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Physics 106 Lesson #9
Electrochemical Cellsand Batteries
Dr. Andrew Tomasch
2405 Randall Lab
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Review: Properties of Electric Charges
• 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)
Franklin
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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.
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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
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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
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Review: Comparing the Electrostatic Force to Gravity
• Gravitational Force (Newton):
• Coulomb Force:
However, the gravitational force can only be attractive!
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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
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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
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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”
• 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
Ampere
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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
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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)
Electron flow
Conventional Current
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Natural Currents:Lightning and the Northern Lights
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Resistance• Life is tough for free
electrons• Resistance:
Repulsion from other electrons
Vibration of atomsImpurities
• Energy is dissipated• Worse at high
temperatures• The symbol for a
resistor:http://regentsprep.org/Regents/physics/phys03/bresist/default.htm