ISNS 3371 - Phenomena of Nature The final will be Tuesday, April 24 at 8:00 AM. NOTICE THE TIME! We will have two review sessions following the class on.

ISNS 3371 - Phenomena of Nature

The final will be Tuesday, April 24 at 8:00 AM. NOTICE THE TIME!We will have two review sessions following the class on Thursday in the lab at the far west end of Founders North. One from 11:00 - and the other from 2:00 - ?. The test will cover any topic we have discussed in class and will be approximately 40% comprehensive and 60% on what we have covered since the last test. Topics include:

Fundamental quantitiesMeasurement unitsVectors, ScalarsMotion - distance, velocity, acceleration, projectilesForceMass, momentum, impulseNewton’s Laws of MotionLaw of Conservation of MomentumTorque, angular momentum, conservation of angular momentumMatter - States of MatterEnergy, WorkLaw of Conservation of EnergyForms of Energy


Transformation of energyPowerGravity - force, acceleration, weight, weightlessnessHeat, TemperatureTransfer of heat - Conduction, ConvectionChange of state of matterGases - gas laws: Boyle, Charles, Guy Lusac, GeneralFirst Law of ThermodynamicsArchimedes’ PrincipleBernoulli EffectWaves, Properties of WavesStanding WavesSound WavesResonance, InterferencePitch vs. Frequency,Loudness vs. IntensityBeatsResonance in open and closed pipesHarmonicsDoppler Effect


Light as a wave, particlePhotoelectric effectElectromagnetic Spectrum - ColorLight interaction with matterRefraction, ReflectionScattering - Blue Sky - Red SunsetsRainbowsPolarizationLenses and mirrorsTelescopesSpectroscopy - emission and absorption spectraThermal radiation/black body spectrumDoppler effect - rotation, extrasolar planetsElectrical charge, voltage, current, powerElectrical circuits (parallel and series), resistanceOhm’s lawAC and DC currentMagnetic fields, moving charges, right-hand ruleInduction, Faraday’s lawMotors and generatorsLenz’s law


Voltage

Electric potential - change in electric potential is work done on charge by an electric (Coulomb) force.

Remember: work = force X distance (over which force is applied)

€

Voltage =Electric Potential

Charge

Analogous to gravitational potential energy: work done by the force of gravity in moving a mass through a certain distance

Units of voltage are Volts (Joules/Coulomb). In a nine volt battery the potential difference between the positive and negative terminals is precisely nine volts.


Current

Establish a potential difference between two points and release some charges - charges will be acted on by the electrical force and start to move - this is a electric current - a flow of electrical charges. Just like the flow of H2O is a water current.

A current is the amount of charge that flows by a given point in a certain time interval. Unit is Ampere (A = Coulombs/second)

In electrical circuits, consist of electrons in the wire moving from lower to higher electrical potential.

By convention, electrical current flows from positive to negative - electrons flow in opposite direction. In other words, a positive current flows from higher to lower electrical potential.

ISNS 3371 - Phenomena of NatureResistivity and Circuits

If a wire is connected between two poles of a battery, a current flows. This is a circuit. The electrons do not move completely freely - there is a resistance to their flow - has to do with number of collisions electrons make with other electrons and atoms. Amount of current dependent of resistivity. Unit of electrical resistance is Ohm ().

Insulator - high resistivityConductor - low resitivitySuperconductor - zero resistivity

Dependent on material, temperature, length of wire

Electrical circuit like hydraulic circuit with flow of water analagous to flow of electrons.

ISNS 3371 - Phenomena of Nature PowerPower is the rate of energy flow. Unit is Watt ( Watt = Joules/second)

Power = Voltage X Current - 1 W = 1 VA

Total amount of energy used = Watt X Time, This is how the power company charges us for electricity - by the kilowatt hour (kWh)

The power rating of an appliance tells us the rate at which it uses electrical energy. For example, a 1500 watt hair dryer uses 1500 joules of electrical energy per second.

Keep a 60 watt light bulb on for a full day:

uses 60 watts x 24 hours = 1440 watt hours = 1.44 kilowatt hours of energy. At 9 cents per kilowatt hour this would cost just under 15 cents.

kilowatt hour is a unit of energy - can convert it to joules:

1.44 kWh = 1440 J/s X 3600 s = 5,184,000 J


QUANTITY SYMBOL UNIT MEANING

Charge Q Coulomb (C)

A basic property of some elementary particles. The electron has a charge of -1.6 X 10-19 C

and the proton has a charge of +1.6 X 10-19 C

Current I Ampere

(A)

(1 A = 1 C/sec)

Rate of flow of charge.

Potential Difference (voltage)

V Volt (V)

(1 V = 1 J/C)

J= Joule

Potential energy difference per coulomb of charge between two points; corresponds to pressure in water flow.

Electrical Quantities


QUANTITY SYMBOL UNIT MEANING

Resistance R Ohm (Ω)

1Ω =

1 V/A

A measure of the opposition to the flow of charge in a particular circuit. For a given voltage, the higher the resistance, the lower the current.

