Resistors, Currents and All That Jazz Chapter 19 – Young & Geller Resistors, Currents and All That Jazz

Date Day Topic 14-Sep Monday Complete Capacitors 16-Sep Wednesday 7AM Problem Session Chapter 19 - DC Circuits 18-Sep Friday Chapter 19 - DC Circuits 21-Sep 23-Sep EXAMINATION #1

7AM Problem Session Chapter 19 - DC Circuits September 23 Date Day Topic 14-Sep Monday Complete Capacitors 16-Sep Wednesday 7AM Problem Session Chapter 19 - DC Circuits 18-Sep Friday Chapter 19 - DC Circuits 21-Sep 23-Sep EXAMINATION #1

Chapter 19 Read the chapter – Important stuff HW assigned

Current I L - + A V

NOTE Electric Current is DEFINED as the flow of POSITIVE CHARGE. It is really the electrons that move, so the current is actually in the opposite direction to the actual flow of charge. (Thank Franklin!)

Charge is moving so there must be an E in the metal conductor!

Electrons “Bounce Around”

(Average) Current Density ANOTHER DEFINITION (Average) Current Density

A closed circuit

Ohm A particular object will resist the flow of current. It is found that for any conducting object, the current is proportional to the applied voltage. STATEMENT: DV=IR R is called the resistance of the object. An object that allows a current flow of one ampere when one volt is applied to it has a resistance of one OHM.

Ohm’s Law

Resistivity and Resistance

Not Everything Follows Ohm’s Law

Neither does this ……

The Battery + -

The Real Deal

A REAL Power Source is NOT an ideal battery V Internal Resistance By the way …. this is called a circuit! E or Emf is an idealized device that does an amount of work E to move a unit charge from one side to another.

A Physical (Real) Battery Internal Resistance

Back to Potential Change in potential as one circuits Represents a charge in space Change in potential as one circuits this complete circuit is ZERO!

Consider a “circuit”. This trip around the circuit is the same as a path through space. THE CHANGE IN POTENTIAL FROM “a” AROUND THE CIRCUIT AND BACK TO “a” is ZERO!!

To remember In a real circuit, we can neglect the resistance of the wires compared to the resistors. We can therefore consider a wire in a circuit to be an equipotential – the change in potential over its length is slight compared to that in a resistor A resistor allows current to flow from a high potential to a lower potential. The energy needed to do this is supplied by the battery.

Series Combinations SERIES Resistors R1 R2 i V1 V2 V

The rod in the figure is made of two materials The rod in the figure is made of two materials. The figure is not drawn to scale. Each conductor has a square cross section 3.00 mm on a side. The first material has a resistivity of 4.00 × 10–3 Ω · m and is 25.0 cm long, while the second material has a resistivity of 6.00 × 10–3 Ω · m and is 40.0 cm long. What is the resistance between the ends of the rod?

Parallel Combination?? R1, I1 R2, I2 V

What’s This??? In this Figure, find the equivalent resistance between points (a) F and H and [2.5]  (b) F and G. [3.13]  

(a) Find the equivalent resistance between points a and b in the Figure. (b) A potential difference of 34.0 V is applied between points a and b. Calculate the current in each resistor.

Back to Potential Change in potential as one circuits Represents a charge in space Change in potential as one circuits this complete circuit is ZERO!

Consider a “circuit”. This trip around the circuit is the same as a path through space. THE CHANGE IN POTENTIAL FROM “a” AROUND THE CIRCUIT AND BACK TO “a” is ZERO!!

To remember In a real circuit, we can neglect the resistance of the wires compared to the resistors. We can therefore consider a wire in a circuit to be an equipotential – the change in potential over its length is slight compared to that in a resistor A resistor allows current to flow from a high potential to a lower potential. The energy needed to do this is supplied by the battery.

Some Circuits are HARDER than OTHERS!

NEW LAWS PASSED BY THIS SESSION OF THE FLORIDUH LEGISLATURE. LOOP EQUATION The sum of the voltage drops (or rises) as one completely travels through a circuit loop is zero. Sometimes known as Kirchoff’s loop equation. NODE EQUATION The sum of the currents entering (or leaving) a node in a circuit is ZERO

Take a trip around this circuit. Consider voltage DROPS: -E +ir +iR = 0 or E=ir + iR rise

START by assuming a DIRECTION for each Current Let’s write the equations.

In the figure, all the resistors have a resistance of 4 In the figure, all the resistors have a resistance of 4.0 W and all the (ideal) batteries have an emf of 4.0 V. What is the current through resistor R?

How Fast ? RC Circuit Initially, no current through the circuit Close switch at (a) and current begins to flow until the capacitor is fully charged. If capacitor is charged and switch is switched to (b) discharge will follow. How Fast ?

What do you think will happen when we close the swutch? Close the Switch I need to use E for E Note RC = (Volts/Amp)(Coul/Volt) = Coul/(Coul/sec) = (1/sec)

Time Constant

Result q=CE(1-e-t/RC)

q=CE(1-e-t/RC) and i=(CE/RC) e-t/RC

Discharging a Capacitor qinitial=CE BIG SURPRISE! (Q=CV) i iR+q/C=0

Power