Day 4 - Electrochemistry and DC Circuits Activity Write

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7 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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Electrochemistry and DC Circuits

1

Electrochemistry and DC Circuits


Goal:

Participants will use different setups to analyze the characteristics of direct current
(DC) circuits and construct an electrochemical cell.


Objectives:


1) Participants will build and use an electrochemical cell.

2
) Participants will use knowledge about the hand crank generators and photovoltaic cells
to construct, test, and draw conclusions about series and parallel circuits.

3) Participants will apply what they know about series and parallel circuits to build an

answer” board with Christmas lights.

4) Participants will use batteries and other power sources to charge and then discharge
capacitors demonstrating their energy storage and how their discharge is dependent on
resistance.


Electrochemistry

Equipment List:

Plastic cups, paper towels, copper strip,
magnesium strip, magnesium sulphate,
copper sulphate, jumper leads, LED, mini
buzzer, vibrating motor, multimeter and
test leads

Answer Board

Equipment List:

Box lid, Christmas lights, pencil,
construction paper,
brass brads, mini test
leads, markers, D cell battery holders, D
cell batteries

Series and Parallel

Equipment List:

Capacitor, hand crank generator,
Christmas light, D
-
cell battery,
photovoltaic cell

Arkansas State Curriculum framework
s:

PS.7.8.1
-

Constru
ct open and closed
electrical circuits (series and parallel)

PS.7.8.3
-

Compare and contrast open and
closed series circuits and parallel circuits


Equipment comes from WalMart, Hobby
Lobby, All Electronics, Radio Shack and
Cynmar.



Background
:

The elect
rochemical reaction we will explore is given by,

,

where
Cu

is the chemical symbol for copper and
Mg

is the chemical symbol for
magnesium. The superscripts denote the ionization of the atoms


that is how many
electrons have been str
ipped away. The reaction above may be broken down into two
half
-
reactions. The first is the
oxidation

half
-
reaction given by

,

and the second is the
reduction

half
-
reaction given by,

,

where an electron is deno
ted by the symbol
e

and has a negative charge. Note that in the
oxidation reaction the electrons are produced and in the reduction reaction the electrons
are consumed. In a battery one terminal is called the
anode

and is marked as negative,
hence this term
inal is connected to the oxidation part of the chemical reaction and from
Electrochemistry and DC Circuits

2

where the electrons originate in the circuit. The other battery terminal is called the
cathode

and is marked positive, hence this terminal is connected to the reduction part of
the c
hemical reaction and thus is where the electrons flow to in the circuit.
a

The atomic ions in the chemical formulas
listed above are floating in water solutions
comprised of magnesium sulphate which
surrounds the magnesium strip and copper
sulphate which s
urrounds the copper strip, see
the figure to the left. The
SO
4

part of the
mixture does not participate in the operation
of the battery.


The salt bridge will be composed of a strip of
paper towel soaked in a table salt and water
solution. This allows cha
rges to flow but
prevents the mixing of the copper and

magnesium ions which would quickly react.


At the magnesium strip the magnesium metal atoms are pulled from the strip and into the
water solution leaving behind two electrons which makes the strip neg
atively charged. At
the copper strip the copper ions in the water attach themselves to the metal strip and
tightly hold on to two electrons in the metal which makes the strip positively charged.
The salt bridge allows the excess negative charge in the copp
er solution to migrate to the
magnesium solution via bonding to the sodium and chlorine ions in the wet paper towel.
The act of separating the electrons from their host atoms requires energy which is
provided by the chemical reactions between the metal str
ips and the sulphate solutions.
This electrical energy is what sends the electrons around a conducting wire from the
anode (negative terminal) to the cathode (positive terminal). A detailed chemical analysis
will reveal that the expected energy per unit ch
arge for this battery is 2 Volts.
b

This
simple battery cannot provide many electrons per second (or current) however we will
see that it can operate some simple devices that require low current and may be combined
with other electrochemical cells to increa
se the total voltage or current.


