Force between parallel conductors; definition of the ampere — practice questions

Question 1 · difficulty L1 · recall

Two long parallel wires carrying currents in the same direction

• A. attract each other
• B. repel each other
• C. exert no force
• D. alternate between attraction and repulsion

Question 2 · difficulty L1 · understanding

Two long parallel wires carrying currents in opposite directions

• A. attract each other
• B. repel each other
• C. exert no force
• D. must twist but not translate

Question 3 · difficulty L2 · understanding

The force per unit length between two long parallel wires separated by d and carrying currents I1 and I2 is

• A. μ0I1I2/(4πd²)
• B. μ0I1I2d/(2π)
• C. μ0I1I2/(2πd)
• D. μ0(I1+I2)/(2πd)

Question 4 · difficulty L2 · application

If the separation between two parallel current-carrying wires is doubled while currents are unchanged, the force per unit length becomes

• A. twice
• B. one-fourth
• C. unchanged
• D. one-half

Question 5 · difficulty L3 · application

If both currents in two parallel wires are doubled at fixed separation, their force per unit length becomes

• A. four times
• B. twice
• C. half
• D. unchanged

Question 6 · difficulty L3 · application

In the traditional force-based definition, two 1 A currents in parallel conductors 1 m apart in vacuum produce a force per metre of

• A. 2 × 10^-6 N m^-1
• B. 2 × 10^-7 N m^-1
• C. 4π × 10^-7 N m^-1
• D. 1 × 10^-7 N m^-1

Question 7 · difficulty L4 · analysis

Three identical long parallel wires carry equal currents in the same direction and are equally spaced in a straight line. The net force on the middle wire is

• A. toward the left wire
• B. toward the right wire
• C. zero
• D. twice the force from one neighbour

Question 8 · difficulty L4 · analysis

Two parallel wires carry 2 A and 3 A and are 0.50 m apart. Which proportional quantity determines their force per unit length?

• A. I1+I2+d = 5.5
• B. I1I2d = 3 A² m
• C. (I1+I2)/d = 10 A m^-1
• D. I1I2/d = 12 A² m^-1

Question 9 · difficulty L5 · analysis

If one of two parallel currents is reversed while magnitudes remain unchanged, the force between the wires

• A. keeps the same magnitude but changes from attraction to repulsion or vice versa
• B. doubles without changing direction
• C. becomes zero
• D. halves and keeps direction

Question 10 · difficulty L5 · analysis

Why can the mutual forces on two parallel current-carrying wires satisfy Newton's third law even though each force is described using a magnetic field created by the other wire?

• A. Magnetic forces are exempt from momentum conservation
• B. The fields and currents give equal-magnitude opposite forces on the two wires
• C. One wire experiences force while the other does not
• D. The forces are equal only when the currents are unequal