HW for E (due Thursday) and C (Friday)

Graph:

I vs. R (with R on the x-axis)
V vs. R (with R on the x-axis)
Try to interpret the graphs.

Also, try these problems:

1. Consider a series circuit, with 3 resistors: 10, 20, and 40 ohms. Voltage is 100 V. Find:

a. total resistance
b. total current
c. current through each resistor
d. voltage "over" each resistor

2. Now try the above problem with the 3 resistors in parallel.

3*. If you have 2 identical resistors in parallel, what do you suppose might always be true (about the total resistance of them, and the current through each)? Could this be generalized to any number of identical resistors in parallel?

C block HW (and E, if you have time)

2 graphs -

Voltage vs resistance

Current vs. Resistance

Interpret these results.

recent notes

Voltage (V) = available energy per coulomb of charge

V = E/q

units are volts (V)


Current (I) = rate at which charge travels in a circuit

I = q/t

units are amperes or amps (A)


Resistance (R) = the ratio of applied voltage to the current through the "resistor"

R = V/I

units are ohms (omega symbol)

The relationship that governs all three is the last expression above, often called Ohm's law and expressed typically as:

V = I R

In short:

Voltage is an energy per charge - sort of like the total energy the battery has, but measured as a ratio of energy per coulomb of charge

Current is how "fast" charge travels, but in coulombs per second (amps)

Resistance is how much the resistor "resists", in terms of a ratio: V/I

HW

C - as identified yesterday; the homework below will be yours for Friday

E - Investigate Ohm's Law. As always, physicsclassroom.com may be a good source.

Is Ohm's law always true?

Problems to try:

1. A 100-ohm resistor is connected directly to a 9-V battery. What is the current through the resistor? What is the current everywhere else in the circuit? What voltage would you read "over" the resistor? If you did this as an experiment, what might you expect to see - how would the measured values compare to the theoretical; why?

2. Now imagine that you have two resistors (50-ohm, 150-ohm) in series (one right after the other) with a 20-V battery. Find the following:

a. the total resistance of the circuit
b. the total current from the battery
c. the current through each resistor
d. the voltage over each resistor
e. draw a schematic for this problem

This problem may seem a little challenging for you - we haven't done one like this before. Try it, no matter what.

On Thursday (E) we will perform the Ohm's Law lab, measuring voltages and currents for several resistors. You'll love it - resistance is futile.

Hw

C block -

As listed earlier - hypothesize the graphical relationship between voltage, current and resistance. Specifically, how does resistance affect current, and how does voltage affect current?

Write a lab hypothesis, focusinng on the graphical nature.

Also, find a definition for resistance and one for power.

Lastly, investigate series and parallel circuits.



E block - the last 2 comments are your HW for the Thursday class.

HW

C block - as before (circuit schematic questions)

E block - as before, if you didn't finish it. Also -

Our next (formal) is about circuits - specifically, the relationship between voltage, current and resistance. In preparation for this -

Be sure you understand the difference between these three concepts/quantities. Know the units, too.

Hypothesize the relationship between current and resistance. Specifically, how does resistance affect current? What might this look like mathematically AND graphically? Formulate this as a lab hypothesis.

We will use electrical meters to measure voltage and current. Be absolutely certain you know the difference.

HW For C, but E should do it after studying for quiz

Investigate how one draws electrical schematics - circuit diagrams.

Draw a schematic for the light bulb, lit by a battery.

Identify different symbols for electrical components. Consider these:

Battery
Resistor
Wire
Diode
Switch
IC (chip)
Anything else interesting

Question 1 from HW

Since 1 proton has a charge of 1.6 x 10^-19, 4 C is made up of 4 divided by this number (that we usually call e).

Same logic for the electron question: -2.5 divided by e

There is another method. There are 6.25 x 10^18 protons per C of charge. So, 4 C would be 4 times 6.25 x 10^18 protons. Similarly, -2.5 C would be 2.5 times 6.25 x 10^18 electrons

hw

E block - Quiz this Friday

Also, research the following: how does a battery work? It may be helpful to distinguish between "wet cell" and "dry cell".

C block

Quiz this Thursday. For tomorrow's class (Tuesday), prepare any questions you might have. Research the following: how does a battery work? It may be helpful to distinguish between "wet cell" and "dry cell".


Formal lab is beginning soon - the topic is circuits. Oh yeah!