alimerzairan
Jan17-11, 12:44 AM
1. The problem statement, all variables and given/known data
Electrical breakdown (e.g, lightning) is caused by an avalanche process. If a free electron gains enough energy between collisions to ionize the neutral molecules when they collide with them, then those two electrons will gain enough energy between collisions to ionize the next neutral molecules they collide with and the ionization of the gas grows exponentially. Conversely, if the electron doesn’t gain enough energy to ionize the molecule, it will lose energy in collisions with neutral molecules and will eventually recombine.
(a) Estimate how large the electric field must be for breakdown to occur. The electron needs to gain ∼ 200 eV of energy between collisions and the maximum cross section for ionization of air molecules is about 3 × 10−16 cm2.
(b) If the electric field is large enough for breakdown to occur, estimate how fast the current channel of free electrons forms.
2. Relevant equations
V = E d
d = mean free path
3. The attempt at a solution
V = 200 V since the electron gains 200 eV.
d is calculated by \frac{V}{N_{A} \sigma} , where N_{A} is Avogadro's number and in STP with V = 22.4 x 10-3 m.3.
Don't know if this exactly right. I get d = 1240.3 nm and an electric field for breakdown to occur of 161.25 \times 10^{6} \frac{V}{m} while I read that it should be around 3 \times 10^{6} \frac{V}{m}.
Also, for part (b) I don't even know what they are asking for.
Any help before Tuesday, January 18 at 1:00 Pacific Time (GMT -0800) would be greatly appreciated.
Electrical breakdown (e.g, lightning) is caused by an avalanche process. If a free electron gains enough energy between collisions to ionize the neutral molecules when they collide with them, then those two electrons will gain enough energy between collisions to ionize the next neutral molecules they collide with and the ionization of the gas grows exponentially. Conversely, if the electron doesn’t gain enough energy to ionize the molecule, it will lose energy in collisions with neutral molecules and will eventually recombine.
(a) Estimate how large the electric field must be for breakdown to occur. The electron needs to gain ∼ 200 eV of energy between collisions and the maximum cross section for ionization of air molecules is about 3 × 10−16 cm2.
(b) If the electric field is large enough for breakdown to occur, estimate how fast the current channel of free electrons forms.
2. Relevant equations
V = E d
d = mean free path
3. The attempt at a solution
V = 200 V since the electron gains 200 eV.
d is calculated by \frac{V}{N_{A} \sigma} , where N_{A} is Avogadro's number and in STP with V = 22.4 x 10-3 m.3.
Don't know if this exactly right. I get d = 1240.3 nm and an electric field for breakdown to occur of 161.25 \times 10^{6} \frac{V}{m} while I read that it should be around 3 \times 10^{6} \frac{V}{m}.
Also, for part (b) I don't even know what they are asking for.
Any help before Tuesday, January 18 at 1:00 Pacific Time (GMT -0800) would be greatly appreciated.