Phys141 Principles of Physical Science

Chapter 8 Electricity and Magnetism

Instructor: Li Ma

Office: NBC 126Phone: (713) 313-7028Email: malx@tsu.edu

Webpage: http://itscience.tsu.edu/ma

Department of Computer Science & PhysicsTexas Southern University, Houston

Oct. 6, 2004

Topics To Be Discussed

Electric Charge and CurrentVoltage and Electric PowerMagnetismElectromagnetismSkip

– §8.3– The Earth’s Magnetic Field in § 8.4– Motors and Generators in §8.5

Electric Charge

Electric charge is a fundamental quantity (mentioned in Chapter 1)

The property of electric charge is associated with certain subatomic particles

Two types of charges:– positive (+)– negative (-)

Electric Charge (cont)

All matter is made up of small particles called atoms

An atom is composed of electrons, protons and neutrons:– Electrons: negatively charged particles– Protons: positively charged particles– Neutrons: neutral particle– Table 8.1 on page 167

Electric Charge (cont)

All three particles have certain massElectrons and protons possess electric

charges– Magnitudes are equal– Natures are different

When we have same number of electrons and protons– Total charge is zero – neutral situation

Electric Charge (cont)

Unit of electric charge: coulomb (C)Electric charge is usually designated by

the letter q– An excess of positive charges: +q– An excess of negative charges: -q

When charge flows, or is in motion, we have electric current

Electric Current

Electric current is defined as the time rate of flow of electric charge

Unit: ampere (A), 1 A = 1 C/s

current =charge

time

I =qt

or

Electric Current (cont)

Electrical conductors: materials in which electric charge flows readily– The outer, loosely bound electrons of the

atoms: ex. metalsElectrical insulators: materials that do

not conduct electricity very well– Electrons are more tightly bound– ex. Wood, glass, plastic, rubber, etc

Electric Current (cont)

Semiconductors: materials that are neither good conductors nor good insulators– ex. graphite (carbon)

Net charge q:– Electrons move randomly and chaotically

=> no net charge flows– More electrons move in one direction =>

net charges flow => current

Electric Force

An electric force exists between any two charged particles

This kind of mutual forces may be either attractive or repulsive, depending on the types of charges (+ or -)

Law of charges:– Like charges repel, and unlike charges

attract

Electric Force (cont)

The law of charges gives the direction of an electric force, what about the magnitude?

Coulomb’s law:– The force of attraction or repulsion

between two charged bodies is directly proportional to the production of two charges and inversely proportional to the square of the distance between them

Electric Force (cont)

Where F is magnitude of the electric force

q1 is magnitude of the first charge

q2 is magnitude of the second charge

r is distance between charges

k is a proportionality constant:

k = 9.0 x 109 N-m2/C2

F =kq1q2

r2

Electric Force (cont)

Compare Coulomb’s law and Newton’s law of universal gravitation:– Both forces depend on the square of the

separation distance– Coulomb’s law depends on charge, whereas

Newton’s law depends mass– Coulomb’s law can give rise to either an attractive

or a repulsive force; on the other hand, the force of gravitation is always attractive

– The electric forces are comparatively much stronger than the gravitational forces

Voltage

It takes work to separate chargesThe more charges to separate, the

more work it takes => electric potential energy

Freely moving charge will move toward the charge of opposite sign:

electric potential energy=> kinetic energy

Voltage (cont)

Voltage, or potential difference, is defined as the amount of work it would take to move a charge between two points, divided by the value of the charge

In other word, voltage (V) is the work (W) per unit charge (q) or the electric potential energy per unit charge

Voltage (cont)

Unit: volt (V), 1 V = 1 J/CVoltage is caused by a separation of

charge

voltage =work

charge

V =Wq

or

Voltage (cont)

Electric potential energy may be used to set up a current

When there is current, it meets with some opposition because of collisions within the conducting material

Resistance (R): the opposition to the flow of charge– Unit: ohm (Ω)

Voltage (cont)

Ohm’s law:

voltage = current x resistance

or V = I·RIt is a simple relationship involving

voltage, current and resistanceOne ohm is one volt per ampere:

R = V / I

Electric Power

When current exists in a circuit, work is done to overcome the resistance of the circuit, and power is expended

Electric power is defined in terms of current and voltage

P = I·V

or P = I2·R

Electric Power (cont)

The power that is dissipated in an electric circuit is frequently in the form of heat

This heat is called joule heat or I2R losses

This heating effect is used in electric stoves, heaters, hair dryers, light bulb, and so on

Magnetism

Two poles of the magnet: one at each end– The north pole (N): the north-seeking pole– The south pole (S): the south-seeking pole

Law of poles:– Like poles repel, and unlike poles attract– N-S poles attract, and N-N poles and S-S poles

repel– The strength of the magnetic force is inversely

proportional to the square of the distance between the poles

Magnetism (cont)

All magnets have two poles: i.e. magnets are always dipoles

Every magnet produces a force on every other magnet

Magnetic field (B): a set of imaginary lines that indicates the direction in which a small compass needle would point if it were placed near a magnet

Magnetism (cont)

Magnetic field is a force field: the field lines are indications of the magnetic force

In a magnetic field pattern:– The arrows in the field lines indicate the

direction in which the north pole of a compass would point

– The closer together the field lines, the stronger the magnetic force

Electromagnetism

There is an electric field around chargesElectric field is the electric force per

unit chargeThe electric and magnetic fields are

vector quantitiesThe interaction of electrical and

magnetic effects is known as electromagnetism

Electromagnetism (cont)

Two basic principles of this interaction:– Moving electric charges (current) give rise

to magnetic fields– A magnetic field may deflect a moving

electric chargeThe source of magnetism is moving and

“spinning” electrons – the 1st principleThere may be magnetic force on a

moving electric charge – the 2nd principle