chapter 6

Question 1

what is charge?

Answer 1

• a basic property of matter
• the unit of charge is coulomb (C) which is defined as the amount of charge carried by a current of 1 ampere (A)
  
  • property of the subatomic particles known as protons and electrons

Question 2

the degree to which a substance readily allows current (moving charge) to be propagated can be quantified in terms of?

Answer 2

• resistivity
• conductivity
  
  • inverse concepts
    
    • high resistivity means that charge does not readily flow through a substance, while high conductivity means that it does

Question 3

Resistivity (rho) and conductivity (sigma) are intrinsic properties of a substance, while the closely related concepts of resistance (R) and conductance (G) refer to the specific properties of a physical conductor made out of a substance:

Answer 3

R = rho(l/A) and G = sigma (A/l)

• l is the length of the conductor
• A is its cross sectioanl area
  
  • in both equations, a large cross sectional area and a short length favour the conduction of charge

Question 4

rho and sigma are material specific constants that are reciprocals of each other:

Answer 4

• rho = 1/sigma and sigma = 1/rho

Question 5

how are conductors defined?

how are insulators defined?

Answer 5

• conductors have low resistance and high conductance
• insulators have high resistance and low conductance

Question 6

The force that exists between 2 charged objects can be defined using?

Answer 6

• Coulomb’s law
  
  • F = kq₁q₂/r²
    
    • ^​F is the electric force rpesent between 2 charges (N)
    • q₁q₂ are two point charges
    • r is the distance between them
    • k is knwon as Coulomb’s constant and is equal to 8.99 x 10⁹ N•m²/C²
      
      • ^​positive values insicate repulsion and negative values indicate attraction

Question 7

A force exists between any 2 point charges according to Coulomb’s law so a single point charge must be capable of exerting a force on any other charge. We describe this in terms of an?

Answer 7

• electric field
  
  • E = F/q
    
    • E is the magnitude of the electric field
    • F is the force experienced by the test charge
    • q is the magnitude of the test charge
      
      • or E = kQ/r²

Question 8

What are field lines used for?

Answer 8

• used to graphically indicate the direction and magnitude of electric fields
  
  • the arrows in field lines point in the direction that a positive test charge would travel in the electric field (away from a positive charge and towards a negative charge)

Question 9

An electric field when you have 2 equal but oppsoute source charges:

Answer 9

• a positive test charge will move away from the positive source charge and towards the negative charge

Question 10

An electric field when we have 2 identical charges (both positive or negative)

Answer 10

• the field lines will never connect the 2 point charges, instead, the field lines emanating from each of the charges will bend away from each other

Question 11

What happens if you were to take the electric field between 2 equal but opposute point charges and stretch out the source charges such that they are distributed over a long linear surface:

Answer 11

• this results in an electric field that is uniform between those surfaces
  
  • this setup is characteristic of devices known as capacitors

Question 12

what is a dipole?

Answer 12

• a linear object with opposite charges on each end

Question 13

when we place a dipole into a uniform electric field, the negatively charged end of the dipole will be pulled towards the positive source charge of the electric field and the positively charged ned of the dipole will be pulled towards the negative source charge of the electric field. This will cause?

Answer 13

• rotation that will align the dipole with the field
  
  • there is also torque because of rotation but the corresponding torques don’t cancel each other out because the relevant directionality of torque is clockwise/counterclockwise
    
    • the forces balance out, the center of the dipole does not move

Question 14

electric fields do work!

Answer 14

W = F•d = kQq/r²(r) = kQq/r

• work that is exerted on a charged particle by or against an electric field and work is the same thing as change in energy so we are dealing with electric potential energy with a formule of:
  
  • U = kQq/r

Question 15

What is electric potential (not the same as Electric PE)?

Answer 15

• denoted by V and measured in volts = 1 J/C so it is a measure of how much energy you need to move a certain amount of charge to get to whatever point in the electric field you’re measuring electric potential at (or the amount of work done by the charge to get there, if that movement is spontaneous)
  
  • V = U/q = (kQq/r)/q = kQ/r
    
    • different distances from a single source charge will be associated with different electric potentials so we can draw lines connecting all points that have the same electric potential (equipotential lines)

Question 16

two points that lie on different equipotential lines (two points with different electric potentials), so the potential difference (deltaV/voltage) between these points can be defined as?

Answer 16

• delta V = V₂ - V₁
  
  • ​or delta V = U₂/q - U₁/q = U₂ - U₁/q = delta U_2-1/q = W_2-1/q
    
    • the net work done by or against the electric field will be the same
    • no work by or against the electric field is done if a charged particles moves around on an equipotential line