Energy contents / Force manifestations


C 3.4  ENERGY CONTENTS / FORCE MANIFESTATIONS


C 3.4.1 Energy contents

The energy stored in a capacitor can be described as

C3-20eq

 

(J) or (Ws) ………………….[C3-20]


C 3.4.2 Force action from electromagnetic fields

Parallel current carrying conductors are surrounded by magnetic fields exerting forces on each other. If currents flow in the same direction the fields (and the conductors) attract each other. If the current flows in opposite directions as in the Figure C3-24 they are repelling each other.

C3-26

 

Figure C3-26. Magnetic force action, F, between conductors with a current flow, I. B = magnetic flux density.

 

If

  • the conductor length l is expressed in m,
  • the current I is expressed in A and
  • the distance a is expressed in m,

the force per meter between the conductors will be:

C3-21eq

 

……………………[C3-21]

 

According to the formula (C3-1) Q = C x V (As). If this expression is derived we obtain dQ/dt = I = C x dV/dt (A). Pulse loads are not unusual, especially in conditions with high voltage gradients, and thus high charging currents also occur which might cause appreciable magnetic fields between close lead patterns, for example.


C 3.4.3 Force action in electrostatic fields

Capacitors are typical examples of applications where electrostatic fields are applied. These fields can generate significant mechanical forces­. If we know the electrode distance d (m) it’s easy to determine the electric field strength E (V/m). Then we can outline the force per unit area, i.e. the pressure that the electrodes exert on the dielectric.

C3-22aeq

 

 

………… [C3-22]

 

Example. Suppose we have an oil impregnated paper capacitor with εr = 5 and the dielectric = 15 μm (0.6 mils) which is loaded with 250VAC. Then the instantaneous

maximum pressure will be

C3-22beq

0.1 kp/cm2!

 

If we instead calculate on a 35 V solid tantalum capacitor with a typical and approximate dielectric thickness of 0.2 μm (0.008 mils) the formula gives at 30 V DC a

pressure of

C3-22ceq

2 N/mm2!

 

It is difficult to determine how much the dielectric is influenced by such forces, especially when the electrodes have such complex configurations, as we shall discuss later. It is the author’s opinion that electrostatic action of such forces here is of vital importance but this subject will be dealt with under the appropriate headline.


rev.1.: P-O.Fagerholt., CLR Passive Components Handbook

© European Passive Components Institute