Question

Chemists (and Astronomers!) like to express lengths in Angstrom units where 1 Angstrom = 10^-10m. roughly the length of a chemical bond. What is the repulsive energy of two protons that are 1 Angstrom unit apart? State your answer in both Joules and Electron Volts. The resistivity of carbon is 3.5x10^-5 Ohm m. Remembering that R=pL/A. how long a piece of carbon, 1 mm in radius would be needed for a 4000 Ohm resistor. What would the electric field be inside the resistor if the two ends were kept at a 5 volt difference in potential? A 2000 Ohm resistor and a 125 mu F capacitor are connected in series and the combination placed across the terminals of a 100 Volt battery. What is the current through the resistor a time 0? What will the current be at long times? What is the charge stored in the capacitor at time 0? What will the charge be at long times?

Answer 1

B)
r = 1*10^-10 m
q1=q2 = 1.6*10^-19 C
U = K*q1*q2/r
     = (9*10^9)*(1.6*10^-19)*(1.6*10^-19) / (1*10^-10)
     = 2.304*10^-18 J
    = (2.304*10^-18)/(1.6*10^-19)
    = 14.4 eV
Answer:
2.304*10^-18 J
14.4 eV

C)
rho = 3.5*10^-5 ohm-m
r = 1*10^-3 m
A = pi*r^2
    = pi*(1*10^-3)^2
    = 3.14*10^-6 m^2
R = 4000 ohm

R = rho*L/A
4000 = 3.5*10^-5 * L/ (3.14*10^-6)
L = 359 m
Answer: Length is 359 m

E = V/L
    = 5/359
     = 0.014 V/m
Answer: Electric field is 0.014 V/m

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