Download Voltage

Voltage
Volt
 The electric potential is related
to the potential energy.
U
V 
q
• Compare to test charge
 The unit of electric potential is
the volt (V).
• 1 V = 1 J/C
q
V
d
q
E
F
Field Measure
 The electric field is most
commonly measured in V/m.
Show that this is consistent
with a measurement in N/C.
 Use the definitions of N and J
to link the two definitions of
electric field.
• 1 N/C = 1 (kg m / s2 ) / C
• 1 N/C = 1 (kg m) / (s2 C)
• 1 V/m = 1 (J/C) / m
• 1 V/m = 1 (N m / C) / m=1 N/C
• 1 V/m = 1 (kg m2 / s2) / (C m)
• 1 V/m = 1 (kg m) / (s2 C)
Electric Work
 A cathode ray tube accelerates
electrons across a potential of
20 kV. Find the speed of the
electrons at the screen.
 The potential can be converted
to an energy.
• qV = (1.6 x 10-19 C)(2 x 104 V)
= 3.2 x 10-15 J.
 The potential energy becomes
kinetic energy.
• qV = ½ mv2
 Solve for the speed v.
• v = 8.4 x 107 m/s
Uniform Field
 A uniform electrical field has
U  qEd
the same magnitude and
direction at all points.
V  Ed
 A charge moving parallel to
the field lines changes
potential by V = Ed.
q
U
d
q
E
F
 A charge moving
perpendicular to the field
lines has no change in
potential.
Conductor Potential
 There is no field within a
conductor.
• External field neutralized by
polarization
q
 A test charge at one end
moved to the other end would
not change potential.
 All points on a conductor are at
the same potential.
q
Voltage Source
 A voltage source is called a
battery.
 A battery attached to a
conducting plate places the
same potential across the
plates.
 A uniform electric field exists
between the plates.
+
-
Internal Field
 A 12-V source is connected to
parallel plates 2.0 mm apart.
Find the magnitude of the field
between the plates.
 The relationship between the
field and voltage is V = Ed.
• Solve for E = V/d
 The field is E =
(12 V) / (0.002 m) = 6.0 kV/m.
+
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