Dipole systems direct current electric field

FIGURE 27.2 Sensitivity distributions, that is, lead fields of lead systems detecting (a) electric and (b) magnetic dipole moments of a volume source. The lead field of the electric lead is shown both with vectors representing the magnitude and direction of the lead field (on the left) and with lead field current flow lines (on the right). 

Before moving on, we note that the static (i.e. measured using direct current) relative permittivity, , is the ratio of permittivities of the medium to fliat of free space (vacuum). It describes how much electric field is generated per unit charge and is a measure of how an electric field is affected by a medium. A high value of e, such as that of water (80.2 at 20 °C) means that it is easy to charge particles in aqueous systems. The permanent dipoles of water can orient themselves and stabilize the charge on particles. 

Tunnelling can occur from tip to sample and from sample to tip. If no bias is applied to the system (i.e. if the electrochemical potentials of the electrons in tip and sample far from the junction are equal) the rates of tunnelling in opposite directions are equal, and no net current flows. (Note that if the tip and sample have different work functions, a finite charge transfer will occur at zero bias to establish a dipole layer at the surfaces, and hence an electric field in the vacuum gap it is the potential difference arising... 

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