Summary

• Electric potential energy
• Electric potential
• Point charges

• Class Quiz    Timer
  
  
• Field and potential inside conductors
  
  
• Capacitance
  • definition and applet
    Example #1
    
    Example #2
    
  • energy storage in capacitors
  • energy storage in fields
    Example #3
    
  • practice
    Example #4
    

gc6 17.q8
If V=0 at a point in space, must E=0 there? ____
If E=0 at a point in space, must V=0 there? ____
A. yes . . . yes
B. yes . . . no
C. no . . . yes
D. no . . . no
Answer







sj6 26.2
Conductors with charges +10µC and -10µC have a potential difference of 10 V. What is the capacitance?
A. 1 pF                B. 10 pF
C. 1 µF                D. 100 µF
Answer







sj6 26.2b
If the charges are increased by a factor of ten, what will happen to the potential difference?
A. decrease by factor of 100
B. decrease by factor of 10
C. stay the same
D. increase by factor of 10
Answer







gc6 17.50
A capacitor has fixed charges on its plates as the separation between the plates is doubled. What happens to the energy stored in the electric field?
A. PE₂ = 4 PE₁                B. PE₂ = 2 PE₁
C. PE₂ = 0.5 PE₁              D. PE₂ = 0.25 PE₁
Answer







sb5 26.47
A capacitor with A=25 cm² and d=1.5 cm is charged to 250 V. What is the charge on its plates?
A. 151 nC           B. 369 pC
C. 519 µC           D. 17.1 mC
Answer







 



D. no . . . no
There is no general relationship between the value of V and the value of E. Instead, the magnitude of E is equal to the rate at which V decreases over a short distance. Consider the point midway between two positive charges. E is 0 there, but V is high. Or, consider the point midway between two negative charges. E is also 0 there, but V is low, because it is negative. Finally, consider the point midway between positive and negative charges of equal magnitude. There E is not 0, because it points towards the negative charge, but V is zero.












 



C. 1 µF













 



D. increase by factor of 10
The capacitance will remain constant as long as the geometry of the capacitor remains constant. Therefore since C = Q/V the potential difference will increase in linear proportion to the amount of charge.












 



B. PE₂ = 2 PE₁
The energy stored in the capacitor is proportional to the charge squared divided by the capacitance, but the capacitance will be cut in half if the plate separation is doubled. Therefore, the energy stored will double. This is because it requires work to pull apart the oppositely charged plates that attract each other.












 



B. 369 pC