SAT II Physics (Gary Graff) Episode 1 Part 8 pps

Magnetic fields form around magnets.The magnetic field exits the north pole of a magnet and enters thesouth pole of a magnet.Magnetic field lines act much the same as electric field line

SERIES AND SERIES PARALLEL CIRCUITS

A source of electrical potential sometimes has a single task, such as tooperate a flashlight bulb But just as often the power source is used toperform many tasks through several different pathways The firstexample we will discuss is the single task or series circuit; and thesecond type is called a parallel circuit

SERIES CIRCUITS

Series circuits have only one pathway in which current can flow.Every unit of charge flowing in a series circuit must pass throughevery position of the circuit

When more than one resistor is in a series circuit, the value of eachresistor is added together to give the total resistance

R t=R1+R2+R3+R…The total resistance for the circuit shown iss 12Ω

Ω Ω ΩΩ

R R t

t

= + +

=

2 4 612

PARALLEL CIRCUITS

Parallel circuits have two or more pathways through which currentcan flow The total value of the current depends on the parallelresistance offered against the current flow Since current and resis-tance are indirectly related, the highest resistance will allow lesscurrent to flow, and the lowest resistance will allow more current toflow

The total resistance of a set of parallel resistors is found in thefollowing manner:

1 1 1 1

R t =R +R +R

1 12

15

110

1401

5 2 1 025

1825

1 21

R

R

R R

in parallel with an extremely small resistor, the total resistance isequal to the smallest resistor When a set of parallel resistors are all

of equal resistance, we can divide the resistance value of one of them

by the total number of resistors in parallel to find the resistance ofthe parallel system

Example

Let’s do a problem where elements of both the series circuit and theparallel circuit are combined

1

1 Find the total resistance for the circuit.Find the total resistance for the circuit

The first step to solve the problem is to find the equivalent resistance

for the R 2 – R 3 – R 4 parallel set of resistors:

1 1 1 1

1 110

18

141

Next we find the equivalent resistance for the R 6 – R 7pair.

1 1 1

1 110

1151

Finally, we add the two parallel equivalent values to the values of R1,

R5, and R8, yielding:

R R t

2 Use Ohm’s La Use Ohm’s La Use Ohm’s Law to fw to fw to find the total curind the total curind the total currrrrrent in the cirent in the cirent in the circuit.cuit

V IR

I V R

3 Find the voltage change between points B and C

This may seem difficult at first, but remember Ohm’s Law The current

passing through R5 is 1A, and the resistance is 3Ω.

V = I R

V = (1A)(3 Ω)

V = 3 V

ELECTRIC CIRCUITS

4 F F Find the vind the vind the voltaoltaoltaggge ce ce changhanghange betwe betwe between points een points een points A and B.A and B

Again, the problem seems more difficult than it really is—it’s anotherOhm’s Law problem The total current passing through the parallel

resistors is 1A, and the equivalent resistance is 4Ω.

Ω

POWER

Power in circuits is the rate at which electric energy is used Thepower capability of the circuit elements is the product of the voltageand the current

Power is an important quantity in circuits The voltage sourcemust have enough power to operate the devices in the circuit

Furthermore, the devices in the circuitry will burn out and open iftheir power capacity is not large enough to perform work at therequired rate Remember, power is the rate at which work is done.The power requirement for a circuit or a circuit element can becalculated if any two of the Ohm’s Law quantities are known

Example

A typical power calculation could require you to find the powerrequirement for a resistor in a circuit

Find the power dissipated by a 4Ω resistor that has a 05A

current passing through it

( )( ).

Ω WattAnother type of power question you could be asked would requireyou to find the total resistance in a circuit, then solve to find eitherthe total power requirement of the circuit or the power requirementfor a single element in the circuit

Example

Find the power requirement for the series–parallel circuit shownbelow:

ELECTRIC CIRCUITS

To find the total power for the circuit you first must find the totalresistance We find that the total resistance for the circuit is 9Ω Withthis information we can now solve for the power required to run the

circuit with P V

R

= 2

P V R

( )Ω Watts

Another question might involve one of the parallel resistors in thediagram above The current in the circuit would be required to solvethe problem

Ω.The next step is to calculate the voltage change across the two

parallel resistors (R2 and R3)

66 5

3 3

Ω

With this information the actual current in R3 can be found by usingOhm’s Law.

