# Permitivity vs permeability

1. Jan 5, 2006

### dervast

Hi i am trying to frigure out the similarities and differences between permitivity and permeability? When do we use the one or another term?

2. Jan 5, 2006

### Staff: Mentor

Permitivity pertains to electric fields, and permeability pertains to magnetic fields. What textbook are you using for this anyway?

3. Jan 7, 2006

### dervast

So its the same thing but the first term is for electricity and the second one for magneticity

4. Jan 7, 2006

### dervast

permitivity is eo and permetabilty is m0 right?

5. Jan 7, 2006

### marlon

It's permiTTivity :)

Although this is a very good question, you really should have asked about the RELATIVE permeability and permittivity.

Why ? Well, because the the "permeability and permittivity" are just the units or the "permeability and permittivity" of the vacuum. If one talks about relativity permittivity of 3.9, than one really means 3.9 times the permittivity of the vacuum.

Now, what are these guys ? Well, they are indeed quite similar since permittivity refers to "electrical phenomena" and permeability refers to "magnetic phenomena".

Let's take the (relative) permittivity. Basically, this quantity refers to how much electrical charge an object can store once you apply a voltage onto it. In other words, it expresses how an object "reacts" through rearrangement of the electronic distribution of the material, if you apply an E-field onto it. As an example, just think of what happens if you apply a voltage onto a medium with 5 plus and 5 minus charges. The charges will separate. There is another, more famous quantity that expresses this : ie the dielectric constant. This actually is the relative permittivity of a dielectric (ie a polarizable medium).

The exact same thing exists for magnetic media. Keep in mind that the electrons are NOT represented by magnetic monopoles ! This is the only assymetry between electrical and magnetic phenomena.

regards
marlon

6. Mar 24, 2009

### cbravo135

One thing that separates the two is that permittivity is a scalar and permeability is a tensor.

7. Mar 25, 2009

### Andy Resnick

That's not true- both the permittivity and permeability may be tensor-valued.

8. Mar 25, 2009

### Bob S

The speed of light, 2.9979 x 108 meters / sec, is equal to the inverse square root of the product of e0 and u0. The impedance of free space, 377 ohms, is equal to the square root of the ratio of u0 / e0. Virtually every material has a dielectric constant and index of refraction > 1, both associated with e0, while very few materials have a relative permeability u > 1 (magnetic materials), associated with u0. e0 is associated with charge, while u0 is associated with currents.

9. Feb 24, 2010

### MarkoASI

Can someone help me to know the same difference but refered to hydraulic flow rate? thanks!

10. Feb 24, 2010

### Glen Bartusch

Where did you get the idea that the impedance of "free space" is 377 Ohms? Since impedance is defined as "the total resistance in [any] circuit," that would mean that the impedance of free space would exceed several million Ohms, provided my DVM probes aren't so close together that they electrically short.

11. Feb 26, 2010

### Claude Bile

Think of permittivity and permeability as "bang for your buck" constants. That is, how much E or B field you get for a given amount of charge at some velocity, v. Permittivity applies to E fields, permeability applies to B fields.

P.S. Glen, Bob S. is correct, the impedance of free space is ~377 ohms.

Claude.

12. Feb 26, 2010

### f95toli

From e.g. any book which includes a chapter on basic antenna theory.
And the 377 Ohms of space can "short" a circuit when you are working at high frequencies, so it is a "proper" impedance;

13. Feb 19, 2012