The SI unit of charge is the Coulomb, named in honor of Charles Augustin CoulombCharles Augustin Coulomb. Charles Augustin Coulomb 1 C = charge on 6.25 x electrons (or protons) 1 e - = 1.60 x Coul = elementary charge

Electric Potential Difference the change in electric potential energy per unit charge V = W/QV = W/Q The SI unit of electric potential VOLT difference is the VOLT, named in Alessandro Volta Alessandro Volta honor of Alessandro Volta.Alessandro Volta One VOLT is the electric potential difference between two points when one Joule of work is done in moving one Coulomb of charge between the points.

Electric potential difference (“voltage”) is always measured across a circuit element. It basically compares the electric potential energy of electrons after they leave a circuit element to what they had before the circuit element. To say that the electric potential difference of a zinc-copper potato cell is 0.75 V means that the cell gives each Coulomb of charge 0.75 Joules of electric potential energy. Other circuit element take away electric potential energy of the electrons to cause light, heat, sound, motion, etc…

electric cell - a device that converts one form of energy to electrical energy Chemical cells convert chemical energy into electrical energy. Chemical cells can be “wet” or “dry”.

Solar cells Solar cells convert light energy into electrical energy. generator A generator converts mechanical energy into electrical energy. battery battery - two or more cells connected in series or in parallel

the flow of charge the flow of charge; can be positive or negative, but usually negative (electrons) through a conducting metal

Electric current is measured in Amperes, in honor of Andre Marie Ampere Andre Marie AmpereAndre Marie Ampere Andre Marie Ampere. One Ampere is the flow of one Coulomb of charge per second. 1 Amp = 1 Coulomb per second = 1 C/s 1 Amp = 1 Coulomb per second = 1 C/s IQt I = Q/t

Electric current is always measured through a circuit element. It basically measures how many electrons pass by a given point in the circuit every second. To say that the current through a circuit element is 3.25 A means that 3.25 Coulombs of charge enter and leave the circuit element every second. Remember that it takes 6.25 x electrons to make up one Coulomb of charge.

Ammeter a device that measures current Voltmeter a device that measures electric potential difference

power = work/time = (work/charge). (charge/time) = electric potential difference. current P (Watts) = V (Volts). I (Amps)

The power of a circuit or a circuit element tells us how many Joules of electric potential energy are being converted into other forms of energy (light, heat, sound, motion, etc…) each second. A light bulb with a power of 60 W means that 60 Joules of electric potential energy are being converted into light and heat each second. The energy to light the bulb is not free. You, or someone you know, must pay for how much energy you use each month in your electric bill.

Analogies of simple circuits are these links: Water circuit analogy link link Air flow link link Various link link Teaching with Analogies link1, link2 link1link2link1link2

Resistance determines the amount of current flow = the ratio of potential difference to current = the ratio of potential difference to current R= V I The SI unit of resistance is the Ohm  Ohm, , named Georg Simon Ohm Georg Simon Ohm in honor of Georg Simon Ohm.Georg Simon Ohm One Ohm One Ohm of resistance is the resistance one Volt such that one Volt of potential difference one Amp is needed to obtain a current of one Amp.

The resistance, in Ohms, of a circuit or circuit element, essentially tells us how many Volts of electric potential difference are needed to produce one Amp of current in the circuit or through that circuit element. To say that the resistance of a circuit element is 175 Ohms means that it would take 175 Volts of electric potential difference to produce a current of one Amp through that circuit element.

The resistance of a circuit element depends on: the length of the conductor 1. the length of the conductor as length increases, resistance increases proportionally the cross-sectional area of the conductor 2. the cross-sectional area of the conductor as area increases, resistance decreases proportionally the resistivity of the conductor 3. the resistivity of the conductor as resistivity increases, resistance increases proportionally

Resistivity The resistivity, , of a conductor is equal to the resistance of a wire 1 cm long and having a cross-sectional area of 1 cm 2. R R = resistance, measured in Ohms =   = resistivity, usually in units of   cm l l = length, measured in cm A A = cross-sectional area, measured in cm 2 Investigate resistivity here here

Ohm’s Law The ratio of potential difference to current is constant. If R = V/I is a constant value for a given resistor, then that resistor is said to obey Ohm’s Law. Click here and here to link to pages describing resistor color codes. here

Click here and here to run here simulations of Ohm’s Law. Many circuit elements do not obey Ohm’s Law. Resistors that get hot, like light bulbs and heating elements, do not keep a constant resistance. Resistance generally increases as objects become hotter.

Resistor Circuits Series 1. total resistance is the sum of the separate resistors separate resistors R T = R 1 + R 2 + R current is the same through each resistor I T = I 1 = I 2 = I 3 = total potential difference is the sum of each V T = V 1 + V 2 + V In other words, in a series circuit, resistance and voltage add, but current stays the same.

R1R1R1R1 R2R2R2R2 R3R3R3R3 E = 12 V RT =RT =RT =RT = VT =VT =VT =VT = IT =IT =IT =IT = R1R1R1R1 R2R2R2R2 R3R3R3R3 V,V PT =PT =PT =PT = R,I,A P,W

R1R1R1R1 R2R2R2R2 R3R3R3R3 E = 12 V R T = 15 Ω V T = 12 V I T = 0.80 A R1R1R1R1 R2R2R2R2 R3R3R3R3 V,V P T = 9.6 W R,I,A P,W

Parallel 1. reciprocal of the total resistance is the 1. reciprocal of the total resistance is the sum of the reciprocals of the separate sum of the reciprocals of the separate resistors resistors 1/R T = 1/R 1 + 1/R 2 +1/R total current is the sum of the current through each resistor through each resistor I T = I 1 + I 2 + I potential difference is the same across each resistor each resistor V T = V 1 = V 2 = V 3 =... In other words, in a parallel circuit, resistance adds as reciprocals, voltage stays the same, and current splits. In other words, in a parallel circuit, resistance adds as reciprocals, voltage stays the same, and current splits.

E = 12 V RT =RT =RT =RT = VT =VT =VT =VT = IT =IT =IT =IT = R1R1R1R1 R2R2R2R2 R3R3R3R3 R, V,VI, PT =PT =PT =PT = R3R3R3R3 R1R1R1R1 R2R2R2R2 P,W

E = 12 V R T = 3.42 Ω V T = 12 V I T = 3.50 A R1R1R1R1 R2R2R2R2 R3R3R3R3 R, V,VI, P T = 42 W R3R3R3R3 R1R1R1R1 R2R2R2R2 P,W

Go to link1, link2, link3, link4, link5, and link6 to view link1link2link3link4link5link6link1link2link3link4link5link6 pages and simulations examining Kirchhoff’s Loop and Junction Rules. Kirchhoff’s Rules Loop Rule: The sum of the potential differences around any closed circuit loop is zero. Junction Rule: The sum of the currents into any circuit junction is zero.

The sites linked here and here here (click on “Circuit here Construction Kit”) allow you to build and test your own series, parallel, and combination circuits. For a complete interactive tutorial For a complete interactive tutorial on electricity and magnetism, go here. here