In [4]:

```
import math
#Variables
phi = 3.4 #Voltage (in electron-volt)
e = 1.6 * 10**-19 #Charge on electron (in Coulomb)
A = 6.0 * 10**4 #Emission constant (in Ampere per meter-square per kelvin-square)
T = 2000.0 #Temperature (in kelvin)
l = 40.0 * 10**-3 #Length (in meter)
D = 0.2 * 10**-3 #Diameter (in meter)
k = 1.38 * 10**-23 #Boltzmann constant (in meter-square kilogram per second-square per kelvin)
#Calculation
b = phi * e /k #Constant
Js = A*T**2*math.exp(-b/T) #Emission current density (in Ampere per meter-square)
S = math.pi * D * l #Emitting surface (in meter-square)
I = Js * S #Emission current (in Ampere)
#Result
print "Emission current is ",round(I,4)," A."
#Slight variation due to higher precision.
```