# I Why cant thelectric field be in circumferential direction?

1. Nov 12, 2016

### garylau

Sorry

in this question i have several things to ask
1.why cant the E field in the circumferential direction?
2.How can i find the direction of B field

3.according to the formula of curl E=-dB/dt and the curlB=u J
did they share the same property so that the direction of E is circumferential (just like B dl=u i which is ampere law so that the direction B is circumferential) and dB/dt is going straight(just the the current is going straight which is enclosed inside the boundary)?

thank

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2. Nov 12, 2016

### Simon Bridge

1. I don't know what you mean by "circumferential".
Clearly it is possible to set up an electric field that goes in a loop: it's called an electric circuit.
But you cannot have a circular field line because the line has to start and end at different potentials.

2. You use a small magnetised needle balanced on a pivot.

3. The similarity in the equations shows that there is a similarity in the equations ...
Note, they should be:
$\nabla\times\vec E = -\partial_t\vec B$ and
$\nabla\times\vec B = \mu_0\left( \vec J +\epsilon_0\partial_t \vec E \right)$
... if $\vec J = 0$ the two equations look even more alike ;)

Check what "curl" means:
https://en.wikipedia.org/wiki/Curl_(mathematics)

3. Nov 12, 2016

### garylau

you can see this picture clearly
the solution states:The magnetic field is “circumferential” in the quasistatic approximation."

.....but i dont know how did the solution concluded "Thus the direction of the electric field is longitudinal" from the simiarlty between the relation between the electric and magnetic fields?

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4. Nov 13, 2016

### Cutter Ketch

What breaks the symmetry between the magnetic and the electric is the lack of magnetic monopoles: north and south magnetic charges that emanate magnetic field in the way positive and negative electric charges emanate electric field. If you had a magnetic wire carrying magnetic monopoles in a straight line they would generate a circumferential electric field in the same way an electric current generates a circumferential magnetic field.

5. Nov 13, 2016

### garylau

But Why did the Electric field outside vanished in this case?

6. Nov 13, 2016

### garylau

But Why did the Electric field outside vanished in this case?

7. Nov 13, 2016

### Simon Bridge

In what case? Please be specific.

8. Nov 13, 2016

### garylau

See the statement inside the red circle.

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9. Nov 13, 2016

### Simon Bridge

Oh so the question had nothing to do with the quoted text? Fine.
To see why the electric field vanishes, try doing the problem.

10. Nov 13, 2016

### garylau

because the total current inside the object is 0?

11. Nov 13, 2016

### Staff: Mentor

That contributes to it, yes, but the main point is the absence of magnetic charges and the fact that the problem assumes a static situation.