BASIC ELECTRONICS PRACTICE
CHARGING AND DISCHARGING CAPACITORS
2.1 Capacitors
Capacitor is a device used to store charge and energy. A capacitor consists of two conductors
which are adjacent but isolated from each other and carry an equal and opposite charge.
Configuration of conductors that act as charge storage. Capacitors are usually used to smooth
out the ripple that arises when alternating current is converted to direct current in the power
supply.
The capacity of the capacitor, which is denoted by C , is the ability of the capacitor to
store charge Q at a potential difference V . It is stated in the equation:
Q
C= .................................................. ...............(2.1)
V
The value C of the capacitor can be enlarged by reducing V the Q fixed one. The value C of a
capacitor depends on the geometry of the conductor, the type of dielectric, the dimensions of
the capacitor and the distance between the two conductors.
One way to charge a capacitor is to place it in the circuit connected to the battery. An
electric circuit is a path by which charge can flow, a battery is a component that provides a
potential difference between the terminals. When the circuit is closed, electrons will flow
toward one of the conducting plates, causing the plate to gain electrons and become
negatively charged. Meanwhile, the other plate loses electrons because the electrons move
towards the battery, so that it is positively charged with the same amount as the negative
plate. When the plates are uncharged, the potential difference between the two plates is zero.
When the plates are oppositely charged, the potential difference increases until it is equal to
the potential difference V between the terminals of the battery. This causes no electric field in
the cable between the two plates. Thus, with a zero electric field, no electrons are flowing, and
the capacitor is said to be fully charged. When charging the capacitor and after charging it, the
charge cannot move from one plate to another through the gap between the two plates. So it
can be assumed that the capacitor is able to store its charge for an unlimited time until it is
connected to a circuit where the charge can be reduced (Halliday & Resnick, 1996) .
With its rechargeable ability, a capacitor can be analogous to a rechargeable battery.
What distinguishes the two is that in one recharge, the battery can last up to hours, while the
capacitor is able to receive and discharge charges in an instant. In addition, in flowing charge, a
chemical reaction will occur in the battery first, while in the capacitor there is no chemical
reaction (Jati, 2010).
2.2 Capacitance
If a charge, for example a proton is released from the positive pole, then the
proton will move to the negative pole. The protons experience acceleration due to the
,electrostatic force generated from the electric field E . The potential possessed by a
charge is proportional to the charge, so:
Q
V= →V ∝ Q............................... ......... .....(2. 2 )
4 π ε° r
Because V is proportional to Q, if the amount of charge Q is doubled to 2Q, then V will
be double the original V, which is 2V. The ratio between Q and v is constant, and this
ratio is called capacitance. Capacitance is represented by the letter C, and is generally
expressed in the following equation:
Q
C= ...... ............................... .......... .. ...............(2. 5 )
V
The capacitance of a capacitor is not affected by Q and V, but is affected by the ratios
where these ratios are:
C=4 π ε ° r .................................................. ........(2. 6 )
So the capacitance of a capacitor is only affected by the geometry and arrangement of the
capacitor (Murdaka, 2010) .
2.3. Types of Capacitors
2.3.1. Parallel Plate Capacitor
Parallel plate capacitors are connected to a source of direct current which provides a
potential difference V 0 , between the two plates is a vacuum and when the charge stored in
the plates is maximum, at that moment the direct current is released. Next, the capacitor is
isolated so that the charge stored on the plate is not lost. If between these plates is replaced
with an insulator, then the potential difference on the two plates changes to V a value smaller
than V 0 . An insulator placed between two conducting plates is called a dielectric. The
dielectric constant is denoted by:
V0
k= .................................................. ...............(2.7)
V
Value k depends on the type of insulator used. Suppose a vacuum insulated parallel plate
capacitor is charged with the entire Qo potential difference V 0and capacity C 0. So after being
filled with another insulator, the potential difference becomes V and the capacity C so that:
C=k C o
A
C=k ε o .................................................. ..........(2.8)
d
This constant k ε 0=ε is called the permittivity of the insulator (Jati & Priyambodo, 2010) .
