Physics II:Electricity & Magnetism

Physics II:Electricity & Magnetism

Sections 21.9 to 21.10Sections 21.9 to 21.10

Tuesday (Day 14)Tuesday (Day 14)

Warm-UpWarm-Up

Tues, Feb 10 Each charge on the next slide is ±q. What will happen to the lines if a 3rd

charge of +q is added to the (1) right side and (2) left side?

Place your homework on my desk: “Foundational Mathematics’ Skills of Physics” Packet (Page 16) Web Assign 21.5 - 21.7

For future assignments - check online at www.plutonium-239.com

Tues, Feb 10 Each charge on the next slide is ±q. What will happen to the lines if a 3rd

charge of +q is added to the (1) right side and (2) left side?

Place your homework on my desk: “Foundational Mathematics’ Skills of Physics” Packet (Page 16) Web Assign 21.5 - 21.7

For future assignments - check online at www.plutonium-239.com

Field Example #1: Each charge below is ±q. What will happen to the lines if a 3rd charge of +q is added to the (1)

right side and (2) left side?

Field Example #1: Each charge below is ±q. What will happen to the lines if a 3rd charge of +q is added to the (1)

right side and (2) left side?

Essential Question(s)Essential Question(s) WHAT PRIOR FOUNDATIONAL MATHEMATICS’ SKILLS ARE

NECESSARY IN PHYSICS II? HOW DO WE DESCRIBE THE NATURE OF ELECTROSTATICS

AND APPLY IT TO VARIOUS SITUATIONS?How do we describe and apply the nature of electric fields in and

around conductors?How do we describe and apply the concept of induced charge and

electrostatic shielding? How do we describe and apply the concept of electric fields?

WHAT PRIOR FOUNDATIONAL MATHEMATICS’ SKILLS ARE NECESSARY IN PHYSICS II?

HOW DO WE DESCRIBE THE NATURE OF ELECTROSTATICS AND APPLY IT TO VARIOUS SITUATIONS?How do we describe and apply the nature of electric fields in and

around conductors?How do we describe and apply the concept of induced charge and

electrostatic shielding? How do we describe and apply the concept of electric fields?

VocabularyVocabulary Static Electricity Electric Charge Positive / Negative Attraction / Repulsion Charging / Discharging Friction Induction Conduction Law of Conservation of Electric

Charge Non-polar Molecules

Static Electricity Electric Charge Positive / Negative Attraction / Repulsion Charging / Discharging Friction Induction Conduction Law of Conservation of Electric

Charge Non-polar Molecules

Polar Molecules Ion Ionic Compounds Force Derivative Integration (Integrals) Test Charge Electric Field Field Lines Electric Dipole Dipole Moment

Polar Molecules Ion Ionic Compounds Force Derivative Integration (Integrals) Test Charge Electric Field Field Lines Electric Dipole Dipole Moment

Foundational Mathematics Skills in Physics Timeline

Foundational Mathematics Skills in Physics Timeline

Day Pg(s) Day Pg(s) Day Pg(s) Day Pg(s)

11

26 3 11 16 16 21

213

147 4 12 17 17 8

322

238 5 13 18 18 9

424†12

9 6 14 19 19 10

5 15 10 7 15 20 20 11

WHAT PRIOR FOUNDATIONAL MATHEMATICS’ SKILLS ARE NECESSARY IN PHYSICS II?

AgendaAgenda

Review “Foundational Mathematics’ Skills of Physics” Packet (Page 16) with answer guide.

Discuss Electric Fields and Conductors Motion of a Charged Particle in an Electric Field

Work on Web Assign

Review “Foundational Mathematics’ Skills of Physics” Packet (Page 16) with answer guide.

Discuss Electric Fields and Conductors Motion of a Charged Particle in an Electric Field

Work on Web Assign

Field Example #2: Each charge below is ±5q. What will happen to the lines if a 3rd charge of +q is added to the (1)

right side and (2) left side?

Field Example #2: Each charge below is ±5q. What will happen to the lines if a 3rd charge of +q is added to the (1)

right side and (2) left side?

Field Example #3: Each charge below is ±10q. What will happen to the lines if a 3rd charge of +q is added to the (1)

right side and (2) left side?

Field Example #3: Each charge below is ±10q. What will happen to the lines if a 3rd charge of +q is added to the (1)

right side and (2) left side?

