A magnetic field produces current in a wire as it pushes electrons in a certain direction until the electrons compress and their electrostatic repulsion counters the magnetic driving force; equilibrium is reached, there is no current. A wire within a constant magnetic field will always reach an equilibrium state in which there is no current in the wire. Since equilibrium is reached so quickly, we see that a current can only be achieved when we change the magnetic field, if we keep changing the strength of the field, we change do not allow the system to reach equilibrium, thereby maintaining a current.
This is entirely different from the reason a current creates a magnetic field which is due to relativity. While intuitively they may seem related, the facts of changing magnetic fields producing current and a current producing magnetic fields have nothing to do with each other. It is a common misunderstanding that you think the magnetic field does not change. Even if the magnetic field does not change in space its position , it changes in the time dimension.
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Figure 3 shows how the field looks and how its direction is given by RHR Figure 3. The field outside the coils is nearly zero. The magnetic field inside of a current-carrying solenoid is very uniform in direction and magnitude.
Only near the ends does it begin to weaken and change direction. The field outside has similar complexities to flat loops and bar magnets, but the magnetic field strength inside a solenoid is simply. Note that B is the field strength anywhere in the uniform region of the interior and not just at the center.
Large uniform fields spread over a large volume are possible with solenoids, as Example 2 implies. What is the field inside a 2. First, we note the number of loops per unit length is. This is a large field strength that could be established over a large-diameter solenoid, such as in medical uses of magnetic resonance imaging MRI.
The very large current is an indication that the fields of this strength are not easily achieved, however. Higher currents can be achieved by using superconducting wires, although this is expensive. There is an upper limit to the current, since the superconducting state is disrupted by very large magnetic fields.
There are interesting variations of the flat coil and solenoid. For example, the toroidal coil used to confine the reactive particles in tokamaks is much like a solenoid bent into a circle. The field inside a toroid is very strong but circular. Charged particles travel in circles, following the field lines, and collide with one another, perhaps inducing fusion.
But the charged particles do not cross field lines and escape the toroid. A whole range of coil shapes are used to produce all sorts of magnetic field shapes. Adding ferromagnetic materials produces greater field strengths and can have a significant effect on the shape of the field. The direction of the magnetic field created by a long straight wire is given by right hand rule 2 RHR-2 : Point the thumb of the right hand in the direction of current, and the fingers curl in the direction of the magnetic field loops created by it.
RHR-2 gives the direction of the field about the loop. A long coil is called a solenoid. The field inside is very uniform in magnitude and direction. Largely because of that poor conductivity, the power the team predicts is small. A cylinder 20 cm long and 2 cm across would generate tens of nanowatts at tens of microvolts. Chyba thinks there could be ways to increase those numbers, but he emphasizes that the first order of business is an experimental test to show that the mechanism really works.
Chyba says that if the mechanism proves correct—and he is adamant that only experiments can say for sure—he hopes engineers will get to work to improve the output. One possibility worth exploring, he suggests, would be a two-layer cylinder in which the slow magnetic material induces a current-generating field geometry in an adjacent material with higher conductivity. This research is published in Physical Review Applied. Applied 6 , A new model of light-matter interactions solves a decades-old problem by reconciling theoretical predictions and experimental observations of polarized light from the Sun.
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