Question

Describe two types of fields pertinent to electronic circuits, and explain why there are no ideal resistors, capacitors or inductors.

Answer 1

The two types of fields are electric and magnetic fields.

An electric field is a field or space around an electrically charged object where any other electrically charged object will experience a force.An electric field is measured by a term known as electric field intensity. If we place a positive unit charge near a positively charged object, the positive unit charge will experience a repulsive force. Due to this force, the positive unit charge will move away from the said charged object. The imaginary line through which the unit positive charge moves, is known as line of force.

Magnetic field

Strong static magnetic fields will have little effect on most electronics, including most of the ones on your list. We routinely placed basic electronics in 1-2T fields, and never had any real problems. There are some exceptions though (this is probably not a complete list):

• Relays work by using a magnet to move a piece of ferrous material to close a switch. Strong fields may open them, close them, or get them stuck in whatever position they are in.
• Inductors and transformers have magnetically soft cores, which couple magnetic flux much better than air. They are designed for a certain flux density based on the properties of the coils in them, and beyond that they saturate, becoming less effective. The field they create adds with the external field, so the external field can cause early saturation. This reduces the inductance of inductors and ferrites, and reduces the efficiency of transformers. It may also introduce harmonic content in transformers.
• Hall sensors obviously measure magnetic field, and will measure the external field. Watch out for them in motors and in some sensors.
• LVDT position sensors probably won't be remotely accurate in an external field.

One simple variation from ideal is that physical devices like resistors, inductors, and capacitors have some level of tolerance around the ideal value (the tighter the tolerance, the more money you pay). Real components never have exactly their specified values.

Real circuit elements deviate from the ideal equations when voltage or current are taken to extremes. The model breaks down at some point and the component could be destroyed. Abstract models of all the ideal components and sources have a limited range in the real world.

A real component is not just that component. Using a resistor as an example: because the wires connected to the ends of a resistor generate a surrounding magnetic field, it will inevitably display some inductive properties. In addition, resistors are made of conducting materials, and are usually located near other conductors. Together these conductors act like the plates of a capacitor, so resistors also display some capacitive properties.

These parasitic effects can be relevant at high frequencies, or when voltage or current changes sharply. If parasitics matter, you can model a component as a combination of ideal elements.