Electrostatic Fields in Matter
Electrostatic Fields in Matter
Electrostatic Fields in Matter
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This leaves a layer of negative charge on the top surface of thedielectric and a positive layer at the bottom.The orig<strong>in</strong>al charge Q, which was placed on the capacitor plates,is referred to as free charge.The charge <strong>in</strong>duced on the dielectric surface is called bound charge.The bound charge generates a new field, <strong>in</strong> the opposite directionto the orig<strong>in</strong>al field. This new field is called the depolarisation field.The total field <strong>in</strong> the dielectric is the sum of the orig<strong>in</strong>al field (fromthe free charges) and the depolarisation field - it will be smaller thanthe orig<strong>in</strong>al field.Bound chargesFree chargesDepolarisation fieldOrig<strong>in</strong>al fieldLook<strong>in</strong>g at equations (6) and (7), we see that when a dielectricis placed between charged plates which are not connected, then Qrema<strong>in</strong>s constant while V decreases; hence C <strong>in</strong>creases. The factorby which C <strong>in</strong>creases is called the dielectric constant K (or therelative permittivity, ɛ r ).-5