Transcript Cells and Batteries Lab

Physics 106 Lesson #9
Electrochemical Cells
and Batteries
Dr. Andrew Tomasch
2405 Randall Lab
[email protected]
Review: Properties of
Electric Charges
Franklin
• Two types of charge: positive and negative
(Ben Franklin in early 1700’s)
• Like charges repel; unlike charges attract
• Charge is conserved
• Charge is quantized (comes in discreet units)
• Objects usually have as much negative
charge in them as they do positive charge →
the total charge is zero (electrically neutral)
Review: Charging a
Conductor by Conduction
Conduction = charging by contact
Rub a Teflon rod
with fur to
separate charge
The process of separating charge
by friction is called triboelectricity.
Review: Conductors and Insulators
We can classify materials
according to their ability to conduct
electrical charge:
– Conductors: charges (free
electrons) move freely (metal)
– Insulators: charge is not readily
transported (glass)
– Semiconductors: electrical
properties in between
Gold is the best
conducting metal
http://www.physicspost.com
Review: Electric Forces
In 1785 Charles Coulomb established the
fundamental law of electric force between
two stationary charged particles:
– Force directed along the line joining the
particles
– Force inversely proportional to the square of
separation distance between particles
– Force proportional to the product of the two
charges
– Force attractive if particles have charges of
opposite sign and repulsive if charges have
same sign
Review: Comparing the
Electrostatic Force to Gravity
• Gravitational
• Coulomb Force:
Force (Newton):
GMm
FG  2
r
FC 
k q1 q2
r
However, the gravitational
force can only be attractive!
2
The Electrochemical Cell
• Used to establish electric
current in a circuit
• Transformation of
chemical energy stored in
the cell to kinetic energy
of the charge carriers
(electrons)
• Two oppositely charged
electrodes (terminals)
• An electrical potential
difference (voltage) exists
between the terminals
• Electric potential is
potential energy per unit
charge.
• Multiple cells connected
together comprise a
battery of cells or battery
A 50’s-style
Carbon-Zinc Cell
Cells in Series and Parallel
• A battery pack consists
of four 1.2-volt cells in
series (end-to-end)..
• The nominal voltage of
the battery is the sum
of the individual cell
voltages = V × 4 = 4.8 V
• With parallel cells,
the voltage stays
the same = 1.2 V
• The amount of
available charge
increases → larger
current
Current: Charge in Motion
• Definition: Current is the
amount of charge moving
past a point per unit time
• Charge flows in a
confined channel (wire)
like a river
• Current is caused by
potential differences
(voltages)
• Charge is measured in
Coulombs (C)
• The unit of current is the
Ampere (A): 1 A = 1 C/s
“One Coulomb per second”
Ampere
• 1 A = lots of moving
charge (1 C = 6.25 x
1018 electrons)!
• Typical house: 200 A
• Direct current (DC):
flows in one direction
• Alternating current
(AC): flows back and
forth
Cells, Batteries & Current
• When connected to a circuit,
the potential difference
between the cell or battery
terminals creates an electric
force on the charges in the
conductor causing them to
move and establishing an
electric current
• The conventional current is
from regions of higher
potential to regions of lower
potential, positive to
negative
Conventional
current
+
Battery
+
Cell
Conventional Current and Electron Flow
• Truth: the particles which
carry charge through
wires in a circuit are
mobile electrons
• Ben Franklin: positive
charges move (oops!)
• The direction of current
is by convention the
direction in which a
positive charge would
move (so current flows
from high potential to
low potential)
Conventional
Current
Electron
flow
Natural Currents:
Lightning and the Northern Lights
Resistance
• Life is tough for free
electrons
• Resistance:
Repulsion from other
electrons
Vibration of atoms
Impurities
• Energy is dissipated
• Worse at high
temperatures
• The symbol for a
resistor:
http://regentsprep.org/Regents/physics/phys03/bresist/default.htm