Exp. #2-1 : Understanding of Coulomb’s Law and Measurement of the Electrostatic
Constant and the Electric Permittivity in Vacuum by Using a Torsion Balance
▣ Theory : Coulomb’s law
▶ Electrostatic force
Magnitude : =
Electrostatic constant in vacuum
=
= 8.99 × 10 Nm /C
Electric permittivity in vacuum
= 8.85 × 10 F/m
Direction :
http://www.listen2will.net
[email protected]
> 0 ⇨ repulsive force
< 0 ⇨ attractive force
PAGE 1/10
General Physics Laboratory II – Exp. #2-1 Lecture
Exp. #2-1 : Understanding of Coulomb’s Law and Measurement of the Electrostatic
Constant and the Electric Permittivity in Vacuum by Using a Torsion Balance
▣ Theory : Capacitance of an isolated sphere
▶ Electric field from Gauss’s law
⋅
∮
,
=
4
= ,
=
−
=
▶ Electric potential
Δ
=
⋅
=
−
=
▶ Capacitance
=
=
⇨ Isolated sphere( → ∞) :
http://www.listen2will.net
[email protected]
=4
PAGE 2/10
General Physics Laboratory II – Exp. #2-1 Lecture
Exp. #2-1 : Understanding of Coulomb’s Law and Measurement of the Electrostatic
Constant and the Electric Permittivity in Vacuum by Using a Torsion Balance
▣ Experimental instruments : High voltage supply
0
2
6
Voltage
adjust knob
ON
OFF
Terminals
kV
(−)
Ground (+)
▶ Connect a charging probe to the (+) or (−) terminal of the high voltage supply.
▶ The actual voltage charging two conductive spheres is the half of the voltage 2 displayed in the
high voltage supply.
http://www.listen2will.net
[email protected]
PAGE 3/10
General Physics Laboratory II – Exp. #2-1 Lecture
Exp. #2-1 : Understanding of Coulomb’s Law and Measurement of the Electrostatic
Constant and the Electric Permittivity in Vacuum by Using a Torsion Balance
▣ Experimental instruments : Torsion balance
Coil
Torsion
wire
Conductive
sphere
▶ Isolated conductive sphere with a radius
Capacitance = 4
Charging voltage
Electric charge =
Magnet
http://www.listen2will.net
[email protected]
PAGE 4/10
General Physics Laboratory II – Exp. #2-1 Lecture
Exp. #2-1 : Understanding of Coulomb’s Law and Measurement of the Electrostatic
Constant and the Electric Permittivity in Vacuum by Using a Torsion Balance
▣ Experimental procedures : Measurement of the torsion constant (1/2)
Mass
▶ Equilibrium condition
Torque due to
Restoring torque
=
the weight of a mass
of the torsion wire
Δ
sin 90° = Δ ,
=
http://www.listen2will.net
[email protected]
Torsion constant
=
PAGE 5/10
General Physics Laboratory II – Exp. #2-1 Lecture
Exp. #2-1 : Understanding of Coulomb’s Law and Measurement of the Electrostatic
Constant and the Electric Permittivity in Vacuum by Using a Torsion Balance
▣ Experimental procedures : Measurement of the torsion constant (2/2)
(unit :
=
)
=
20
40
50
Slope =
70
90
Δ
http://www.listen2will.net
[email protected]
PAGE 6/10
General Physics Laboratory II – Exp. #2-1 Lecture
Exp. #2-1 : Understanding of Coulomb’s Law and Measurement of the Electrostatic
Constant and the Electric Permittivity in Vacuum by Using a Torsion Balance
▣ Experimental procedures : Measurement of the electrostatic force vs. the distance |
Measurement of the electrostatic force vs. the charge (1/2)
▶ Equilibrium condition
Torque due to
Restoring torque
=
the electrostatic force
of the torsion wire
sin 90° = Δ ,
Δ
Electrostatic force
http://www.listen2will.net
[email protected]
=
PAGE 7/10
General Physics Laboratory II – Exp. #2-1 Lecture
Exp. #2-1 : Understanding of Coulomb’s Law and Measurement of the Electrostatic
Constant and the Electric Permittivity in Vacuum by Using a Torsion Balance
▣ Experimental procedures : Measurement of the electrostatic force vs. the distance |
Measurement of the electrostatic force vs. the charge (2/2)
Case
(
(i)
6.0
Constant
5.0 ~ 20.0
Variable
(ii)
3.0 ~ 6.0
Variable
7.0 / 10.0
Constant
(
)
(ii)
(i)
)
=
= 7.0 cm
= 10.0 cm
http://www.listen2will.net
[email protected]
PAGE 8/10
General Physics Laboratory II – Exp. #2-1 Lecture
Exp. #2-1 : Understanding of Coulomb’s Law and Measurement of the Electrostatic
Constant and the Electric Permittivity in Vacuum by Using a Torsion Balance
▣ Experimental procedures : Calculation of the electrostatic constant and
the electric permittivity in vacuum
Slope =
http://www.listen2will.net
[email protected]
=
PAGE 9/10
General Physics Laboratory II – Exp. #2-1 Lecture
Exp. #2-1 : Understanding of Coulomb’s Law and Measurement of the Electrostatic
Constant and the Electric Permittivity in Vacuum by Using a Torsion Balance
▣ Problems
1. If protons with a mass 1 g and electrons with a mass 1 g are separated by a distance 1 m, show that
the electrostatic force of about 1.5 × 10 N is exerted between two.
2. Explain the physical principle about the role of magnets and coils mounted in the torsion balance.
3. Explain why the attractive force is exerted between charged and uncharged conductive spheres.
http://www.listen2will.net
[email protected]
PAGE 10/10
General Physics Laboratory II – Exp. #2-1 Lecture