MCAT Physics Question 68: Answer and Explanation
Home > MCAT Test > MCAT physics practice tests
Test Information
- Use your browser's back button to return to your test results.
- Do more MCAT physics practice tests.
Question: 68
8. If the magnetic field a distance r away from a current-carrying wire is 10 T, what will be the net magnetic field at r if another wire is placed a distance 2r from the original wire and has a current twice as strong flowing in the opposite direction?
- A. 0 T
- B. 15 T
- C. 20 T
- D. 30 T
Correct Answer: D
Explanation:
The safest way to answer this question is to quickly draw a diagram:
Notice right away that between the two wires, the direction of the magnetic field is the same: into the page. Therefore, because the vector direction is the same, we can just focus on the magnitudes of the two magnetic fields. We know that B1 = 10 T at a distance r. Consider the relationships in the equation 
Magnetic field and current are directly proportional, whereas magnetic field and distance are inversely proportional. Therefore, doubling the current will result in double the magnetic field as the first wire, or 20 T. The overall magnitude of the magnetic field is 10 T + 20 T = 30 T into the page.