Bell Ringer From yesterday’s lab… …Do you believe holiday decoration lights are wired in series or in parallel?

Bell Ringer What is the resistance of a 25W light bulb when attached to a 120V outlet?

Electric Circuits

The Basics  Circuit: Any path along which electrons can flow  Electrons will only flow if the circuit is complete with no gaps  Open Circuits are broken and do not allow the flow of electrons  Closed Circuits are complete and allow the flow of electrons

Types of Circuits  Devices within a circuit can be connected in one of two ways:  In Series: Devices form a single pathway for electrons to flow  In Parallel: Devices form branches, each of which is a separate path for the flow of electrons

Series Circuits

 All current passing through each device is the same  Ohm’s Law (I = V/R) applies to the entire circuit, AND each individual device within the circuit  Drawback: If one device in the circuit fails, the entire circuit will fail  i.e.) If one light goes out, they all go out

Parallel Circuits

 Each device connects the same 2 points in the circuit  Voltage is the same across each branch  The total current in the circuit is the sum of the currents in each branch  More parallel branches = less circuit resistance = MORE CURRENT

Parallel Circuits  Advantage: If one device in the circuit fails, the rest of the circuit still functions  i.e.) If one light goes out, the others stay lit

Schematic Diagrams  Simple diagrams which describe electric circuits  Connecting Wire  Battery  Resistor  Capacitor  Open Switch  Closed Switch

Schematic Diagrams - Examples  Two series 3  resistors in parallel with a 1  F capacitor, connected to a 12 V battery  Three parallel 2  F capacitors connected in series to a 10  resistor and 1.5 V battery  Commonly used symbols

Bell Ringer  Draw the following Schematics  Two series resistors in parallel with a capacitor, connected to a battery  Three parallel capacitors connected in series to a resistor and battery

Equivalent Resistance  Combining resistors in a compound circuit:  Equivalent Resistance: The value of the single resistor that would represent the resistance of all of the devices in a circuit R 1 = 8  R 2 = 8  R 3 = 8 

Equivalent Resistance  For series circuits: R eq = R 1 + R 2 + R 3 + …  For parallel circuits: 1 R eq 1 R1R1 1 R2R2 1 R3R3 =++ …

Bell Ringer Calculate the Equivalent Resistance for the circuit below… R 1 = 8  R 2 = 8  R 3 = 8 

Bell Ringer What is the equivalent resistance for the circuit below?

Bell Ringer How much power is dissipated by a 24,000Ω as 120V are sent across it?

Parallel Circuits and Overloading  Lines: Low resistance wires connecting wall outlets to generators at the power utility  Usually carry 110 V to 120 V  When more devices connected in parallel:  Resistance is lowered  Current is increased

Parallel Circuits and Overloading  If this current exceeds the safe amount for the line, the circuit is said to be overloaded  Resulting heat can melt insulation and cause a fire

Preventing Overloading  Fuses: Devices connected in series to prevent the overloading of circuits  A fuse is constructed of a thin wire that will melt if the current becomes to great, therefore breaking the circuit

Short Circuits  Short Circuit: Insulation wears away, causing wires to touch, shortening the circuit  This bypasses the normal resistance of the circuit  Draws very large currents

Poteniometer Poteniometer is a variable resistance Found on many household devices Rotating disc changes the length of the resistor