A current element dl points up the page. The unit vector rhat from that element to the field point points right. Which way does its Biot-Savart contribution point?

• Into the page, perpendicular to both vectors
• Out of the page, perpendicular to both vectors
• Up the page, parallel to the current element

One current element makes a nonzero field contribution. Hold its current, length and angle to the field-point direction fixed, excluding parallel and antiparallel orientations. How does the contribution's magnitude depend on distance r?

• It grows in proportion to the square of distance
• It falls in proportion to one over distance alone
• It falls in proportion to one over distance squared

At a fixed point away from an infinitely long straight wire, all its current elements contribute magnetic field in the same direction.

A circular wire loop of radius R carries steady current I in vacuum. Which expression gives its field magnitude at axial distance x from its centre?

• mu0 I R^2/[2(R^2+x^2)^(3/2)]
• mu0 I R^2/[2(R^2+x^2)^(1/2)]
• mu0 I R^2/[2(R^2+x^2)^(5/2)]

An infinitely long straight wire carries unchanged nonzero steady current. At four times the original perpendicular distance, what fraction of the original field magnitude remains? Give a decimal.

Two long, thin, parallel wires in vacuum carry nonzero steady currents in opposite directions. What magnetic interaction occurs?

• They attract and pull toward one another
• They repel and push away from one another
• Their magnetic forces cancel on each wire

For two long parallel wires, both current magnitudes double and their separation doubles. Their current directions stay unchanged. By what factor does magnetic force magnitude per unit length change?

A semicircular wire arc of radius R carries steady current I in vacuum. What field magnitude does this arc alone produce at its centre, excluding every other circuit part?

• mu0 I/R
• mu0 I/(2R)
• mu0 I/(4R)