, The dielectric in a capacitor has limitations, because a large electric field strength can
result in ionization of the dielectric so that the dielectric which was originally an insulator can
turn into a conductor, and cause the dielectric to breakdown so that the capacitor leaks. The
maximum electric field Ebd strength that can still be used in a capacitor dielectric is called the
dielectric strength. This dielectric strength depends on the physical structure of the capacitor.
If the distance between parallel plates is d then it is Ebd stated in kV/mm and fulfills the
relation:
V bd
Ebd = .................................................. ..........(2.9)
d
(Jati & Priyambodo, 2010)
2.3.2. Ball Capacitor
This capacitor consists of two concentric conductors. The inside of the ball is radius a
overall charged +Q that is homogeneously distributed on the surface of the ball, and the outer
radius of b the charged −Q . The inner and outer spheres have the same charge but different
signs, in such conditions the capacitor stores an electric charge Q (Halliday & Resnick, 1996) .
2.3.3. Coaxial Cylindrical Capacitor
This capacitor is made of 2 concentric cylindrical conductors. the two cylindrical
conductors have length l , each radius a , the medium is vacuum or air, and each has its own
charge +Q and – Q scattered on the surface of the cylinder (Halliday & Resnick, 1996) .
2. 4 . Dielectric
Dielectrics are materials that do not have free electrons. If a dielectric is not affected
by an electric field, then the positive and negative charges do not separate.
If a dielectric is affected by an electric field, the negative charge on the dielectric will
be pulled in the opposite direction to the direction of the electric field, while the positive
charge will be pulled in the same direction as the electric field. So the positive and negative
charges are separated. The influence of positive charges on the dielectric neutralize each
other, so that the effect is on the charges on the edge of the dielectric. With the presence of
induced charges on the edges of the dielectric, the electric field strength will be smaller.
σ σi
E= − .................................................. ........(2.10)
ε0 ε0
σ XE
E= − .................................................. .....(2.11)
ε0 ε0
The dielectric constant is defined as ke :
CHARGING AND DISCHARGING CAPACITORS
2.1 Capacitors
Capacitor is a device used to store charge and energy. A capacitor consists of two conductors
which are adjacent but isolated from each other and carry an equal and opposite charge.
Configuration of conductors that act as charge storage. Capacitors are usually used to smooth
out the ripple that arises when alternating current is converted to direct current in the power
supply.
The capacity of the capacitor, which is denoted by C , is the ability of the capacitor to
store charge Q at a potential difference V . It is stated in the equation:
Q
C= .................................................. ...............(2.1)
V
The value C of the capacitor can be enlarged by reducing V the Q fixed one. The value C of a
capacitor depends on the geometry of the conductor, the type of dielectric, the dimensions of
the capacitor and the distance between the two conductors.
One way to charge a capacitor is to place it in the circuit connected to the battery. An
electric circuit is a path by which charge can flow, a battery is a component that provides a
potential difference between the terminals. When the circuit is closed, electrons will flow
toward one of the conducting plates, causing the plate to gain electrons and become
negatively charged. Meanwhile, the other plate loses electrons because the electrons move
towards the battery, so that it is positively charged with the same amount as the negative
plate. When the plates are uncharged, the potential difference between the two plates is zero.
When the plates are oppositely charged, the potential difference increases until it is equal to
the potential difference V between the terminals of the battery. This causes no electric field in
the cable between the two plates. Thus, with a zero electric field, no electrons are flowing, and
the capacitor is said to be fully charged. When charging the capacitor and after charging it, the
charge cannot move from one plate to another through the gap between the two plates. So it
can be assumed that the capacitor is able to store its charge for an unlimited time until it is
connected to a circuit where the charge can be reduced (Halliday & Resnick, 1996) .
With its rechargeable ability, a capacitor can be analogous to a rechargeable battery.
What distinguishes the two is that in one recharge, the battery can last up to hours, while the
capacitor is able to receive and discharge charges in an instant. In addition, in flowing charge, a
chemical reaction will occur in the battery first, while in the capacitor there is no chemical
reaction (Jati, 2010).
