EE Pro for TI - 89, 92 Plus
Equations - Capacitors & Electric Fields
15
17.1 Point Charge
The two equations in this topic calculate the radial electric field Er and the potential V at a point located a distance
r away from a point change Q. The first equation shows the inverse square relationship between Er and r, while
the second equation shows the inverse relationship between the potential V and distance r. The equations have
been generalized to include
ε
r, the relative permittivity of the medium.
Er
Q
r r
=
⋅ ⋅ ⋅ ⋅
4
0
2
π ε ε
Eq. 17.1.1
V
Q
r r
=
⋅ ⋅ ⋅ ⋅
4
0
π ε ε
Eq. 17.1.2
Example 17.1 -
A point charge of 14.5E-14_coulomb is located 2.4_m away from an instrument measuring
electric field and absolute potential. The permittivity of air is 1.08. Compute the electric field and potential.
Entered Values
Computed results
Solution -
Both equations are needed to solve this problem. Press
„
to display the input screen, enter
all the known variables, and press
„
to solve for the unknown values. Note that
ε
0, the permittivity of
free space does not appear as one of the variables that needs to be entered. It is entered automatically by
the software. However,
ε
r
, the relative permittivity must be entered as a known value.
-PQYP8CTKCDNGU3
'AEQWNQOD
T
AO
ε
r
= 1.08
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A8O
8
A8
17.2 Long Charged Line
An infinite line with a linear charge density,
ρ
l, (coulombs per unit length) exerts a radial electric field, Er, a
distance r away from the line. The equation has been generalized to include
ε
r, the relative permittivity of the
medium.
Er
l
r r
=
⋅ ⋅ ⋅ ⋅
ρ
π ε ε
2
0
Eq. 17.2.1
Example 17.2 -
An aluminum wire suspended in air carries a charge density of 2.75E-15_coulombs/m. Find
the electric field 50_cm away. Assume the relative permittivity of air to be 1.04.