Review for Friday's Test

Electric and Magnetic Fields

• know the difference between gravitational fields and electric fields
• know the difference between gravitational force and electric force
• what's the diff between a field and a force ?????????
  

HINT: are they vectors? what direction do these vectors take? are masses or charges involved? what are the units for these quantities?

• Coulomb's law - know how to apply it, what constant is used (k=9 x 10⁹),
• what is the relationship between electric force and distance between the charges?
• how is this law similar to Newton's law of gravitation and how is Coulomb's law different from Newton's law of gravitation
• don't forget charges have a unit called the Coulomb;
• charge on a proton is equal to the charge on an electron which is 1.6 x 10^-19

Strength of an Electric Field

• E = F_e /q [field intensity around a test charge; remember a test charge is very tiny and positive]
• E = kq/d² [field intensity due to a single charge]
• E = V/d [field intensity between 2 oppositely charged plates]
• The above 3 formulas all solve for electric field, units being N/C or V/m

Electric Potential/Electric Potential Difference/Electric Potential Energy

• what the heck is potential??? ratio of potential energy per charge
• potential difference is the difference between two potentials-we tend to use potential difference in calculations
• electric potential energy is the amount of energy a charge has due to its position; see the notes on gravitational potential energy and the similarity to understanding e.p. energy
  
• PE = W = qV does this make sense to you? can you explain why potential energy is equal to work????
• know how to solve problems on charged particles that exist between two charged parallel plates (questions 1-4 on Moving Charges worksheet)

Magnetic Fields

• when a charged particle enters a magnetic field its direction changes; an electron will circle one way and a proton will circle in the opposite way
• a magnetic field only changes the direction of a charged particle where an electric field accelerates a charges particle [nice to know this difference]
• the magnetic force acting on a charged particle is given by the formula F_B= Bqv, where B is the magnetic field (measured in teslas), q is the charge (measured in coulombs) and v is the velocity of the charged particle (measured in m/s)
• when a charged particle enters a magnetic field, the magnetic force acting on this charged particle is equal to the centripetal force acting; we can equate these two formulae to solve for unknowns.

F_B= Bqv F_c= mv²/R

F_B = F_c, therefore,

Bqv = mv²/R

Bq = mv/R

m = BqR/v let's you solve for the mass of the charged particle

You can rearrange this formula to solve for the radius of curvature, magnetic field, velocity, etc
See problems 8 - 16 on the Moving Charges worksheet that deal with these formulae given above.

Technologies that use combined electric and magnetic fields are older type of televisions that have a cathode ray tube [not plasmas or LCD TV's], mass spectrometers, loudspeakers-think of objects that have magnets and electric currents.