Transcript Electric Potential, Voltage and Battery

Electric Potential, Voltage and Battery (CH
16/17)
EXPERIMENT NO. 2
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Electric Potential Energy
Electric Potential Difference, Voltage
Equipotential Surfaces
Electric Potential Energy between Charges
What have we learned?
What is a Battery?
How does a battery produce electric current?
Organic Cell used in the Experiment
Electrode Potentials
Organic Cell Experiment
Quiz No. 2
Homework No. 2
New Terminology
Glen Canyon Dam and Hydroelectric Power Plant 1956
Glen Canyon Dam produces electrical power through the use of the gravitational
force of falling or flowing water.
It is 710 ft. high and 300 ft. wide at its base. It has an installed capacity of 1,296 MW.
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Electric Potential Energy
An electric field E, between plates A and B
is a space where electric charges q, are acted
upon by an electric force F.
E = F/q [V/m]
B
d
q
E = F/q
Suppose electric charge q is moved against
electric field, E from the negative plate A to the
positive plate B.
An external electric force required to accomplish
this is
F = Eq
The work done is
Work = Fxd
A
By moving the charge from plate A to plate B the
external force has increased the charge’s electric
potential energy, U.
ΔU = UA - UB = Eqxd
Electric Potential Difference
The electric potential difference ΔV between any two points in space
(commonly called Voltage) is the work done per unit positive charge
between those two points, or the change in electric potential energy per
unit positive charge. In equation form, this relationship is
ΔV = ΔU/q or ΔV = Work/q
ΔV = (Eqd)/q = Ed
[J/C or V]
Potential difference is commonly called voltage, and the symbol is just V.
The potential difference depends on characteristics of charged plates that
is, the electric field they produce (E) and their separation (d).
The positively charged plate is at a higher electric potential than the
negatively charged one by an amount ΔV or simply Voltage, V.
Electric Potential Surfaces
Equipotential surfaces also called equipotential
are surfaces on which a charge has the same
potential energy. They are perpendicular to the
electric field on which it takes no work to move
charges from one point to another.
ΔV = Work/q [J/C or V]
ΔV = Ed
Example 16.1 Moving a Proton, Parallel Plates and Potential Difference
Plates are 1.5 cm apart. Electric field is uniform with a magnitude is 1500 NC.
a)What is the change in the proton’s electric potential energy?
b)What is the electric potential difference (voltage) between the plates?
Discussion:
a)The change in potential energy can be computed from the work done to move the charge.
b) The potential difference between the two plates can be determined by dividing the work
done by the charge moved.
Electric Potential Difference Due to a Point Charge
The potential difference between two points in
an electric field is determined by the same
equation ΔV = ΔU/q or ΔV = Work/q. Because
the electric field strength varies, the math is
complicated because the work is done by a
varying force. Therefore, the potential difference
between two points at distances rA and rB is
ΔV = kq/rA - kq/rB
[k = 9.00x109 Nm2/C2]
•Electric potential increases as we consider locations
nearer the positive charges or further away from
negative charges.
•Electric potential decreases as we consider locations
further from positive charges or nearer to negative
charges.
ΔV = kq/r
What have we learned?
Electricity is the set of physical phenomena associated with the presence or flow of an electric
charge.
Electric current is a flow of electrically charged particles. I = q/t, [A]
Electric charge, q is the fundamental physical property of matter like atoms that causes it to
experience a force when close to other electrically charged matter, positive or negative. All atoms
are made up of still tinier particles: neutrons, protons (+) and electrons (-)
Electric field is an electric charge produced without movement. (See above the electric field
surrounding a positive and negative charge.) E = F/q [V/m]
Electric potential or Voltage is the capacity of an electric field to do work on an electric charge, to
move it between two specified points. The difference between two electric potentials is Voltage,
[Volt or V] The reference point is the Earth or Ground. ΔV = ΔU/q or ΔV = Work/q [J/C or V]
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What is a Battery?
Four cells are in parallel.
Battery cell converts stored chemical energy into
useful potential electrical energy.
Once an external connection is made between its
positive and negative terminals a chemical
reaction is initiated that generates electrons at
electrode to supply the current to the external
circuit.
Four cells are in series.
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A battery is a device that stores electrical energy.
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How does a battery produce electric current?
 An electrolyte and two unlike metal electrodes
cause ions of both metals to dissolve.
 One electrode (cathode) becomes more
negatively charged than the other (anode).
 The anode is a higher potential than the
cathode. It receives electrons (negative electric
charges). By convention, the anode is
designated the positive terminal and the
cathode the negative.
