Answer:
B = 1.37 mT
Step-by-step explanation:
Given that,
The magnitude of the electric field, E = 480 N/C
The speed of the proton,
![v=3.50 * 10^5\ m/s](https://img.qammunity.org/2022/formulas/physics/college/uvzxoory90eekqor4migh1jmicyzb6cdtp.png)
We need to find the magnitude of the magnetic field. In a velocity selector, the electric field is balanced by the magnetic field. So,
![qE=qvB](https://img.qammunity.org/2022/formulas/physics/college/oempkjs035l5tzdx7isrlx466muug1yi81.png)
Where
B is the magnetic field
![B=(E)/(v)\\\\B=(480)/(3.5* 10^5)\\\\B=1.37* 10^(-3)\ T\\\\or\\\\B =1.37\ mT](https://img.qammunity.org/2022/formulas/physics/college/tfomovc7r54wl4lk52hhpf7wjhkii9jdq8.png)
So, the magnetic field is equal to 1.37 mT.