CASTLE Unit 2: Reading 3
More about capacitors
As you already know, a capacitor consists of two layers of conducting material separated by a
layer of an insulator. The name comes from the “capacity” of this three-layer device to store
electrical energy through the rearrangement of the charge on its plates.
When a capacitor has been connected to a battery as in Activities 2.1 and 2.2, it is often said to
be “charged.” This is an unfortunate term as it might imply that prior to connection to the battery
the capacitor contained no charge. This is not the case. Capacitor plates, like all conductors,
always contain mobile electric charge. The process of “charging” a capacitor does not add charge
to the capacitor. The battery provides the energy to the circuit to move mobile charge from one
plate to the other. This rearrangement of electric charge on the capacitor allows the capacitor to
store energy in the electric field between its plates.
Capacitance is measured in a unit called the FARAD, named after the British scientist Sir
Michael Faraday (1791-1867). The blue capacitor in your CASTLE kit has a capacitance of
0.025 farad, or 25,000 microfarads, µF.
Your teacher will have some larger capacitors, which are to be shared by the class in a number of
activities. These have a capacitance of 0.1 farad, or 100,000µF — four times as much as the blue
capacitor.
NOTE: Sometimes capacitor plates may pick up stray charge that needs to be removed. A good
way to avoid this problem is to keep a wire connected to the terminals of your capacitor when
you are not using it. Also, after using a capacitor you will want to “reset” it so you can start all
over again. To do this, touch a wire simultaneously to both of the capacitor terminals.
Electrical Energy
The term “energy” is probably one you often use. However, if you attempt to define it, you may
find it difficult to do so.
“Energy” is the ability to make something happen. We have identified a number of things that
happen in circuits – charges move, compasses deflect, bulbs heat and give off light. What is the
source of the energy that makes these things happen? In most of the circuits we have observed,
the source of the energy has been the battery. In some circuits, however, there was no battery
present. When a capacitor discharges, it can make these same things happen, so it must also have
been a source of energy, at least temporarily.
In some circuits, a Genecon was used, but the source of energy was the energy stored in your
muscles. The cranking action transformed the energy from muscle storage to the energy of
moving charges and bulbs releasing light.
You know that batteries eventually wear down, and may become “dead”. This means that they
no longer have sufficient energy stored in them to make something happen in a circuit. Some
batteries are called “rechargeable” and can be re-used. This is an incorrect term, however, since
the batteries’ task was never to supply charge to the circuit – it was already there! These batteries
would more properly called “re-energizeable”.
©Modeling Workshop Project 2011
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C2 Reading 3, v3.2
In the circuit, charge originates in every conductor, and constantly re-cycles around the circuit.
However, energy leaves the energy source and travels one-way, leaving the circuit as heat
thermal and light energy radiating from the bulbs (the receivers of the energy). The energy
source might be the stored chemical energy in a battery, or the stored energy in muscles used to
crank a Genecon, the stored electrical energy in a charged capacitor.
Stored Energy and Rechargeable Batteries
A fresh battery (such as a single D-cell) generally contains two substances, which we will refer
to as A and B. These are high-energy substances (like some foods are known to be excellent
sources of energy), which means that
that energy is stored in the chemical
bonds within these substances. These
These two chemicals react inside the
the battery cell and release the energy
energy that pushes charges around the
the circuit. The reaction is ‘rolling
down’ the energy hill when energy is
is released from the cell. The equation
equation for the reaction is:
A + B → C + D + energy
Eventually, however, substances A and
B will be used up – completely
converted into low-energy substances
substances C and D. When only
substances C + D are left, we refer to
to the battery as ‘dead’.
©Modeling Workshop Project 2011
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C2 Reading 3, v3.2
Some batteries, however, are designed so
that the chemical reaction is reversible. By
pumping energy back into the system from
another source, the chemical reaction can
be forced in the opposite direction. The
diagram on the right below demonstrates
energy being absorbed into the battery,
‘climbing the energy hill’. The equation for
the reaction is:
Energy + C + D → A + B
The battery is now ‘refreshed’ or ‘reenergized’ and can be used again to pump
charges around a circuit.
(The term ‘rechargeable’ is misleading,
since a battery never runs out of charge.
Charge is present everywhere in the circuit,
and is not supplied by the battery. The battery simply supplies the energy needed to make the
charge move through the circuit. The term “re-energizable battery” would be more accurate.)
©Modeling Workshop Project 2011
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C2 Reading 3, v3.2