Now, a wire’s field cannot produce a force on itself. The field that we drew is caused by the left wire,
but produces a force on the right-hand wire. Which direction is that force? Use the right-hand rule for the
force on a charged particle. The charges are moving up, in the direction of the current. So point up the
page, and curl your fingers toward the magnetic field, into the page. The right wire is forced to the LEFT.
Newton’s third law says that the force on the left wire by the right wire will be equal and opposite.^2 So,
the wires attract, answer A.
Often, textbooks give you advice such as, “Whenever the current in two parallel wires is traveling in
the same direction, the wires will attract each other, and vice versa.” Use it if you like, but this advice can
easily be confused.
Mass Spectrometry: More Charges Moving Through Magnetic Fields
A magnetic field can make a charged particle travel in a circle. Here’s how it performs this trick.
Figure 20.7a Positively charged particle moving in a magnetic field directed out of the page.Let’s say you have a proton traveling through a uniform magnetic field coming out of the page, and the
proton is moving to the right, like the one we drew in Figure 20.7a . The magnetic field exerts a
downward force on the particle (use the right-hand rule). So the path of the particle begins to bend
downward, as shown in Figure 20.7b .