L 26 Electricity and Magnetism [3] Monday, October 31, 2005; Posted: 5:12 a.m. EST (10:12 GMT) • Electric circuits WACO, Texas (AP) – • what conducts electricity • what doesn’t conduct electricity A pastor performing a baptism was • Current voltage and resistance electrocuted inside his church • Ohm’s Law Sunday morning when he adjusted • Heat in a resistor – power loss a nearby microphone while • Making simple circuit connections standing in water, a church employee said.

Frog's leg Batteries Alessandro Volta • Professor of Physics at the University of • in 18th century Luigi Galvani a professor of Pavia realized that the electricity was not in anatomy at the University of Bologna the frog’s leg but the twitching was the found that a freshly dissected frog leg result of touching it with two different metals hung on a copper hook twitched when • Volta had discovered the first battery. touched by an iron scalpel. • Lemon battery • The two metals had to be different. • Galvani thought that he had discovered the secret life force

Batteries Î use chemical energy to produce electricity Inside a Duracell 1.5 Volt battery

• two dissimilar metals immersed in a conducting Metal Cap fluid (like an acid for example) cause a chemical plastic case reaction which can produce electric current. +

Carbon center Zinc outer electrode electrode

copper zinc -Bottom Electrolyte electrode electrode electrode paste

acid

1 Current– flow of electric charge Electric current (symbol I) If I connect a battery to the ends of the copper bar the electrons in the copper will • Electric current is the flow of electric be pulled toward the positive side of the charge q (Coulombs) battery and will flow around and around. Î this is called current –flow of charge q

• It is the amount of charge q that passes a copper given point in a wire in a time t, I = q / t • Current is measured in amperes An electric circuit! • 1 ampere (A) = 1 C / 1 s Duracell +

Potential difference or Voltage Voltage and current are not the (symbol V) same thing • Voltage is what causes charge to move in • You can have voltage, but without a path a conductor (connection) there is no current. • It plays a role similar to pressure in a pipe; to get water to flow there must be a An pressure difference between the ends, this electrical pressure difference is produced by a pump outlet • A battery is like a pump for charge, it provides the energy for pushing the charges around a circuit

voltage

Electrons pass through an obstacle Electrical resistance (symbol R) course in a conductor

• Why is it necessary to keep pushing the atoms charges to make them move? • The electrons do not move unimpeded through a conductor. As they move they keep bumping into the atoms which either slows them down or bring them to rest electron path • This continuous opposition to the motion of the electrons is called resistanceÆ R The resistance (R) is a measure of the degree to which the conductor impedes the flow of current. Resistance is measured in Ohms (Ω)

2 Current, Voltage and Resistance Direction of current flow OHM’S LAW

R • Ohm’s law is a simple Resistance R resistor relation between these three important circuit parameters An electric circuit! • Ohm’s law: Current I • I = Voltage/ Resistance Duracell = V / R Battery voltage V + • V in volts, R in ohms, I in amps •V = I R The electrons go one way but the current other forms •R = V / I of Ohm’s Law goes the other way by convention.

Examples Heat produced in a resistor

• (1) If a 3 volt flashlight bulb has a • The collisions between the electrons and resistance of 9 ohms, how much current the atoms in a conductor produce heat. will it draw • The amount of energy converted to heat • I = V / R = 3 V / 9 Ω = 1/3 Amps per second is called the power loss in a • (2) If a light bulb draws 2 A of current resistor when connected to a 120 volt circuit, what • If the resistor has a voltage V across it and is the resistance of the light bulb? carries a current I the power dissipated is • R = V / I = 120 V / 2 A = 60 Ω given by Æ Power P = I × V or I2 × R

Heat produced in a resistor example •Power Î P = I ×V or I2 × R • How much current is drawn by a 60 Watt • Power is measured in Watts = amps × volts light bulb connected to a 120 V power • All wire is rated for the maximum current that line? it can handle based on how hot it can get • Solution: P = 60 W = I x V = I x 120 • To carry more current you need wire of a so I = 0.5 Amps (A) larger diameter Æ this is called the wire • What is the resistance of the bulb? gauge, the lower the gauge the more current it can carry • Solution: V = I RÆ 120 V = ½ A x R • Using extension cords can be dangerous! so R = 240 Ω, or R = V/I

3 extension cords and power strips

• extension cords are rated for maximum current Æ you must check that whatever is plugged into it will not draw more current than the cord can handle safely. • power strips are also rated for maximum current Æ since they have multiple imputs you must check that the total current drawn by everything on it does not exceed the current rating

Simple direct current (DC) Electric circuits electric circuits • a circuit must provide a closed path for the Exercise: given a battery, some wire and a current to circulate around light bulb, connect them so that the bulb is on. • when the electrons pass through the light bulb they loose some of their energy Æ the conductor The battery polarity (resistor) heats up +/- does not matter, • we refer to conductors as resistors because they Either way the bulb impede (resist) the flow of current. Will be on. • the battery is like a pump that re-energizes them 1.5 V each time they pass through it • the current flows in the direction that is opposite to the direction that the electrons travel (this is Ben Franklin’s fault!).

connecting batteries What is DC? Æ do’s and don’ts • With DC or direct current the current always flows in the same direction don’t connect a wire from the + side to the – side, this shorts out the battery and will make it get hot • this is the type of current you get when and will shorten its lifetime. you use a battery as the voltage source. • the direction of the current depends on + how you connect the battery Do not do this • the electricity that you get from the power company is not DC it is AC (alternating). Duracell

4 dueling batteries Proper connections Do not do this Connecting two 1.5 volt batteries

gives like this gives 3.0 volts.

+ Duracell Duracell Duracell Duracell + + +

The batteries are trying to push currents in opposite directions Æ they are working against each other. This does not work.

Batteries in parallel Longer lasting power

Duracell This connection still + gives 1.5 volts but Duracell Duracell since there are 2 Duracell + + + batteries it will provide electrical current Duracell Duracell for a longer time + +

This connection provides 3.0 volts and will 1.5 V D Cell provide power for a longer amount of time

5