In the context of the magnetic field produced by straight currents, what causes the magnetic field around a long, straight wire?

• A. The wire's electrical resistance by itself, even with no current
• B. A static accumulation of charge on the wire with no current
• C. The presence of an external electric field alone, even if no charges move
• D. The motion of electric charges in the wire, i.e., an electric current

Answer: (D)

Verified step by step guidance

1. Understand that a magnetic field is generated by moving electric charges, which means that stationary charges or static electric fields do not produce a magnetic field.
2. Recall the Biot-Savart Law, which relates the magnetic field \(\mathbf{B}\) at a point in space to the current \(I\) flowing through a conductor: \(\mathbf{B} = \frac{\mu_0}{4\pi} \int \frac{I d\mathbf{l} \times \mathbf{\hat{r}}}{r^2}\), where \(d\mathbf{l}\) is an element of the wire, \(\mathbf{\hat{r}}\) is the unit vector from the wire element to the point, and \(r\) is the distance.
3. Recognize that the magnetic field around a long, straight wire is due to the electric current, which is the flow of charges, not due to the wire's resistance or static charge accumulation.
4. Note that the presence of an external electric field alone, without moving charges, does not create a magnetic field; magnetic fields arise specifically from moving charges or changing electric fields.
5. Conclude that the magnetic field around a long, straight wire is caused by the motion of electric charges (electric current) flowing through the wire.