Section 21.9Section 21.9

How do we describe and apply the nature of electric fields in and around conductors?How do we explain the mechanics responsible for the absence of

electric field inside of a conductor?Why must all of the excess charge reside on the surface of a

conductor? How do we prove that all excess charge on a conductor must

reside on its surface and the electric field outside of the conductor must be perpendicular to the surface?

How do we describe and apply the nature of electric fields in and around conductors?How do we explain the mechanics responsible for the absence of

electric field inside of a conductor?Why must all of the excess charge reside on the surface of a

conductor? How do we prove that all excess charge on a conductor must

reside on its surface and the electric field outside of the conductor must be perpendicular to the surface?

Section 21.9Section 21.9

How do we describe and apply the concept of induced charge and electrostatic shielding?What is the significance of why there can be no electric

field in a charge-free region completely surrounded by a single conductor?

How do we describe and apply the concept of induced charge and electrostatic shielding?What is the significance of why there can be no electric

field in a charge-free region completely surrounded by a single conductor?

21.9 Electric Fields and Conductors

The static electric field inside a conductor is zero – if it were not, the charges would move.

The net charge on a conductor is on its surface.

Charge ball suspended in a hollow metal sphere

Charge ball suspended in a hollow metal sphere

ObservationsThe hollow sphere had a charge on

the outside.The charged ball still had a charge.

ConclusionsThe charged ball on the inside

induces an equal charge on the hollow sphere.

ObservationsThe hollow sphere had a charge on

the outside.The charged ball still had a charge.

ConclusionsThe charged ball on the inside

induces an equal charge on the hollow sphere.

21.9 Electric Fields and Conductors

The electric field is perpendicular to the surface of a conductor – again, if it were not, charges would move.

Charge ball placed into a hollow metal sphere

Charge ball placed into a hollow metal sphere

ObservationsThe hollow sphere had a charge on

the outside.The charged ball no longer had a

charge.Conclusions

The charge resides on the outside of a conductor.

ObservationsThe hollow sphere had a charge on

the outside.The charged ball no longer had a

charge.Conclusions

The charge resides on the outside of a conductor.

Applications of E-fields and conductors: Faraday Cages

Applications of E-fields and conductors: Faraday Cages

Faraday cages protect you from lightning because there is no electrical field inside the metal cage (Notice (1) it completely surrounds him and (2) the size of the gaps in the fence (it is not a solid piece of metal).

Faraday cages protect you from lightning because there is no electrical field inside the metal cage (Notice (1) it completely surrounds him and (2) the size of the gaps in the fence (it is not a solid piece of metal).

Section 21.10Section 21.10

How do we describe and apply the nature of electric fields in and around conductors?How do we determine the direction of the force

on a charged particle brought near an uncharged or grounded conductor?

How do we describe and apply the nature of electric fields in and around conductors?How do we determine the direction of the force

on a charged particle brought near an uncharged or grounded conductor?

Section 21.10Section 21.10

How do we describe and apply the concept of induced charge and electrostatic shielding?How do we determine the direction of the force

on a charged particle brought near an uncharged or grounded conductor?

How do we describe and apply the concept of induced charge and electrostatic shielding?How do we determine the direction of the force

on a charged particle brought near an uncharged or grounded conductor?

Section 21.10Section 21.10

How do we describe and apply the concept of electric field?How do we calculate the magnitude and

direction of the force on a positive or negative charge in an electric field?

How do we analyze the motion of a particle of known mass and charge in a uniform electric field?

How do we describe and apply the concept of electric field?How do we calculate the magnitude and

direction of the force on a positive or negative charge in an electric field?

How do we analyze the motion of a particle of known mass and charge in a uniform electric field?

Electron accelerated by an electric fieldElectron accelerated by an electric field

An electron is accelerated in the uniform field E (E=2.0x104N/C) between two parallel charged plates. The separation of the plates is 1.5 cm. The electron is accelerated from rest near the negative plate and passes through a tiny hole in the positive plate. (a) With what speed does it leave the hole? (b) Show that the gravitational force can be ignored. [NOTE: Assume the hole is so small that it does not affect the uniform field between the plates]

An electron is accelerated in the uniform field E (E=2.0x104N/C) between two parallel charged plates. The separation of the plates is 1.5 cm. The electron is accelerated from rest near the negative plate and passes through a tiny hole in the positive plate. (a) With what speed does it leave the hole? (b) Show that the gravitational force can be ignored. [NOTE: Assume the hole is so small that it does not affect the uniform field between the plates]

Electron accelerated by an electric field (a) With what speed does it leave the hole?

Electron accelerated by an electric field (a) With what speed does it leave the hole?

