Ch5: Electrostatics and Magnetism

Electrostatics

Study of stationary charges and associated forces

Attractive Forces

Between charges with opposite signs

Repulsive Forces

Between charges with same sign

Grounding

Charge return to earth

Static Electricity

Charge buildup

Low humidity = easier charge separation

Unit of Charge (e)

1\.6E-19 Coulomb (C)

Proton: +e

Electron: -e

Law of Conservation of Charge

Charge cannot be created nor destroyed

Insulator

Hold charge

No charge distribution on surface

e linked closely with nuclei

Nonmetals

Insulator Ex

Dielectric material in capacitors

Isolate charge to prevent grounding

Conductor

Transfer charge

Distribute charge on surface

Nuclei with sea of free e

Metals and ionic solutions

Coulomb’s Law

Electrostatic force (Fe) between 2 charges

Point along line connecting charge centres

\-Fe

Point towards other charge (attractive)

\+Fe

Point away from other charge (repulsive)

Coulomb’s Constant (k)

8\.99E9 N•m^2/C^2

Electric Field (E)

Set up by electric charge

Exert force on other charges in field (depend on q)

Vector

Test Charge (q)

Charge in electric field

Source Charge (Q)

Create electric field

Field Lines: Positive Charges

Radiate out

Field Lines: Negative Charges

Radiate in

Electric Field: Positive Test Charge

Force same direction as field lines

Electric Field: Negative Test Charge

Force opposite direction as field lines

Electric Field Lines

Imaginary lines representing positive test charge movement direction in electric field

Field Lines: Dense

Stronger field

Field Lines: Dispersed

Weaker field

Net Electric Field

Sum of electric fields at point

Electric Potential Energy (Eel)

Depend on relative position of charges

Work to move test charge from infinity to electric field

Eel: Like Charges

\+Eel

Increase the closer charges are

Eel: Unlike Charges

\-Eel

Decrease the closer charges are

Eel: Stability

Decrease Eel = increase stability

Electric Potential (V)

Scalar

Charge Eel magnitude/charge magnitude

Measured in volts V (J/C)

Potential Difference/Voltage (∆V)

Difference in V from moving test charge between points

Path independent

Measured in V

∆V: +q

\-∆V (high to low potential)

Eel decrease

∆V: -q

\+∆V (low to high potential)

Eel decrease

Equipotential Line

Line of uniform V (∆V = 0)

Perpendicular to electric field lines

Moving Charge Between Equipotential Lines

Conservative forces (not dependent path)

Electric Dipole (p)

From equal and opposite charges separated by small distance

Transient or permanent

Vector (-q -> +q)

Perpendicular Dipole Bisector: Electric Potential

V = 0 (90ª to dipole axis)

Perpendicular Dipole Bisector: Electric Field

Point in opposite direction to p (+q -> -q)

Dipole: Equilibrium

Translational equilibrium

NOT rotational equilibrium (Electric forces act in opposite directions)

Dipole: Torque

CW from opposite forces

Dipole reorient to align with E

Magnetic Field

From moving charge/magnets

Measured in tesla (T) or gauss

Tesla and Gauss

1 T = 1 N•s/m•C

1 T = 10^4 gauss

Diamagnetic Materials

Atoms with all paired e

No net magnetic field

Repelled by magnet (antimagnetic)

Paramagnetic Materials

Atoms with unpaired e

Net magnetic dipole moment (random orientation)

No net magnetic field

Weak magnetization

Ferromagnetic Materials

Atoms with unpaired e

Net magnetic dipole moments (random orientation)

No net magnetic field

Strong magnetization

Bar Magnet

Ferromagnetic

Electric field from N to S

Opposite poles attract, like poles repel

Magnetic Field (B): Straight Wire

Concentric ring shape

Magnetic Field (B): Circular Wire Loop

Only measure B at centre of loop

B: RH Rule

Thumb: Current dir

Fingers: Curl in B dir

Magnetic Force (FB)

External electric field act on other moving charges

Object not affected by own magnetic field

Lorentz Force

Sum of electrostatic and magnetic forces

FB: Moving Charge

Charge moving parallel/antiparallel to magnetic field experience 0 magnetic force

FB: Current-Carrying Wire

Current = flow of + charge

FB: RH Rule

Thumb: Velocity dir of + charge

Fingers: Magnetic field dir

Palm: FB for positive charge

Back of Hand: FB for negative charge

FB and v

Always perpendicular

FB cause uniform circular motion (FB = Fc)