Electricity – Notes


Basic Ideas


Electric charge is a fundamental quantity like mass, distance, or time.


Charge is observable and measurable by the force it exerts on other charges.


There are two types of charges:  positive and negative.


Like charges repel one another:  positive repels positive, negative repels negative.


Opposite charges attract one another:  positive attracts negative, negative attracts positive.


The variable q or Q is used to represent an amount of charge (may be positive or negative).


The SI unit of charge is the Coulomb, abbreviated C.


Since all matter contains protons and electrons, there is charge present (often in great quantities) in every object.  Typically, however, the number of protons in an object essentially equals the number of electrons – adding the amounts of charge gives a total of zero – the object is said to have no net charge and to be neutral.  An object is charged or has a net charge when there are unequal numbers of protons and electrons.  This occurs almost always as a result of electrons being transferred to or from an object.


Charge is conserved!  The net amount of electric charge produced in any process is zero.



Means of becoming charged:


Conduction is the transfer of charge from one object to another, usually as a result of contact.

Induction involves the rearrangement of charge within an object due to the presence of an external charge (or electric field).  There is no contact.



Insulators vs. Conductors


Both insulators and conductors can possess charge.


Key difference:  charge can travel freely through conductors.  Charge cannot travel freely through insulators, but rather tends to be “locked” in place.


Explanation:  It is now known that electrons are carriers of charge.  In conductors (metals) the electrons are not tightly bound to nuclei  and easily “roam” from one atom to another.  In insulators (e.g. rubber, plastic, wood, etc.) electrons are held more tightly in orbits around nuclei and do not easily move from one atom to another.



Coulomb’s Law


The force one charge, q1, exerts on another, q2, has a magnitude given by:


                        F = k q1 q2



where r is the distance between q1 and q2 and k is a constant.


The direction of this force is either toward or away from the other charge – depending on whether it is attraction or repulsion.


k = 9.0 ´ 109 N m2/C2 



Quantization of Charge


The smallest possible amount of charge is that on an electron or proton.  This amount is called the fundamental or elementary charge, e.


                        e = 1.602 ´ 10-19 C


An electron has charge:  q = -e = -1.602 ´ 10-19 C


A proton has charge:  q = +e = 1.602 ´ 10-19 C



Furthermore, any amount of charge greater than the elementary charge is an exact integer multiple of the elementary charge!  Weird, eh?


                        q = ne,  where n is an integer


For this reason, charge is said to be “quantized”.  It comes in quantities of 1.602 ´ 10-19 C.



Electric Potential


V = W/q    or    V = E/q   


V = Electric potential  (A.K.A. Voltage, Potential Difference, Electromotive Force or EMF)

W = Work done to move charge between two points

E = Potential energy due to position (separation) of charge

q = Amount of charge


SI unit for electric potential:  the Volt


1 Volt = 1 Joule/1 Coulomb     or     V = J/C  (The number of volts indicates the number of joules work or energy per coulomb.)



Electric Current


            I = Q/t


I = electric current – rate at which charge flows in a certain pathway

Q = amount of charge flowing past a certain point

t = time


SI unit for electric current:  the Ampere


1 Ampere = 1 Coulomb/1 second     or     A = C/s  (The number of amperes indicates the number of coulombs of charge flowing per second.)



Electric Power


            P = VI


The power for an electrical device is equal to the voltage times the current.  This value will indicate (in Watts) the rate at which the device transforms energy.





            R = V/I    more commonly written as:  V = IR    (known as Ohm’s Law)


R = electric resistance (this is the “resistance” to flow of charge through a device)

An object or device with greater resistance will require a greater voltage to produce a certain amount of current.


SI unit for resistance:  the Ohm


1 Ohm = 1 Volt/1 Ampere     or     W = V/A    (The number of ohms indicates how many volts are required to produce 1 ampere of current.)


For certain materials and devices the resistance will be constant over a wide range of voltages and currents; such a device or material is said to be “ohmic”.  Ohmic materials include metals like copper, silver, etc.  Common carbon based resistors are also ohmic.  Nonohmic devices have a resistance that changes depending on voltage and current.  A light bulb filament is nonohmic because its resistance increases as its temperature increases.  Semiconductors, such as diodes and transistors, and electric motors are also nonohmic.