 This potential difference (V, Voltage) between
the two electrodes causes a current or a flow of
charges (electrons), in the wire. Simultaneously
in the cell, the positive ions flow from the
cathode to the anode.
I = q/t; [A = C/s]; q = nxe; e =1.6x 10-19 C or As]
While the positive ions flow from the cathode to the anode, simultaneously the
electric potential between two electrodes causes an electric current to flow in the
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wire connected to the light bulb.
Organic Cell’s Essential Electric Components
 Anode is the negative electrode where the electrons are released to the external circuit.
 Cathode is the positive electrode that acquires electrons from the external circuit. (A
cathode is an electrode through which electric current flows out of a polarized electrical
device. The direction of electric current is, by convention, opposite to the direction of
electron flow-thus electrons are considered to flow toward the cathode electrode while
current flows away from it.)
 Electrolyte is the medium that provides the ion transport mechanism between the
cathode and anode.
 A Cation is a positively charged ion, i.e., one that would be attracted to the cathode in
electrolysis. It has fewer electrons than protons. The opposite is Anion.
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About the Organic Cell used in the Experiment?
 An organic cell is an item that contains an electrolyte and two electrodes producing
electricity by chemical reaction.
 In our experiment well use lemon and potato as organic electrolytes (other fruit are also
effective like banana and cucumber that are derived from a living organism). Lemons juice is
an electrolyte called citric acid. Potato has an electrolyte made of Sodium (Na), Potassium (K)
and Chloride (Cl) ions (phosphoric acid).
 Copper and Zinc are two metals that serve in our experiment as electrodes that attract
positive (Co) and negative (Zn) ions during a chemical reaction in the electrolyte. Copper
electrode is positive at +0.34 V that attracts negative (Zn) or (Ni) ions during the chemical
reaction.
 Zinc and Nickel electrodes have free electrons with a potential voltage of -0.76 V and -0.25 V.
 (Magnesium (Mg), Lead (Pb), Nickel (Ni) and Cadmium (Cd) are used commercially.)
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Standard Electrode Potentials in Aqueous Solution at 25°C.
Standard Potential
K+(aq) + e  K(s)
Ca2+(aq) + 2e  Ca(s)
Na+(aq) + e  Na(s)
Mg2+(aq) + 2e  Mg(s)
Al3+(aq) + 3e  Al(s)
2H2O (l) + 2e  H2(g) + 2OH (aq)
Zn2+ (aq) + 2e  Zn(s)
Cr3+ (aq) + 3e  Cr(s)
Fe2+ (aq) + 2e  Fe(s)
Cd2+ (aq) + 2e  Cd(s)
Ni2+ (aq) + 2e  Ni(s)
Sn2+ (aq) + 2e  Sn(s)
Pb2+ (aq) + 2e  Pb(s)
Fe3+ (aq) + 3e  Fe(s)
Li+(aq) + e  Li(s)
(Volts)
-3.04
-2.92
-2.76
-2.71
-2.38
-1.66
-0.83
-0.76
-0.74
-0.41
-0.40
-0.23
-0.14
-0.13
Standard Potential
(Volts)
-
2H+ (aq) + 2e  H2(g)
Sn4+ (aq) + 2e-  Sn2+(aq)
Cu2+ (aq) + e-  Cu+(aq)
ClO4-(aq) + H2O (l) + 2e-  ClO3-(aq) + 2OH- (aq)
AgCl (s) + e-  Ag (s) + Cl-(aq)
Cu2+ (aq) + 2e-  Cu (s)
ClO3-(aq) + H2O (l) + 2e-  ClO2-(aq) + 2OH- (aq)
IO-(aq) + H2O (l) + 2e-  I-(aq) + 2OH- (aq)
Cu+ (aq) + e-  Cu(s )
I2(s) + 2e-  2I-(aq)
ClO2-(aq) + H2O(l) + 2e-  ClO-(aq) + 2OH- (aq)
Fe3+ (aq) + e-  Fe2+(aq)
Hg22+ (aq) + 2e-  2Hg (l)
Ag+ (aq) + e-  Ag (s)
0.00
0.15
0.16
0.17
0.22
0.34
0.35
0.49
0.52
0.54
0.59
0.77
0.80
0.80
-0.04
(aq)=aqueous or water solution; (s)=solid phase; (l)=liquid phase
Aluminum (Al), Zinc (Zn), Nickel (Ni), and Copper (Cu) will be used in experiment
Table of Standard Electrode Potentials taken from hyperphysics.phy-astr.gsu.edu/hbase/tables/1310/19/13
Electrode Potential determines the output voltage of an electro-chemical cell.
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Electrode Potential determines the output voltage of an
electro-chemical cell
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Each chemical element is characterized by its excess (-) or lack of (+) free electrons.
