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Resistance

Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.

  • Intro 0:00
  • Question 1 0:12
  • Question 2 0:53
  • Question 3 1:44
  • Question 4 2:31
  • Question 5 3:21
  • Question 6 4:06

Transcription: Resistance

Hi everyone and welcome back to Educator.com. 0000

In this mini-lesson, we are going to go through page 1 of the APlusPhysics worksheet on resistance and you can find a link to that worksheet down below the video. 0002

So let us dive right in. 0011

Number 1 -- At 20 degrees C, 4 conducting wires made of different materials have the same length and the same diameter. 0012

Which wire has the least resistance? 0019

Remember resistance is ρL/A. If they have the same length and the same diameter, their length and area are going to be the same. 0022

We need the one that is going to have the least resistivity, so we are looking for the material with the lowest resistivity. 0031

To determine that, I need to go and do a little research. 0037

I need to go look up on a table of resistivity's of materials to find out that of these four materials, the one that has the lowest resistivity is going to be Number 2 - Gold. 0040

Number 2 -- The graph below represents the relationship between the current in a metal conductor and the potential difference across the conductor. 0053

Find the resistance of the conductor. 0060

Well, in this case, the slope is going to give us current over potential difference, but the inverse of that is going to give us the resistance. 0063

So, 1/the slope is going to be resistance or we could just take a couple of points since this is linear and determine it that way. 0075

So let us do it the easy way. Resistance = V/I by Ohm's Law. 0085

We have a potential difference of 4 volts where we have a current of 2 A. 0090

That is just going to give us a resistance of 2 ohms, so the answer to number to must be answer Number 2. 0095

Number 3 -- The diagram below represents a lamp, a 10 volt battery, and length of nichrome wire connected in series. 0105

As the temperature of the nichrome is decreased, what happens to the brightness of the lamp? 0112

Remember brightness is going to be proportional to the power through the lamp, which is current times voltage. 0117

As temperature decreases, the resistance of the nichrome wire is going to go down. 0123

As that resistance goes down -- it is a series circuit -- our total current must be going up.0130

As well we are going to have more of the voltage dropped across the lamp, so the voltage across the lamp goes up, so current and voltage going up means the power is going to go up, therefore the brightness of the lamp should increase. 0135

Number 4 -- A student conducted an experiment to determine the resistance of a light bulb. 0151

As she applied various potential differences to the bulb, she recorded the voltages and corresponding currents and constructed the graph below. 0157

The student concluded that the resistance of the light bulb was not constant. 0164

What evidence from the graph supports the student's conclusion? That should be pretty obvious. 0168

Remember we said if we have a graph of current and potential difference with potential difference on the x-axis, 1/slope is the resistance. 0173

The slope is changing, therefore the resistance must be changing, so we have a changing slope, which implies that we have a changing resistance. 0184

Number 5 -- Looking at the same graph -- according to the graph, as the potential difference increased, what happened to the resistance of the light bulb?0202

As the potential difference increased -- as we get over here from over here -- well, in this case, remember R = V/I or the slope in this case is going to be 1/the resistance. 0210

Well, here you have a big slope and here you have a little slope.0229

If you have a little slope over here, you must have a bigger resistance, so the correct answer here must be Number 2 - It increased. 0232

Finally, on to Number 6 -- While performing the experiment, the student noticed that the light bulb began to glow and became brighter as she increased the voltage. 0244

Of the factors affecting resistance, which factor caused the greatest change in the resistance of the bulb during her experiment? 0253

Well, in this case, remember that R = ρL/A and what we would expect is that as the voltage is increasing, probably having the temperature increase because we have more power going through here. 0259

As temperature goes up, resistivity goes up, and as resistivity goes up, resistance goes up, so what is the biggest factor causing that? 0273

I would say that it is an increase in temperature causing resistivity to increase. 0283

All right. That concludes page 1 of the worksheet on resistance. 0300

If that went great -- Terrific -- go ahead and move on and if you struggled with this, it is probably time to go back and review the lesson on resistance and resistivity. 0304

Thanks so much for your time everyone. Make it a great day!0313