Physics 106 Lesson #9

Electrochemical Cellsand Batteries

Dr. Andrew Tomasch

2405 Randall Lab

atomasch@umich.edu

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

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 Force (Newton):

• Coulomb Force:

However, the gravitational force can only be attractive!

2r

GMmFG

2

21

r

qqkFC

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”

• 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

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)

Electron flow

Conventional Current

Natural Currents:Lightning and the Northern Lights

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