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Electrodes have a potential to release FREE electrons (-) to those that don’t have any FREE
electrons (+)
Ex 1: Zinc, Zn
= -0.76 V
Ex 2: Nickel, Ni
= -0.23 V
Ex 3: Copper, Cu = +0.34 V
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Voltage difference between Cu and Zn electrodes in a lemon electrolyte can be calculated
ahead of an experiment.
Voltage difference between Cu to Zn electrodes = +0.34V – (-0.76V) = 1.0V
(Important note: A cathode is an electrode through which electric current flows out of (and
electrons into) a polarized electrical device. The direction of electric current is, by
convention, opposite to the direction of electron flow. Therefore the electrons flow into (and
current out of) the cathode electrode Cu and out of (and current into) anode electrode Zn.)
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What is Needed to Conduct this Experiment?
1. Electrolyte (natural fruit: lemon, potato (other fruit can be used)
2. Electrodes to insert into the fruit (one inch a part, half an inch deep)
3. Voltmeter to measure voltage
4. A Pair of Red and Black Copper Wires (with alligator clips to connect electrodes
with a voltmeter)
5. A Team of two Students (to make connections and write down the measurements)
6. Report and Discuss Results
7. Use the napkin to clean electrodes and save it all in the plastic bag.
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Organic Cell Experiment Test No. 1
Measure the voltage across Copper and Zinc electrodes placed parallel to each other
in a fruit one inch apart and ½ inch deep!
Connections:
Use black alligator clips to connect the Copper electrode (-) with voltmeter (-);
Use red alligator clips to connect the Zinc electrode (+) with voltmeter (+);
Set the voltmeter in the 20 VDC range and turn it ON;
Measurement:
Read the voltage and make a note: xxx Volts
Record:
Report test readings and notes in the Data Sheet
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Test No. 1 Potato CELL is an Organic CELL that can
Generate Electricity
VOLTMETER
Set on 20 VDC scale
ZINC electrode
COPPER electrode
Use Potato instead of Lemon in this test only!
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Test No. 2 Lemon CELL is an Organic CELL that generates the
same Electric Potential as does the Potato Cell
VOLTMETER
Set on 20 VDC scale
ZINC electrode
COPPER electrode
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Test No. 3 TWO LEMON CELLS Connected in Series Produce
TWO TIMES Voltage of a Single One
VOLTMETER
SET ON 20 VDC SCALE
1.78 volt
COPPER electrode
ZINC electrode
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Test No. 4 Using Nickel instead of Zinc Electrode the Lemon
CELL generates LOWER Electric Potential
VOLTMETER
Set on 20 VDC scale
NICKEL electrode
COPPER electrode
.45 V
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Organic Cell Experiment Test Results Data Sheet
Name_____________________________
Purpose:
Develop an understanding the role an electrolyte and electrodes have in a the
process of generating battery’s electric potential.
Test No. 1 Estimate: ____ Volts; Measured: ___V; Electrode: Cu/Zn; Electrolyte: 1 Potato
Test No. 2 Estimate: ____ Volts; Measured: ___V; Electrode: Cu/Zn; Electrolyte: 1 lemon
Test No. 3 Estimate: ____ Volts; Measured: ___V; Electrode: Cu/Zn; Electrolyte: 2 Lemon
Test No. 4 Estimate: ____ Volts; Measured: ___V; Electrode: Cu/Ni; Electrolyte: 1 Lemon
1)Explain why the electric potential is the same in test No. 2 as in Test No.
1?_____________________________________________________________________
2)Explain why the electric potential measured in test no. 3 is double what was measured in
the test no. 2___________________________________________________________
1)Explain why the electric potential measured in the test no. 4 is one half of what you have
measured in the test no. 3:________________________________________________
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Quiz No. 2
Due Date:
1.
October 11, 12, 2016
Name:__________________________
If the electric potential is constant inside and throughout an object, what is the electric field in the
object?
A.
Homework No. 2
Due Date:
1.
2.
October 11, 12, 2016
Name:_________________________
The Si unit of electric potential is: a) joule, b) newton, c) newton-meter, d) joule per coulomb? A)
If two locations are at the same potential, does it take net work to move a charge from one location to
another? A.: Equipotential surfaces are those surfaces in which a) the potential is constant, b) the electric
field is zero, c) the potential is zero. A.:
New Terminology
Hydroelectricity--- production of electrical power through the use of the gravitational force of falling or
flowing water.
Dam–----------------- a thin concrete structure, with a concave side of the curve downstream.
Electric potential-- the capacity of an electric field to do work on an electric charge, to move it between
two specified points. The difference between two electric potentials is Voltage, [Volt or V] The reference
point is the Earth or Ground.