2.2 Capacitance
If a charge, for example a proton is released from the positive pole, then the
proton will move to the negative pole. The protons experience acceleration due to the
,electrostatic force generated from the electric field E . The potential possessed by a
charge is proportional to the charge, so:
Q
V= →V ∝ Q............................... ......... .....(2. 2 )
4 π ε° r
Because V is proportional to Q, if the amount of charge Q is doubled to 2Q, then V will
be double the original V, which is 2V. The ratio between Q and v is constant, and this
ratio is called capacitance. Capacitance is represented by the letter C, and is generally
expressed in the following equation:
Q
C= ...... ............................... .......... .. ...............(2. 5 )
V
The capacitance of a capacitor is not affected by Q and V, but is affected by the ratios
where these ratios are:
C=4 π ε ° r .................................................. ........(2. 6 )
So the capacitance of a capacitor is only affected by the geometry and arrangement of the
capacitor (Murdaka, 2010) .
2.3. Types of Capacitors
2.3.1. Parallel Plate Capacitor
Parallel plate capacitors are connected to a source of direct current which provides a
potential difference V 0 , between the two plates is a vacuum and when the charge stored in
the plates is maximum, at that moment the direct current is released. Next, the capacitor is
isolated so that the charge stored on the plate is not lost. If between these plates is replaced
with an insulator, then the potential difference on the two plates changes to V a value smaller
than V 0 . An insulator placed between two conducting plates is called a dielectric. The
dielectric constant is denoted by:
V0
k= .................................................. ...............(2.7)
V
Value k depends on the type of insulator used. Suppose a vacuum insulated parallel plate
capacitor is charged with the entire Qo potential difference V 0and capacity C 0. So after being
filled with another insulator, the potential difference becomes V and the capacity C so that:
C=k C o
A
C=k ε o .................................................. ..........(2.8)
d
This constant k ε 0=ε is called the permittivity of the insulator (Jati & Priyambodo, 2010) .
, The dielectric in a capacitor has limitations, because a large electric field strength can
result in ionization of the dielectric so that the dielectric which was originally an insulator can
turn into a conductor, and cause the dielectric to breakdown so that the capacitor leaks. The
maximum electric field Ebd strength that can still be used in a capacitor dielectric is called the
dielectric strength. This dielectric strength depends on the physical structure of the capacitor.
If the distance between parallel plates is d then it is Ebd stated in kV/mm and fulfills the
relation:
V bd
Ebd = .................................................. ..........(2.9)
d
(Jati & Priyambodo, 2010)
2.3.2. Ball Capacitor
This capacitor consists of two concentric conductors. The inside of the ball is radius a
overall charged +Q that is homogeneously distributed on the surface of the ball, and the outer
radius of b the charged −Q . The inner and outer spheres have the same charge but different
signs, in such conditions the capacitor stores an electric charge Q (Halliday & Resnick, 1996) .
2.3.3. Coaxial Cylindrical Capacitor
This capacitor is made of 2 concentric cylindrical conductors. the two cylindrical
conductors have length l , each radius a , the medium is vacuum or air, and each has its own
charge +Q and – Q scattered on the surface of the cylinder (Halliday & Resnick, 1996) .
2. 4 . Dielectric
Dielectrics are materials that do not have free electrons. If a dielectric is not affected
by an electric field, then the positive and negative charges do not separate.
If a dielectric is affected by an electric field, the negative charge on the dielectric will
be pulled in the opposite direction to the direction of the electric field, while the positive
charge will be pulled in the same direction as the electric field. So the positive and negative
charges are separated. The influence of positive charges on the dielectric neutralize each
other, so that the effect is on the charges on the edge of the dielectric. With the presence of
induced charges on the edges of the dielectric, the electric field strength will be smaller.
σ σi
E= − .................................................. ........(2.10)
ε0 ε0
σ XE
E= − .................................................. .....(2.11)
ε0 ε0
The dielectric constant is defined as ke :