An electric circuit is simply a path for the electrons to flow from the anode to the
cathode. Common electronic devices contain many pathways and objects for the electrons
to interact with. We will focus on three of these:

the battery, the resistor and the
capacitor. A resistor is a device that resists (or slows down) the flow of electrons and a
capacitor is a device that stores electrons. Additionally, the circuit pathways can be
broken down into two fundamental arrangemen
ts: series or parallel. A series circuit is
essentially a single loop where the circuit elements are placed one after the other around
the loop. A parallel circuit consists of the circuit elements arranged like the rungs on a
ladder with connecting wires
along the ladder rails. See Figure 1 for a circuit schematic



a

In our modern era we have a much better understanding of atomic and subatomic physics/chemistry than
Ben Franklin did when he postulated that the
positive

charge was the one that moved!

b

A volt is a unit of energy per unit charge or Joules per Coulomb.

The unit “Volt” is named after the Italian
Alessandro Volta

(1745
-
1827) who built the first electrochemical cell.

Electrochemistry and DC Circuits

3

that illustrates series and parallel circuits. In the figure the battery is denoted by the
symbol:

and the resistor by the symbol:
. A capacitor has the
circuit symbol:
, notice the slight di
fferences between the capacitor and battery
symbols.




Figure 1


Series and Parallel Circuit Schematic


The chemical reactions in the battery produce excess electrons with high energy which
can then flow through the circuit’s wires and resistors. You ca
n think of the battery as the
source of a mountain river. A mountain top river has high gravitational potential energy
just as our battery produces electrons with high electrical potential energy. In the series
circuit the river (electrons) flows through a

cascade of waterfalls (resistors) one right
after the other. With each waterfall the gravitational potential energy drops yet the
amount of current flow is undiminished. The amount of water (or electrons) that leave the
top of the mountain (battery) retur
ns at the base. In a parallel circuit the river (electrons)
splits into three branches with three separate waterfalls (resistors) in which each waterfall
takes the river (electrons) from high energy to low energy all at once. The river
(electrons) then rec
ombines as it flows back to the base of the mountain (battery). Each
circuit can be represented by a single resistance (or waterfall) given by the following
formulas which come from a more detailed analysis of the circuit’s energy and current:


Equivalent
Series Circuit Resistance:

Equivalent Parallel Circuit Resistance:


Thus, in a series circuit the total resistance is dominated by the largest resistance while in
a parallel circuit, the total resistance is d
ominated by the smallest resistance. This makes
sense if we think back to our waterfall analogy. If there is a large waterfall in a series of
Electrochemistry and DC Circuits

4

waterfalls then the large waterfall brings the river through the largest change in
gravitational potential energy
(down the mountain) yet the amount of water flowing (the
electrons) is the same in each waterfall. However, in a parallel arrangement of waterfalls
the total change in gravitational potential energy is the same but each waterfall can carry
different amount
s of water (electrons). Thus, the waterfall that carries the most water (i.e.
has the least resistance) will dominate the flow of the water down the mountain.


A battery usually has two ratings: 1) its voltage (energy per unit charge) and 2) its amp
-
hours

(total charge available). Typical battery voltages are 1.5V, 6V, 9V and 12V.
Batteries can also be placed in series or parallel arrangements, as illustrated in the Figure
2 below. When batteries are placed in series with the cathode abutting the anode of
the
next battery the total voltage of all the batteries is just the sum of the voltages of each
individual battery but the total charge available is limited to the battery with the smallest
amp
-
hour rating. Since the batteries are in series the total curre
nt (rate of charges moving
through the circuit) must be the same through each battery. If identical batteries are
placed in parallel with all the cathodes connected together the total voltage is the same
but the total charge is increased. Since the batteri
es are in parallel the total current from
them is increased. Thus, identical batteries in series are like individual steps that lead to a
great height or gravitational potential energy but in this case its electrical potential energy
but only a few people
(charges) can climb the steps at a time. However, identical batteries
in parallel is like having a very wide stairway that is only one step high, many people
(charges) can climb the step but they don’t have a large potential energy.






