ISNS 3371 - Phenomena of Nature Circuits in Series Resistance (light bulbs) on same path Current has one pathway - same in every part of the circuit Total.

ISNS 3371 - Phenomena of Nature

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.

Magnetic Field Generation by a Moving Charge


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.

Force on a Moving Charge

A beam of electrons is deflected by a magnetic field.

If the beam is confined to a wire, then the wire feels a force.


This magnetic force on a wire is the idea behind how electric motors work. Current is passed through a loop of wire (or many loops - called an armature). The currents on either side of the loop are in opposite directions - the forces are therefore in opposite directions - in this picture, up on one side and down on the other. Causes the loop to rotate. The current is reversed every half revolution by stationary contacts - brushes - on the shaft.

Electric Motors


Magnetic forces on charged particles 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.

Current Induction

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.


Electromagnetic induction is the principle behind electric generators - motors in reverse.

Motor - electrical energy is turned into mechanical energy

Generator - mechanical energy is turned into electrical energy.

In modern power generators, an armature is forced to spin in strong magnetic fields by a turbine - in turn spun by steam or falling water

Voltage is induced in a rotating loop when it is rotated in a magnetic field.

Electric Generators


Lenz's Law

When an electric field is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced electric field is such that it produces a current whose magnetic field opposes the change which produces it. The induced magnetic field inside any loop of wire always acts to keep the magnetic field in the loop constant.

If the magnetic field is increasing, the induced field acts in opposition to it.

If the magnetic field is decreasing, the induced field acts in the direction of the applied field to try to keep it constant.

ISNS 3371 - Phenomena of Nature Circuits in Series Resistance (light bulbs) on same path Current has one pathway - same in every part of the circuit Total.

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