Transcript ch_24_poss_elmo

Examples
•
In a 9 volt battery, typically used in IC circuits, the positive terminal has a
potential 9 v higher than the negative terminal. If one micro-Coulomb of
positive charge flows through an external circuit from the positive to negative
terminal, how much has its potential energy been changed?
V 
q
U
 Vb  Va  0  9
q
U  9q
= -9V x 10-6 C
U = -9V x 10-6 Joules
U = -9 microJoules
Potential energy is lower by 9 microJoules
Examples
A proton is placed in an electric field of E=105 V/m and released. After going 10
cm, what is its speed?
Use conservation of energy.
a
E = 105 V/m
d = 10 cm
b
v
+
V = Vb – Va = -Ed
v
U = q V
U + K = 0
K = (1/2)mv2
K = -U
(1/2)mv2 = -q V = +qEd
2qEd
m
2(1.6  10 19 C)(10 5 Vm )(0.1m)
1.67  10 23 kg
v  1.4  104
m
s
Electric potential due to a positive point charge
Hydrogen atom.
•What is the electric potential at a distance of 0.529 A from
the proton?
+
r
r = 0.529 A
1A =10-10 m
• V = kq/r = 8.99*109 N m2//C2 *1.6*10-19 C/0.529*10-10m
• V = 27. 2 J/C = 27. 2 Volts
What is the electric potential energy of the electron at that point?
U = qV= (-1.6 x 10-19 C) (27.2 V)= - 43.52 x 10-19 J
or - 27.2 eV where eV stands for electron volts
Total energy of the electron in the ground state of hydrogen is - 13.6 eV
Also U= 2E = -27.2 eV. This agrees with above formula.