Concept #1: Magnetic Field Produced by Straight Currents

Example #1: Find Field due to Two Perpendicular Currents

Example #2: Find Zero Magnetic Field

The magnetic field at a certain distance from a wire carrying 2-A current is 4 μT. What will be the magnetic field if the current in the wire is doubled?
A) 8 μT
B) 6 μT
C) 4 μT
D) 2 μT

Three very long, straight, parallel wires each carry currents of 4 A, directed out of the page in the drawing in Figure 3. The wires pass through the vertices of a right isosceles triangle of side 2 cm. What is the magnitude of the magnetic field at point P at the midpoint of the hypotenuse of the triangle?

In the figure, the two long straight wires are separated by a distance of d = 0.40 m. The currents are I1 = 1.0 A to the right in the upper wire and I 2 = 8.0 A to the left in the lower wire. Find the magnitude and direction of the magnetic field at the center point O, and also point P, a distance d/2 = 0.20 m below the lower wire? (NOTE: μ0 = 4π x 10-7 T•m/A.)

A long straight wire carries a current of I = 3.00 A directed to the left, as shown in the sketch. A small sphere with negative charge q = -5.00 x 10-4 C is moving from the wire. When the sphere is 0.080 m from the wire, its speed is v = 9.00 m/s. What are the magnitude and direction of the magnetic field produced by the wire at the location of the sphere, 0.080 m below the wire? Write the direction in the blank provided and also show the direction on the sketch.
Ans. B =
direction =

A long straight wire carries a current of I = 3.00 A directed to the left, as shown in the sketch. A small sphere with negative charge q = -5.00 x 10-4 C is moving from the wire. When the sphere is 0.080 m from the wire, its speed is v = 9.00 m/s. What are the magnitude and direction of the force that the wire exerts on the small sphere when the sphere is at the position shown in the sketch? Write the direction in the blank provided and also show the direction on the sketch.
Ans. F =
direction =

The magnetic field created by a long straight wire carrying a current iA) decreases by a factor of four as the distance doubles.B) increases by a factor of four as the distance doubles.C) remains unchanged as the distance doubles.D) decreases by a factor of two as the distance doubles.E) increases by a factor of two as the distance doubles.

A very long straight current-carrying wire produces a magnetic field of 20 mT at a distance d from the wire. To measure a field of 5 mT due to this wire, you would have to go to distance of A) 2dB) 8dC) d√2D) 4dE) 16d

Two long straight parallel wires carry currents as shown in the sketch. Point A is midway between the two wires, 0.20 m from each wire. What are the magnitude and direction of the net magnetic field at point A due to the two wires?

A long, straight wire lies along the y-axis and carries current in the positive y-direction. A positive point charge moves along the x-axis in the positive x-direction. The magnetic force that the wire exerts on the point charge is inA. the positive x-direction.B. the negative x-direction.C. the positive y-direction.D. the negative y-direction.E. none of the above.

Two long wires carrying currents I 1 and I2 in the directions shown, with I 2 = 4I1, cross at right angles in the xy plane. Consider the points A = (2d, d) and B = (2d, −d), where d is a positive number. Find the ratio of the magnitudes of the magnetic fields at these two points, i.e., what is |BB/BA|?
A) 9/7
B) 1
C) 5/3
D) 4
E) None of these

The figure represents two long, straight, parallel wires carrying equal currents extending in a direction perpendicular to the page. The current in the right wire runs into the page and the current in the left runs out of the page.What is the direction of the magnetic field created by these wires at location a, b and c? (b is midway between the wires.)
1. down, zero, down
2. up, down, up
3. up, zero, up
4. down, down, up
5. up, zero, down
6. down, zero, up
7. down, up, down
8. up, up, down

Four long parallel wires each carry 10.0 A of current in the direction shown. The wires are at the corners of a square that is 10.0 cm on a side. What is the magnetic field at the center of the four wires?A. 0B. 1.13 x 10-4 TC. 5.66 x 10-5 TD. 8.00 x 10-5 TE. 2.00 x 10-5 T

In the figure below, the two long straight wires are separated by a distance of d = 0.40 m. The currents are I1 = 1.0 A to the right in the upper wire and I2 = 8.0 A to the left in the lower wire. What is the magnitude and direction of the magnetic field at point P, that is distance d/2 = 0.20 m below the lower wire?Note: μ0 = 4π x 10-17 T x m/AA. B = 7.7 x 10-6 T, directed into the plane of the page.B. B = 8.3 x 10-6 T directed into the plane of the page.C. B = 7.7 x 10-6 T directed out of the plane of the page.D. B = 8.3 x 10-6 T directed out of the plane of the page.

Two long parallel wires carry currents of 20 A and 5 A in opposite directions. The wires are separated by 0.20 m. What is the magnitude of the magnetic field midway between the two wires? (μ0 = 4π x 10-7 T•m/A)A) 3.0 x 10-5 TB) 1.0 x 10-5 TC) 2.0 x 10-5 TD) 4.0 x 10-5 TE) 5.0 x 10-5 T

The figure shows four different sets of insulated wires that cross each other at right angles without making electrical contact. The magnitude of the current is the same in all the wires, and the direction of current flow are as indicated. For which (if any) configuration will the magnetic field at the center of the square formed by the wires be equal to zero?
A) A
B) B
C) C
D) D
E) The field is not equal to zero in any of these cases.

At what distance from the central axis of a long straight thin wire carrying a current of 5.0 A is the magnitude of the magnetic field due to the wire equal to the strength of the Earth's magnetic field of about 5.0 x 10-7 T (μθ = 4π 10-7 T•m/A)A) 1.0 mB) 2.0 mC) 4.0 mD) 5.0 mE) 3.0 m