Figure 2


Series and Parallel Battery Arrangements


Ohm’s Law is an important relationship between the energy per charge (voltage), the rate
at which charges are flowing (current) and the resistance. Some materials maintain a
constant resista
nce irrespective of the current flowing through them, however, most
materials will change their resistance if a large enough current flows. Ohm’s Law is
primarily used for the case when the resistance is a constant and states:
V=IR
, where
V

is
Electrochemistry and DC Circuits

5

the voltage
(energy per charge),
I

is the current (or rate at which charge is flowing) and
R

is the resistance in Ohms (Greek letter capital omega,

). Ohm’s law can be used for the
whole circuit as well as for individual parts. If you know two of the three quantities

in the
equation you can solve for the third. Another important equation computes the rate at
which energy is used in the circuit


otherwise known as the power,
P=IV
. An electronic
device uses some energy and electrons at some rate given by the power. To
run the
device with a battery, hand crank generator or photovoltaic cell the power source must be
able to match the power requirements of the device.


A capacitor is a device for storing electrons. In this context you can think of it as a lake or
pond tha
t can be filled or drained by water flowing into or out of it. A capacitor has the
circuit symbol:
, notice the slight differences between the capacitor and battery
symbols. The capacitor can be charged by connecting it to a battery, the hand crank
genera
tor or the photovoltaic cell. The stored charge can then be released when the
capacitor is connected to other circuit elements.


A multimeter is a device that can measure a multitude of things (hence its name!).
Typical multimeters have a dial knob that al
lows the user to select the quantity they wish
to measure: voltage, current or resistance. Multimeters can measure both voltage and
current that is steady (DC) or alternating (AC). Care must be made to select the
appropriate range on the dial and to have t
he measurement leads plugged into the
appropriate sockets. The photos below show how the multimeter should be arranged for
the desired measurement.



Resistance Measurement

(meter in parallel)


Voltage Measurement

(meter in parallel)


Current Measurem
ent

(meter in series)




Electrochemistry and DC Circuits

6

Electrochemistry Activity:

1) Begin by gathering the equipment needed
to build the electrochemical cell as
illustrated in the photo to the left.


2) We will make one molar solutions of
copper sulphate (root killer) and magnesium

sulphate (hydrangea plant supplement). Use
the triple beam balance to find the mass of
the plastic cup then mass out 35 grams of
magnesium sulphate , 45 grams of copper
suplate and 20 grams of table salt. The exact
amounts are not critical but try to get

close.
Put the chemicals in separate plastic cups.


3) You will need approximately 350 ml (350
grams) of water in each cup which you can
mass using a plastic cup and the triple beam
balance.


4) Mix the water with the chemicals in the plastic cups until n
o solid parts remain. Clip
the metal strips into the appropriate solution using the clothes pins


Copper strip in the
blue copper sulphate solution and the silver
-
looking magnesium strip in the clear
magnesium sulphate solution.


5) The battery is not com
plete until a salt bridge links the two solutions. Soak a strip of
paper towel in the table salt solution and then place it over the edges of the two cups as
illustrated in the photo above. Alligator clips on the metal strips complete the
construction of t
he battery!


6) Use the multimeter to measure the voltage between the battery terminals. Note that the
anode (negative terminal) is the copper strip and the cathode (positive terminal) is the
magnesium strip. What happens to the voltage measurement if you

switch which
multimeter led touches which battery terminal? What does this measurement mean?


7) Your battery is ready to use. Clip the appropriate leads to the terminals on the smallest
buzzer. Do you hear it buzzing? Remove the small buzzer and attach t
he battery to the
small calculator. Does the calculator work properly? Will the single battery power the
larger buzzer, electric motor or the small
l
ight
e
mitting
d
iode (LED)? The LED has a flat
spot on the plastic housing that denotes the cathode wire.