Homework

• WebAssign: Simple Electricity(1)

• Read Chapter 22 (Notes)

A flow of charged particles in a closed system. (Which can

be very large…)

Electricity

Electron EnergyKinetic Energy

Schematic

Current (I): Definition

Potential Difference

Battery Internal Resistance

Circuit

kWH and Usage

Ohms / Mhos

V: Definition

Franklin

RT = R1+ R2 + …

RT = 1/R1+1/R2 +1/R3 + …

R = (T)L/A

P = IVParallel

V = IR

P = I2R

Graphs:VI(Linear)IV(Linear)

V=IR Light bulbR(t)

Work

Resistance (R): Definition

Series

Positive and Negative Current

SuperConductor

Ohm’s Law Tools:

VoltmeterAmmeter

VOMDMM

RheostatPotentiometer

Battery

Material Resistivity / Reference

Tables

R=R(T)

Voltage, Current & CircuitsVoltage is the potential difference that drives the movement of charged particles. (Named after Volta, who rediscovered the battery)

Current is the flow of charged particles.

An electric circuit is composed of three elements:

1) conductor

2) potential difference (V), supplied by • battery, solar cell, thunderstorm, …

3) closed loop

Voltage, Current & CircuitsVoltage is the potential difference that drives the movement of charged particles.

Voltage is analogous to height in a gravitational field:

gh

Multiply by m mgh – and you now have gravitational potential energy (Joules).

Electrical potential energy is W = qV = Joules. The unit of V is Joules / coulomb

Current• Electric current is the rate of flow of charge

• I = Δq/ Δt

• Ampere = Coulomb per second (passing by a point in a circuit)

• Ampere made discoveries regarding the interrelationships between electricity and magnetism in the early 1800’s

Current – which way does it go?Two types of current: • Conventional current: Current flows from the positive terminal to the

negative terminal

• Electron flow: Follow the electrons! Which are going the wrong way (by definition)

• Why are these not the same?– What famous American guessed wrong?

• When was the electron discovered?

Current – which way does it go?Two types of current: • Conventional current: Current flows from the positive terminal to the

negative terminal

• Electron flow: Follow the electrons! Which are going the wrong way (by definition)

• Why are these not the same?– What famous American guessed wrong?

• When was the electron discovered?– 1898 (JJ Thompson) with his cathode ray tube. Long after current was

defined.

How do I visualize what is going on in an electric circuit?

How do I visualize what is going on in an electric circuit?

What is driving the current?What is driving the flow?

Ancient Videohttp://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/default.htm

http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/CurrentBasics.htm

http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/OhmsLawEquation.htm

Ohm’s Law: Ohm's Law …says that, for many materials under a wide range of conditions, the voltage, V, and current, I, are linearly related, which implies resistance, R, is independent of V and I.

Linear equation with zero intercept: y = mxV = IR

The slope is the resistance, R

Resistance• Resistance

– the ratio of V/I

• Units: Ohms– Its inverse is conductance, unit is mhos

• Georg Ohm promulgated his famous law in 1827– He was a high school Physics teacher!

Which is the dependent and which the independent variable?

The slope is the inverse of the resistance, 1/R

Ohm’s Law V = IR

Why do I care?

• Fix wiring problems in my home*

• Fix wiring problems in my car

• Fix my kid’s toys

• Figure out why my circuit breaker / fuse blows

• Do my job at work (the one I have now, not the one I had for the last 27 years)

*Disclaimer: Consult your parents or a qualified electrician before trying this yourself.

Ohm’s Law Mnemonic

V = IR

V----- = I R

V----- = R I

Ohm’s Law

Ohm’s Law: Ohm's Law …says that, for many materials under a wide range of conditions, the voltage, V, and current, I, are linearly related, which implies resistance, R, is independent of V and I.

When does it not apply?

•Circuit elements that change temperature•Examples?

•Circuit elements with large capacitance or strong magnetic fields (that are changing)

•Semiconductors: materials that are natural insulators that are made to be somewhat conductive

•Diode – like a one-way path

Example

Units:

Volt

Current

Resistance

Example

A 10V battery is placed across a 5 Ohm resistor. What is the current?

A 10A current flows through a 10 Ohm resistor. What is the voltage?

A 10V battery drives a 1A current through a resistor. What is the resistance?

Power

• Power is the rate of doing work (Watts)

• Power = Work / time

• Power = Volt * q / t but q/t = I

• Electrical Power = Voltage * Current = VI

• Electrical Energy = Power * Time = VIt

Resistance and Ohm’s Law

• Resistance is the impedance to the flow of charge

• Resistance is defined as the ratio of potential difference to current

• R = V/I Unit is the Ohm, Ω

• A device is said to obey Ohm’s Law of its resistance is independent of the Voltage

Find Resistance of a material• Depends on four factors

– R ∞ Temp– R ∞ RHO (ρ) resitivity– R ∞ Length– R ∞ A-1 (cross-sectional Area)

Low resistance

• Short

• Fat

• cold

High Resistance

• Long

• Thin

• Hot

Problem

• Find the resistance of a 20 meters length of Aluminum with a diameter of 12 mm

Givens

• Length 20 m

• RHO =2.82 * 10 -8 Ω * m

• Area = π r 2

• D= 12 mm = .012 m r = .006 m

• A = π (.006m)2 = m2

Diagramming circuits

• Series Connection: – Within the circuit, – ammeters are always in the circuit, they measure flow of

charge

• Ammeters should always have low resistance

• Parallel Connection– Provides multiple paths for flow– Voltmeter is always placed parallel to the device

you wish to measure the potential difference and have very high resistance

More Electric Energy Equations

• P = W/t Watt

• P = VI = V2/R =I2R

• Electric Energy (W)= Pt = VIt = V2/Rt =I2Rt

• Joules for all types of Energy

Kirchoff’s Junction Rule

• Conservation of charge

Kirchoff’s Loop Rule

• Conservation of Energy

• The sum of the changes in potential around any closed path (loop) of a circuit must be zero.

Series Circuit

• Current : One path for the flow of charge• It=I1=I2=I3

• Resistance must increase because length increases

• Rt-=R1+R2+R3

• Voltage must add to Zero• Vt-V1-V2-V3=0 Vt=V1+V2+V3

Series Diagram

VIRP Table

• Vt = V1 = V2 =

• It = I1 = I2 =

• Rt = R1= R2 =

• Pt = P1 = P2 =

Practice Series Circuits

/www.stmary.ws/physics/home/animations3/electricity/ElectricPowerChallenge.html.net/ed1_files/circuits1.html

http://www.stmary.ws/highschool/physics/home/notes/electricity/circuits/default.htm

Parallel Circuit

• More than one path for the flow of charge

• More room for the flow of charge so Resistance goes down

• Voltage must stay the same since all charges have the same drop

Parallel Equations

Parallel Diagram