Elements of a Circuit

Learning Goals:

Describe how to create a circuit.Understand the role voltage and current play in a circuit.

Requirements of a Circuit

1. Closed conducting loop (ex: usually wire)2. Power or energy source, also called an emf

(ex: batteries, generators, power outlet)3. A load, something that uses energy (ex: light

bulb, resistors, any electrical appliance)

http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

Types of Circuits

Closed Circuit – a complete conducting loop through which charge can travel

Open Circuit – a path where charge does not flow

Short Circuit – a closed circuit that does not contain a load (very dangerous)

Electric Potential Energy

• Charges obtain electric potential energy just like objects gain gravitational potential energy.

• What do you have to do to give an object gravitational

potential energy? • Does the object “want” to move toward Earth or away

from it? • Do you move the object in the direction of the

gravitational force or against it?

As work is done against gravity the barbell gains potential energy.

Work

Gravitational Force

Now replace Earth with a positive charge and replace the small object with a small positive charge. As work is done against the electric force the charge gains potential energy.

Work

Electric Force

•When work is done on the small charge it gains electric potential energy.

• Batteries do work on charges causing a gain or change in potential energy.

Electric Potential -

• Electric potential energy per charge– has a value of 0 at the negative terminal of a

battery– the positive terminal on a 6V battery is 6V higher

in electric potential than the negative terminal

Voltage (V)-

• Change in electric potential energy per charge

- measured in Volts (V)Equation:

V = ΔPE

q

• Ex: When 1 Coulomb of charge moves from the negative to the positive terminal of a 6V battery the potential energy increases by ___.

• Ex: When 3 Coulombs of charge moves from the negative to the positive terminal of a 6V battery the potential energy increases by ___.

Current (I) – the rate at which charge flows past a point- Units: Amperes (A)- Equation: I = Q

t- conventionally, it is the flow of positive charge- flows from the positive terminal to the

negative terminal of the battery in the external part of the circuit.

Actual Charge Movement

• Electrons are the charges that actually move in the wires. (We know protons don’t move!)

• They take a zigzag path in the wires because they continuously colliding with other particles.

• The net velocity of the electrons is called drift velocity and it is very, very slow but eventually they make it all the way around the circuit.

• How can the current in a circuit be high when the charges take so long to make it around?

• How can a light switch turn on the lights so quickly if the charge moves so slow?

• What is the difference between direct current (DC) and alternating current (AC)?

Ex: If the current in a wire is 8 mA, a. how long does it take for 2.0 C of charge to pass

a point in this wire? I = 8mA = 0.008 A or 8x10-3A I = Q/t

Q = 2C 8x10-3 = 2/tt = ? t = 250sb. How many electrons pass the point in the time you found in part a?n = ? Q = nqQ = -2C -2 = n (-1.6x10-19)qe- = -1.6x10-19C n = 1.25x1019 electrons

Resistance and

Ohm’s LawAlessandro Volta Andre Ampere

Georg Ohm

Resistance

– a measure of how much a material resists the flow of electrons – SI unit is Ohms (Ω) (Greek letter omega)–Good conductors have a low resistance• Ex: copper 0.00000004 Ohms

–Poor conductors have a high resistance • Ex: dry human skin 500,000 Ohms

Factors that Affect Resistance in a Wire

1. Resistance increases as the length of the wire increases

2. Resistance increases as the width decreases

- wire gauge describes its width

American Wire Gauge (AWG)

3. Resistivity of a materialMaterial

Resistivity(ohm•meter)

Silver 1.59 x 10-8

Copper 1.7 x 10-8

Gold 2.4 x 10-8

Aluminum 2.8 x 10-8

Tungsten 5.6 x 10-8

Carbon 3.5 x 105

Polyethylene 108 - 109

Glass 1010 - 1014

Hard Rubber 1013

R = r _L_ A

r is resistivityL is lengthA is cross-sectional area

(width)

http://phet.colorado.edu/en/simulation/resistance-in-a-wire

Analogy: Water experiences more resistance as pipes get smaller and longer, so current decreases.

Ohm’s Law

Resistance = Voltage

Current

R = V

I

More commonly seen in this form:

V = IR

Now go to physicsclassroom.com and answer the questions about Resistance and Ohm’s Law.

http://www.physicsclassroom.com/Class/circuits/u9l3b.cfm

Answer the questions on the website above and check your answers then hit next section: Ohm’s Law on the bottom of the site to go to the next page. Answer the questions about Ohm’s Law and check your answers.