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# Physics field question ?

(a) An positron (electron with a positive charge) starts at rest and accelerates through an electric field established by a set of parallel plates with a potential difference of 35 V. What is the speed of the positron the instant before it hits the negative plate?

(e = 1.6 × 10-19 C, melectron = 9.1 × 10^-31 kg)

(b) Instead of hitting the negative plate, the positron, travelling East, escapes the parallel plates through a small hole and enters a magnetic field of 0.75 T directed downward. What will be the magnetic force (magnitude and direction) on the charge?

(c) Once the positron has entered the magnetic field, it is in circular motion. What is the radius of the positron's circular path?

### 1 Answer

- ?Lv 72 months ago
I suppose we are to assume that the positron starts quite near the positive plate! -- though the question doesn't say so! The final kinetic energy of the particle will be 35 electron-volts, which is 5.6 x 10^(-18) J, and this is (1/2)mv^2 if relativistic effects can be ignored. Dividing out mass of a positron (= mass of electron), you get 6.154 x 10^12 m^2/s^2; dividing out the "1/2" you get 12.31 x 10^12 m^2/s^2; and taking the square root you get 3.5 x 10^6 m/s. This is slow enough that you can ignore relativistic effects, as this speed is only about 1% of the speed of light.