I V R

.Ω

Next we calculate the power

CAPACITORS

Capacitors (previously mentioned) are parallel plate devices that arecapable of storing electric charge and releasing (discharging) it asneeded Their effectiveness is enhanced by placing a non-conductive

material, called a dielectric, between the two plates The energy (in

the form of the charge) stored by a capacitor is E=1CV

2

2.Capacitors placed in an electric circuit do not allow a steadycurrent to flow Instead, they build charge between their two platesover a period of time until the potential of the capacitor is almostequal to the applied voltage When a capacitor is fully charged in a

DC circuit, current cannot flow until the capacitor is discharged,because the capacitor acts like an open circuit element

Capacitors used in an electric circuit can be in series or in

parallel The total capacitance (C t) for capacitors in a parallel circuit isfound by adding the values of the capacitors in parallel with oneanother You should note that this technique is the opposite of thetechnique we used to find the value of the resistance of parallelresistors

ELECTRIC CIRCUITS

The six capacitors in the diagram have values of 2, 3, 4, 5, and 6 microfarads, respectively The total capacitance is found by adding themtogether.

1 1 1 1 1 1

1 1

2

13

14

15

161

AMMETERS AND VOLTMETERS

Ammeters and voltmeters are used to monitor electric circuits.Ammeter

Ammeters s s measure the current flowing in a circuit They have alow resistance, which keeps them from interfering with the circuit.This is necessary because they are hooked in a series with a circuitbeing measured, and a large resistance would decrease the currentbeing measured with the meter

VVVoltmeteroltmeteroltmeters s s are used to measure voltage changes in a circuit.They have a high resistance and are used in parallel with the circuitbeing measured The high resistance in the voltmeter causes all butthe tiniest fraction of current to pass through the circuit beingmeasured, which prevents the voltmeter from interfering with thecircuit

MAGNETS AND MAGNETIC FIELDS

The properties of naturally occurring magnetic rocks have beenknown for several thousand years The Chinese knew that a piece ofiron could be magnetized by putting it near lodestone, and sailorshave been navigating with magnetic compasses for nearly a thousandyears

Some characteristics of magnets that you should know andremember are:

1 Magnets have poles The north–seeking pole is the north pole ofthe magnet The south–seeking pole is the south pole of themagnet

2 Like poles of magnets repel one another and unlike poles ofmagnets attract one another

3 Magnets can induce demagnetized ferrous materials to becomemagnetized

4 Temporary magnets cease acting like magnets as soon as thepermanent magnet is removed

5 Permanent magnets retain their magnetism for a long time

MAGNETS AND MAGNETIC FIELDS

A magnet field is said to exist in the region where a compassexperiences a force upon it Magnetic fields form around magnets.The magnetic field exits the north pole of a magnet and enters thesouth pole of a magnet.

Magnetic field lines act much the same as electric field lines

when magnetic poles are brought near one another That is, the

magnetic lines of force do not cross. You could say that the repulsiveforce the north pole of a magnet exerts on another similar magneticpole is a manifestation of the lines of force not crossing one another

The attraction between unlike poles also can be explained in amanner similar to the electric field

Some magnets exert large repulsive or attractive forces on othermagnets regardless of how physically close the two magnets are toone another The way these magnets repel or attract is an indication

of their magnetic field strength, but this does not give an accuratemeasure of the actual strength of the magnetic field

A more accurate method for determining the magnetic field

strength is to use a term called magnetic flux The number of

magnetic field lines within a given area is called magnetic flux, and it

represents the strength of the magnetic field B Magnetic fields exert

a force on current carrying wires The force the wire experiences isthe product of the length of the wire in the field, the magnitude of thecurrent, and the strength of the magnetic field

F=B IL⊥Note the perpendicular sign behind the B The maximum force is

exerted when a current carrying wire is perpendicular to themagnetic field The direction of the force exerted on the wire can befound by using the right-hand rule

A 5m length of wire carries a 6A current within a magnetic field The

wire is at right angles to the field and experiences a force of 45N.What is the strength of the magnetic field?