F =qE F =ma⇒ a =F

m=

qE

m

v2 =v02

=0+ 2aΔx⇒ v = 2aΔx

v = 2qEm

⎛⎝⎜

⎞⎠⎟Δx

v = 21.60 x 10-19 C( ) 2.0 x 104 NC( )

9.1 x 10−31 kg( )0.015 m( )

v =1.0 x 107 ms

Electron accelerated by an electric field (b) Show that the gravitational force can be ignored. Electron accelerated by an electric field

(b) Show that the gravitational force can be ignored.

FE =qE

FE = 1.60 x 10-19 C( ) 2.0 x 104 NC( )

FE =3.5 x 10−15 N

FG =mg

FG = 9.1 x 10-31 kg( ) 9.8 ms2( )

FG =8.9 x 10-30 NNote that FE is 1014 times larger than the FG.Also note that the electric field due to the electron does not enter the problem since it cannot exert a force on itself.

Applications of an electron accelerated by an E-Field: Mass Spectrometer

Applications of an electron accelerated by an E-Field: Mass Spectrometer

Mass Spectrometers are used to separate isotopes of atoms. The charged isotopes (a.k.a. ions) are accelerated to a velocity by the parallel plates (located from S

to S1)

Mass Spectrometers are used to separate isotopes of atoms. The charged isotopes (a.k.a. ions) are accelerated to a velocity by the parallel plates (located from S

to S1)

Projectile Motion of a Charged Particle:Electron moving perpendicular to E

Projectile Motion of a Charged Particle:Electron moving perpendicular to E

Suppose an electron is traveling with a speed, v0 = 1.0x107m/s, enters a uniform field E at right angles to v0. Describe the motion by giving the equation of its path while in the electric field. Ignore gravity.

Suppose an electron is traveling with a speed, v0 = 1.0x107m/s, enters a uniform field E at right angles to v0. Describe the motion by giving the equation of its path while in the electric field. Ignore gravity.

F =qE=−eE F =ma⇒ a =F

m=−

eE

m

y =v0y=0

t+ 12 ayt

2=−eE

2mt 2

x =v0xt+ 12 ax

=0t2 ⇒ t =

x

v0 x

=x

v0

⇒ t 2 =x

v0

⎛

⎝⎜⎞

⎠⎟

2

=x2

v02

= −eE

qmv02 x2 This is the equation of a

parabola (i.e. projectile motion).

Electrons moving perpendicular to E: The discovery of the electron: J.J. Thomson’s

Experiment

Electrons moving perpendicular to E: The discovery of the electron: J.J. Thomson’s

Experiment J. J. Thomson’s famous experiment that allowed him to discover the electron. J. J. Thomson’s famous experiment that allowed him to discover the electron.

Applications of an electron moving perpendicular to E: Cathode Ray Tube (CRT)

Applications of an electron moving perpendicular to E: Cathode Ray Tube (CRT)

Television Sets & Computer Monitors (CRT)Television Sets & Computer Monitors (CRT)

Applications of an electron moving perpendicular to E: Mass Spectrometer

Applications of an electron moving perpendicular to E: Mass Spectrometer

Mass Spectrometers are used to separate isotopes of atoms. The charged isotopes (a.k.a. ions) are accelerated to a velocity by the parallel plates (located at the -

& + plates)

Mass Spectrometers are used to separate isotopes of atoms. The charged isotopes (a.k.a. ions) are accelerated to a velocity by the parallel plates (located at the -

& + plates)

Applications of an electron moving perpendicular to E: e/m ApparatusApplications of an electron moving perpendicular to E: e/m Apparatus

e/m Apparatus e/m Apparatus

Applications of an electron moving perpendicular to E: e/m ApparatusApplications of an electron moving perpendicular to E: e/m Apparatus

e/m Apparatus e/m Apparatus

SummarySummary

Using your kinematic equations, determine the equation that relates y to v0, g, , and x?

HW (Place in your agenda): “Foundational Mathematics’ Skills of Physics” Packet (Page 17) Web Assign 21.12 - 21.14

Future assignments: Electrostatics Lab #3: Lab Report (Due in 1 class)

Using your kinematic equations, determine the equation that relates y to v0, g, , and x?

HW (Place in your agenda): “Foundational Mathematics’ Skills of Physics” Packet (Page 17) Web Assign 21.12 - 21.14

Future assignments: Electrostatics Lab #3: Lab Report (Due in 1 class)

How do we use Coulomb’s Law and the principle of superposition to determine the force that acts between point charges?