# Homework Help: Electron's Speed in Field and Charge

1. Jan 21, 2007

### Soaring Crane

1. The problem statement, all variables and given/known data

An electron is released from rest in a uniform electric field of 1.25 kN/C. After 20 ns, the electron's speed will be approximately

a. 2.5 x 10^-5 m/s

b. 3.9 x 10^3 m/s

c. 3.0 x 10^8 m/s

d. 2.5 x 10^3 m/s

e. 4.4 x 10^6 m/s

2. Relevant equations

Vector E = (m_e*a)/q_e

v_f = v_i +a*t

3. The attempt at a solution

a = (E*q_e)/m_e = [(1250 N/C)*(1.60*10^-19 C)]/(9.11*10^-31 kg) = 2.1954*10^14 m/s^2

v_f = v_i +a*t, where v_i = 0 m/s

v_f = a*t = (20*10^-9 s)*(2.1954*10^14 m/s^2) = 4.39*10^6 m/s (E.) ???

1. The problem statement, all variables and given/known data

How many electrons must be transferred to a body to produce a charge of 125 nC?

a. 1.25 10^-7
b. 7.81 10^11
c. 1.28 10^12
d. 3.45 10^11

2. Relevant equations

Total q / q_e = # of electrons

3. The attempt at a solution

(125*10^-9 C)/(1.60*10^-19 C) = 7.813*10^11 electrons ???

Thanks.

2. Jan 21, 2007

### Kurdt

Staff Emeritus
Both correct as far as I can see.