Electric current-----a flow of electrically charged particles. I = q/t, [A]
Electric field---------an electric charge produced without movement. (See above the electric field
surrounding a positive and negative charge.
Power----------------- the rate at which electric energy is transferred by an electric circuit. The unit of
electric power is one Watt (W).
Electrical Energy--- a physical quantity that expresses over time the rate at which electric power is being
transmitted
Battery---------------- converts stored chemical energy into useful electrical energy. Once an external
connection is made between its positive and negative terminals a chemical reaction is initiated that
generates electrons at the anode to supply the current of the battery to the external circuit. Not all
batteries can be recharged.
Electrodes------------ are the two parts of a battery where the battery’s chemical reactions take place. The
electrodes extend to the outside of the battery, where they connect to its external circuit.
Anode----------------- is the negative electrode where the electrons are released out of (and current into)
the external circuit.
Cathode--------------- the positive electrode that acquires electrons into (and current out of) the external
circuit.
Electrolyte------------ the medium that provides the ion transport mechanism between the cathode and
anode.
An ion------------------ an atom or molecule in which a total number of electrons is not
equal to the total number of protons, giving the atom a net positive or negative charge.
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SI-International System of Units
Symbol
J
I
q
E
ρ
P
E
H
B
μ
μ0
F
v
Ω
in
T
N
Wb
J
P
Kg
Name
Work
Electric current
Electric charge
Electromotive force, Potential difference
Resistivity
Electric power
Electric field strength
Magnetic field strength
Magnetic flux density, Induction (F/qv)
Permeability
Permeability of free space
Force of motion
Velocity
Resistance
Inch
Tesla
Newton
Weber
Joule
Mechanical power
Kilogram mass
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Unit
SI Conversion
Joule
kgm/s2 = Nm = Ws = CV
Ampere
A = C/s
Coulomb
C=A
Volt
J/C = W/A = Nm/C
Resistivity
Ωm
Watt
W = VA = Nm/s = kg m/s3
Volt per meter
V/m
Ampere per meter A/m
Tesla
T = Wb/m2
henry per meter Tm/A= H/m = kgm/(As)
4 π x 10-7
N/A2
N
1 N = 1 kg m/s2
m/s
Ohm
V/A = 1 Ω = 1 kg·m2·s-3·A-2
Inch
0.0254 m or 25.4 mm
Tesla
Vs/m2 = N/Am = Wb/ms2
Newton
Kgm/s2
Weber
1 V·s = 1 T·m2 = 1 J/A;
1Nm; 1kgm/s2 ; 2.78 x 10-7 kWh; 2.39 x 10-4 kcal; 9.48 x10-4 BTU
1hp
550 ft.-lb/s = 1 W
2
1.0 kg x 9.8m/s
1N
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Coulomb’s Law and Electric
Force
Following Benjamin Franklin’s work, electrical research advanced by leaps.
Quantitative measurements were carried out in 1785 by the French
physicist Charles-Augustin de Coulomb (1786----1806).
He showed that attraction (or repulsion) between given (electric) charges, q varied inversely as the
square of their distance, r from each other. It became known as the Coulomb’s Law where Electric
force, F between two electric charges between two points is:
F = k(q1 x q2)/r2 ; k = 9.00 x109 Nm2/C2
F = [(Nm2/C2)x C2/m2] = [N]
W= Fxm [Nm]
Example: Two point charges q1 = -1 nC and q2 = +2 nC; distance, r between them is 0.30 m. What is the electric force,
F on each particle?
A: F = 9.00 x 109 Nm2/C2 x (1x10-9 C x 2x10-9 C)/0.09 m2
= 18.0 x 10-9 N/0.09
F12
0.30m
F21
-9
-2
= 18.0 x 10 N/9x 10
= 0.20 x 10-6 N;
F = 0.20 μN
Coulomb’s Law and Electric Force Exercises
Exercise 21, Ch. 15
Compared to electric force the gravitational force between two protons is a, about the same, b,
somewhat larger, c, very much larger, d, very much smaller. A.: 21 d
Battery Electromotive Force, emf and Terminal Voltage, V
VRI
Vemf
Vemf = VRI + VrI
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By Kirchhoff’s Loop Theorem Vemf – VRI – VrI = 0
Vri
Vemf – VRI – VrI = 0
Vterm = Vemf– VrI
Vterm = VRI
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Electric Potential Energy for a Pair of Point Charges
U1,2 = kq1q2/r1,2 (U = zero at r = ∞)
Electric Potential Energy for more than two Point Charges
U1,2,3 = U1,2 + U2,3 + U1,3