Electromechanical systemsElectromechanical systems

Class 9 – Electricity

Dr. Marisela Strocchia X 2310Marisela.strocchia@sheridanc.on.caOffice hours: Tuesdays from 11 to 1 pm

Fridays from 11 to 12 m

Agenda

• Next theory class on Tuesday 23 in room B212F from 12 to 3 pm

• Last class review:– Gas properties

• Electricity• Exercises• Homework

Last class review: Properties of Gases

What does happen when a gas is heated?When a gas is heated, the increased kinetic energy causes the volume

to increase, the pressure to increase, or bothCharles’ law– If the pressure on a gas is constant, the volume is directly proportional

to its absolute (Kelvin or Rankine) temperature V/T = V’/T’

Where: V = Original volume; T = Original temperature V’ = Final volume; T’ = Final temperature

Boyles’ law– If the temperature of a gas is constant, the volume is inversely

proportional to its absolute pressure V/ V’ = P’/P or VP = V’P’

Where: V = Original volume; P = Original pressure V’ = Final volume; P’ = Final pressure

• Boyles’ law experiment

Last class review: Properties of Gases

Density and PressureFrom Boyle’s law: V/ V’ = P’/P or VP = V’P’From density concepts D = m/V;

V = m/DV’ = m/D’

Substituting in Boyle’s lawm/D/ m/D’ = P’/P

D/D’ = P/P’

Where: V = Original volume; V’ = Final volume; P = Original pressure; P’ = Final pressure m = mass; D = Original density; D’ = Final density;

Last class review: Properties of Gases

Most of the time it is very difficult to keep the pressure constant or the temperature constant

Charles’ law and Boyles’ law combinedVP/T = V’P’/T’

Where: V = Original volume; T = Original temperature V’ = Final volume; T’ = Final temperature

P = Original pressure; P’ = Final pressure BOTH Pressure and Temperature MUST be absolute

Matter and electricity

Neutrons and protons are tightly packed in the nucleus, the electrons surround the nucleus. If an electron is removed, the atom is left with a + charge, that is an excess of protons. If an extra electron is added, the atom has a – charge, that is an excess of electronsMOVIE

The force F between two points charges q1 and q2 is directly proportional to the product of their magnitude and inversely proportional to the square of the distance separating them r 2

F= k q1 q2 /r

Where F = Force of attraction or repulsion (N) 9 2 2

K = 9x 10 Nm/C q1 and q2 = electric charges (C = Coulombs)

r = distance between the charges (m)

Coulomb’s law

The flow of charge in an electric circuit is often compared to the flow of water in a hydraulic system.

Ohm’s law

Series circuits

Series circuits

In a series circuit the emf or E (electromotive force, is "that which tends to cause current (actual electrons and ions) to flow.“ More formally, emf is the external work expended per unit of charge to produce an electric potential difference across two open-circuited terminals. The electric potential difference is created by separating positive and negative charges, thereby generating an electric field.)

Parallel circuits

A water system may be compared to a parallel electric circuit.

Parallel circuits

Parallel circuits

In a parallel circuit the emf or E (electromotive force, is "that which tends to cause current (actual electrons and ions) to flow.“)

Circuit A can be replaced by circuit B, where R4 is the equivalent resistance (R4 = R1 + R2). Then, circuit B can be replaced by circuit C, where R5 is the equivalent resistance (1/R5 = 1/R4 + 1/R3).

Compound circuits

Circuit A can be replaced by circuit B, where R4 is the equivalent resistance (1/R4 = 1/R2 + 1/R3). Then, circuit B can be replaced by circuit C, where R5 is the equivalent resistance (R5 = R4 + R1).

Compound circuits

Summary

• Problems: – 6 p 454– 5 p 468– 3 p 475– 5 p 479

Homework

• Ewen, Schurter and Gundersen (2009). Applied Physics. New Jersey: Prentice Hall

• Rex, A., Wolfson, R (2009). College Physics. Boston: Adison-Wesley

Bibliography