physics exam 2 - ch21

voltage

difference in electric potential between the two terminals

capacitator on the circuit

stores charge and electric potential energy

conservation of energy

W = ΔUg

stationary source charges

are repelled by charges q

• a hand must push to move q closer to the source charges

hand does work

transferring energy into system of charges

ΔU_elec = W

U_elec

electric potential energy

• of a charge can be determined by computing how much work it took to move the charge to the position

work

energy transferred into a system by pushing on it

a charged particle’s potential energy is

proportional to its charge

electric potential (V)

potential to create an electric potential energy if charge is placed at the point

• tells us how the source charges provide q with potential energy

• created by the source charge

• exists at every point in space

electric potential energy (J)

U = equal to the amount of work done

interaction energy of a charged particle with the source charge

• farther from +, U gets smaller

• farther from -, U gets bigger

electric potential

only dependent on distance from source charge

• other moving charges don’t matter

• that potential is always there in space based on source charges

a positive charge at a lower V

loses energy

a negative charge at a lower V

gains energy

conservation of energy equation

K_f + (U_elec)f = K_i + (U_elec)_i

K_f + qV_f = K_i + qV_i

K_f - K_i = U_i - U_f

ΔK = -ΔU = -qΔV

motion of charges

ΔK = -qΔV

• for a positive charge

  • positive K → speeds up

  • negative K → slows down

• negative charges are the opposite

positive charge

speeds up from high to low potential

• slow down from low to high

negative charge

speeds up from low to high potential

• slow down from high to low

a positive and negative charge are released from rest in a vacuum and as they move towards each other

a negative potential energy becomes more negative

1 electron volt = 1 eV

a unit of energy

kinetic energy gained by the electron as it accelerates through a potential difference of 1 volt

K = -qΔV = -(-e)(1) = eV

V_o

potential at the surface of a sphere

= Q/4piE_oR

potential outside the sphere charged to potential V_o

V = RV_o/r

as distance from center increases, the potential decreases

R = radius of sphere

r = distance from center to point

an electric potential is created

separating a positive charge from a negative charge

• requires work for the separation

metal wires connecting items

every point has the same electric potential

batteries

can create a fixed potential difference using chemicals

voltmeter

can measure potentials with 2 inputs

electric energy can be

transformed to other types of energy (kinetic, thermal, etc)

when a + charge moves from - to +

potential energy increases

kinetic energy decreases

electric potential inside a parallel-plate capacitor

uniform electric field

V_c = V₊ - V_- = Ed

equipotential surfaces

the potential of planes is the same throughout the plane

variables of a parallel-plate capacitor

V_c = potential difference between + and -

d = distance between the plates (m)

E = electric field

Q+ = charge on + plate

Q- = charge on - plate

x = distance of the point from the negative plate

V = 0 and x = 0

at the negative plate of a parallel-plate capacitor

V = V_c and x = d

at the positive plate of a parallel-plate capacitor

potential of an electric dipole

sum of potentials of positive and negative charges

in a capacitor, electric field gets stronger and potential differencee increases

as the charge on the electrodes increases

potential difference between electrodes

is proportional to their charge

C

capacitance = constant of proportionality

• depends on shape, size, and spacing of electrodes

capacitor

• can be charged by a battery

• holds the charge even if battery is removed later

a charged capacitor stores energy

as electric potential energy