455

N(6N

A current-carrying wire is a source of a magnetic field too This is thereason the wire has a force exerted on it when it is in a magneticfield

Let’s place a compass beside a wire hooked to a battery with anopen switch

Notice that with the switch open the compass points correctly

to the north When the switch is closed the compass points in adifferent direction

A force must affect the compass to cause it to point in a differentdirection The current in the wire is the source of this force Thedirection of the magnetic field around the current-carrying wire can

be found by using a variation of the right-hand rule

1 Point the thumb of your right hand in the direction of the rent

2 Grasp the wire and wrap you fingers around the wire

3 Your fingers point in the same direction as the orientation of themagnetic field around the wire (see below)

MAGNETS AND MAGNETIC FIELDS

When two current-carrying wires are side by side, their magneticfields interact, causing the wires to attract one another or to repelone another.

Notice the arrows in diagram A Think of this situation as

opposite magnetic fields, that attract one another The arrows indiagram B head in the opposite direction Think of this situation aslike magnetic fields that repel one another

Electric currents consist of moving charges It is only natural toexpect a charged object to experience a force when it is movingwithin a magnetic field The right-hand rule can be applied to chargedparticles within a magnetic field Unlike the current in a wire thatfollowed its pathway in the wire, a charged particle can have itspathway changed by the force applied to it

Reviewing the right-hand rule:

1 Point the thumb of your right hand in the direction the particle

The charged particle has a charge q on it, and its velocity is v.

The definition of current is a number of moving charges Thus, wecan say F=B IL⊥ for a current-carrying wire

The current in a wire is made of a number of individual chargesthat are moving at a high velocity For a single charged particle, thecharge on the particle and the velocity with which it is moving can beequated to the current and the length of the wire This leads to thefollowing equation, which describes the force exerted on a movingparticle in a magnetic field.

F=B qv⊥

Unless the particle leaves the magnetic field, the direction of theforce on the particle keeps changing as the path of the particlechanges The particle will move in a circular path as the force on itremains constant in magnitude, and the direction continually changes.Any time an object moves in a circular path, a centripetal force

(F c) is involved In fact, the force applied to the charged particle bythe magnetic field is a centripetal force

r c

MAGNETIC INDUCTION

More than 99% of the electrical power used in the United States isgenerated by converting mechanical energy into electrical energy for

heating, lighting, cooking, etc This process is called electromagnetic

induction and was discovered in 1831 by Michael Faraday and JohnHenry

A current can be induced in a wire in several ways One is toplace a closed loop of wire in a magnetic field and move the wire.Remember our discussion of the force applied to a current-carryingwire? The movement of the wire within the magnetic field causes aforce to be generated in the wire, which in turn forces free electrons

MAGNETS AND MAGNETIC FIELDS

The direction of an electric current induced into a wire can be dicted by using the right-hand rule.

1 Point the thumb of the right hand in the direction of motion ofthe wire

2 Point the fingers of the right hand in the direction of the netic field

3 Your palm will point in the direction of the force and the duced current

in-The magnitude of the induced current is dependent on severaldifferent things

1 The strThe strThe strength of the maength of the maength of the magnetic fgnetic fgnetic field.ield.ield A stronger magnetic fieldhas more flux density, meaning more field lines to cross

The induced current will be larger when the magnetic field isstronger

2 The rate at which the wire is moved in the magnetic field.The rate at which the wire is moved in the magnetic field.The faster the wire is moved through the magnetic field, themore magnetic field lines will be cut, generating a larger current

3 The length of the wirThe length of the wirThe length of the wire in the mae in the mae in the magnetic fgnetic fgnetic field.ield.ield Again, the longerthe wire, the more magnetic field lines it will cut through,

generating more current flow

The movement of a wire in a magnetic field causes the freeelectrons to move because their electrical potential is raised Wealready know that current flows from high potential to lower

potential This difference in potential is called the induced EMF, which

is in volts We also know that the length of the wire (L), the strength

of the magnetic field (B), and the rate at which the wire is moved in

The following represents the relationship between the inducedvoltage and the moving magnetic field in which a wire is placed (Thewire may be moved in a stationary magnetic field too.)

EMF = BLV

Power-generating plants induce current into lengths of wire on alarge scale Electrical power is generated by using long loops of wirethat are wrapped in such a manner that they are in continuouscontact with magnetic field lines The loops of wire are rotatedwithin the magnetic field at a constant rate to produce the type ofelectricity called alternating current or AC

A simplified drawing of an AC generator is shown below

The current and voltage produced in the AC generators is notsteady as in DC systems Both current and voltage increase anddecrease each time the loop of wire in the magnetic field turns a full360° The diagram below shows a comparison between AC and DCvoltages

MAGNETS AND MAGNETIC FIELDS