# Magnetic Fields Confusion

Hello everyone

Ive got myself into a muddle over the forces involved in magnetic fields.

I understand that if a current carrying wire is perpendicular to a magnetic field then it will experience a force according to Flemings Left Hand Rule.

Now if a wire is to move in the same direction as the motion 'induced' above. at right angles to the same magnetic field (this time with no current flowing in it) the emf induced is in the opposite direction to the current in the top example. (If you try to use flemings left hand rule here the current goes in the complete opposite (wrong) direction.

What causes this change in direction? Surely both situations are identical, since the direction of motion and the direction of the magnetic field are the same in both, yet the current in one is in the opposite direction to the induced current in the other situation? Is it a product of Lenz Law?

Hopefully you can understand what ive said!

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Doc Al
Mentor
I understand that if a current carrying wire is perpendicular to a magnetic field then it will experience a force according to Flemings Left Hand Rule.
OK. Now ask yourself: If the wire were to move in the direction of the force, what direction would the induced current flow? Would it be in the same direction as the original current or opposite?

vanhees71
Gold Member
2019 Award
I've never heard of a left-hand rule. That's very confusing. Usually everything is right-handedly oriented. In the context of electromagnetism the most important rules are those for the vector product (in the magnetic part of the Lorentz force) and the relative orientation of area and its boundary in the integral form of Faraday's Law, which reads in the most general case of moving wires

$$\mathcal{E}=\int_{\partial F} \mathrm{d} \vec{x} \cdot (\vec{E}+\vec{v} \times \vec{B}) = -\frac{\mathrm{d}}{\mathrm{d} t} \int_{F} \mathrm{d}^2 \vec{F} \cdot \vec{B}=-\frac{\mathrm{d}}{\mathrm{d} t} \Phi_{\vec{B}}.$$

Doc Al
Mentor
I've never heard of a left-hand rule.
There are many mnemonic rules (unfortunately) for various situations. (See: "[URL [Broken] Left Hand Rule[/URL])
That's very confusing.
I agree. The only rule I ever use is the right hand rule for computing cross products.

(But I don't think confusion over the hand rule is the issue here.)

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Ok so in fact in the first situation an emf would be induced too in the opposite direction to the original current. So does this mean if you were to connect an ammeter to the wire in the first situation you would notice a drop in current?