere is a current of one coulomb per One ampere is a current of one coulomb per

second.second.

The BatteryThe Battery

The battery behaves in a way like a The battery behaves in a way like a capacitor.capacitor.

As charge is withdrawn from one side As charge is withdrawn from one side of the battery the battery replaces it of the battery the battery replaces it via a chemical reaction in the battery via a chemical reaction in the battery cell.cell.

The battery therefore can maintain a The battery therefore can maintain a constant potential difference or constant potential difference or voltage between its terminals.voltage between its terminals.

Electricity made a giant leap in 1800

when Volta invented the

electric cell and the battery (of

cells).

For the first time investigators had

a source of steady electric

current. Alessandro Volta, (1745-1827)

Pretty Pretty ComplicateComplicatedd

Lots of sources onthe internet to findout how these thingsreally work.

The Commercial BatteryThe Commercial Battery

Something that we ALL Something that we ALL Know!Know!

The Circuit – A ThunkerThe Circuit – A Thunker

A

I1

I2

The Circuit – A ThunkerThe Circuit – A Thunker

A

Current must be the same!Current must be the same!

A IV

IRV

)Ohm (Unit I

VR

R Resistance Define

Ohm Sweet OhmOhm Sweet Ohm

The more resistance, the less the The more resistance, the less the current.current.

Like stepping on a garden hose.Like stepping on a garden hose. If you stepped in two spots on a garden If you stepped in two spots on a garden

hose, the flow would even be less.hose, the flow would even be less. The “resistance” would double.The “resistance” would double.

So let’s look at a few bulbs.So let’s look at a few bulbs.

ExampleExample

10 ohm6 volts

AR

VI

IRV

6.010

6

Add a BulbAdd a Bulb

B

C

6V10

10Total = 20

I=V/R=6/20=0.3 amperes … ½ the previous current

Parallel CircuitParallel Circuit