Question

# What we do to measure the mass of subatomic particles is to shoot them into a magnetic field and measure the resulting circular path. We set up the magnetic field

region with a current in two Helmholz coils (see your homework problem 30.61). Basically, this means that the field has equal contributions from each coil; each coil produces a magnetic field measured some distance z away from the center of N loops of wire: Here, we have 2 coils (R = 15.5 cm) whose B fields add together midway between them (z=7.75 cm from each coil), and we have N = 130 turns for each coil. 1. В =__________ If the current is 2 A in each coil, calculate the total magnetic field due to both. 2. v= _________ The electrons are introduced into the magnetic field region by accelerating them through a voltage of 300 V. By conserving energy, write down an equation for the speed of the electrons as a function of their mass 3. me=___________ OK, remember we said the radius of the circular path would be for electrons. When doing this experiment, you observe that the r = 4 cm. So if you substitute your expression for v, you should be able to solve for the mass of the electron! 4._______ Look up the exact (accepted) value for me and just calculate the percentage difference between what we got and the "real" value. Not bad for measuring tiny masses on a tabletop! r=\frac{m v}{e B} \boldsymbol{B}=\frac{\boldsymbol{N} \mu_{3} \boldsymbol{R}^{2}}{2\left(\boldsymbol{R}^{2}+\boldsymbol{z}^{2}\right)^{3 / 2}} \boldsymbol{I}

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