# The Tribocharging Model

For repeated contacts, a saturation charge level is reached which is a function of both charge generation and dissipation processes. In most ESD (electrostatic discharge) problems, triboelectrication is the prime charge generation process.

In EDEM, you must specify the work function for each material. If work functions between two materials are different, tribocharging (triboelectric charging) will take place. If work functions are the same there will be no charge transfer.

The following equation is based on work by Greason (2000) and describes the change in charge:

where q is a charge of a sphere at time t, α and β are the time constants of the charge generation and dissipation respectively. α is in the range of 0.02–0.085 s-1 for polymers and in the range 0.002–0.005 s-1 for metals. β is neglected in calculations because charge dissipation through atmospheric ion impingement is slow (Hogue, et al. 2007).

And qs is a saturation charge, which can be defined as:

where σ is a surface charge density which is approximately equal to 2.66e-5 cm-2 for air standard pressure and temperature, and As is the surface area of a particle.

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