Bell Ringer l From yesterdays lab lDo you

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

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

Electric Circuits

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

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

Series Circuits

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

Parallel Circuits

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

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

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

Schematic Diagrams - Examples series 3 W resistors in parallel with a 1 m. F capacitor, connected to a 12 V battery Ø Two parallel 2 m. F capacitors connected in series to a 10 W resistor and 1. 5 V battery Ø Three Ø Commonly used symbols

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

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

Equivalent Resistance Ø For series circuits: Req = R 1 + R 2 + R 3 + … Ø For parallel circuits: 1 Req = 1 R 1 + 1 R 2 + 1 R 3 …

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

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

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

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

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

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

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

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