Power P Watt (W)

(1 W =

1 VxA)

Rate of energy flow.


Ohm’s Law states that the amount of current in a circuit is proportional to the voltage across the circuit and inversely proportional to the resistance in that circuit

I=V/R

or

V = IR

Voltage = Current x Resistance

Ohm’s Law

ISNS 3371 - Phenomena of NatureElectrical Shock

“It’s not the voltage but the current.”

The current is what actually causes a shock - human body has resistance of ~500,000 with dry skin - ~100 wet! Requires conducting path.

Can cause: (1) burning of tissue by heating, (2) muscle contractions, (3) disruption of cardiac rhythms.

Current (A) Effect

0.001 Can be felt

0.005 Is painful

0.010 Causes spasms

0.015 Causes loss of muscle control

0.070 Goes through the heart - fatal after more than 1 second


Circuits in Series

•Resistance (light bulbs) on same path•Current has one pathway - same in every part of the circuit•Total resistance is sum of individual resistances along path•Current in circuit equal to voltage supplied divided by total resistance•Sum of voltages across each lamp equal to total voltage•One bulb burns out - circuit broken - other lamps will not light (think of

string of old Christmas lights)

ISNS 3371 - Phenomena of NatureWater Analogy for Series Circuits

ISNS 3371 - Phenomena of Nature Parallel Circuits

•Bulbs connected to same two points of electrical circuit•Voltage same across each bulb•Total current divides among the parallel branches - equals sum of current in each branch - current in each branch inversely proportional to resistance of branch•Overall resistance of circuit lowered with each additional branch•Household wiring (and new Christmas light strings) designed in parallel - too many electrical devices on - too much current - trip fuse/breaker

ISNS 3371 - Phenomena of NatureWater Analogy for Parallel Circuits


AC vs DC Current

Direct Current (DC) - electricity that you get from batteries - current (movement of electrons) flows in one direction - from positive (high potential) to negative (low potential) - note: electrons actually flow from negative to positive.

Alternating Current (AC) - electricity that comes from the wall plug in your home - current alternates in direction flowing first one way then the other - electrons move back and forth in wire


System of AC electricity in the US cycles the current from positive to negative and back to positive 60 times a second - 60 Hz.

Standard household wiring design has two 120 volt "hot" wires and a neutral which is at ground potential

- two 120 volt wires are obtained by grounding the centertap of the transformer supplying the house so that when one hot wire is swinging positive with respect to ground, the other is swinging negative- allows the use of either hot wire to supply the standard 120 volt household circuits.

Major advantage of AC over DC - AC voltages can be transformed to higher or lower voltages.

- high voltages used to send electricity over great distances from the power station can be reduced to a safer voltage for use in the house- higher voltage in transmission means less loss of power


Magnetism


The situation with forces due to magnetism is more complicated that electrical/Coulomb forces.

-despite numerous searches, no evidence of magnetic charges exist. In other words, there are no particles which create a radial magnetic field (usually termed monopoles by physicists) in the way an electric charge creates a radial field.

Regions called magnetic poles produce magnetic forces - a north magnetic pole never exists without a south magnetic pole - together called a dipole

Cutting a magnet in half will not isolate a single north or south. One magnet becomes two, then four, and so on.

As for charges - like poles repel and opposite poles attract.


Magnetic fields can be created by moving electric charges, i.e. currents produce fields.

Follows right-hand rule: point thumb of right hand in direction of current - magnetic field curls around wire in direction of curled fingers

If the current flows in a loop, the magnetic field produced is like a bar magnetic - curl fingers of right hand in direction of current flow - north pole is in direction of thumb.


The electron spins on its axis, giving rise to a electron current in the direction of rotation.

The electron is like a magnetic dipole, a miniature magnet, with a north end and a south end.

In most substances, electrons spin in random directions - magnetic fields cancel. For iron and other magnetic substances, the spin magnetism is not canceled. Can be permanently magnetized by placing in strong magnetic field and permanently aligning atoms - can be demagnetized by dropping magnet and jostling atoms out of alignment.

Electromagnetic produced by wrapping coil around iron bar - magnetic field produced that aligns atoms in bar - more coils or more current - larger magnetic field and greater atomic alignment


There is amazing reciprocity in nature - if moving electric charges create magnetic fields, then magnetic fields must exert forces on moving electric charges, just as electric charges create and are ``acted upon by'' magnetic fields. Force is perpendicular to magnetic field and direction of motion.

Leads to the induction of current in a wire moved through a magnetic field - or a magnetic field moved past a wire

Pass a magnet through a coil of wire - produce a current

Faraday’s Law - The induced voltage in a coil is proportional to the number of loops multiplied by the rate at which the magnetic field changes within those loops.

Induction

ISNS 3371 - Phenomena of Nature The final will be Tuesday, April 24 at 8:00 AM. NOTICE THE TIME! We will have two review sessions following the class on.

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