capacitor revision

You recently completed the topic on capacitors in dc circuits, finishing off with a detailed study of the graphs obtained for current & voltage against time when a capacitor is charged or discharged through a series resistor.  There are some additional notes and practice questions at the end of this post but please watch the embedded video clips first.

This introduction to capacitors from the nice people at Make Magazine is a good starting point.

The S-cool revision site has some helpful notes and illustrations on capacitor behaviour; try page 1 (how capacitors work) and page 2 (charging and discharging).

There is information on charging and discharging capacitors on BBC Bitesize.

Here is a video that covers some of the areas covered in your printed notes. Ignore the maths at the end of each section of the film, you won’t need it.  Notice how the man in the film uses a lightbulb, rather than an ammeter, to show when the current is large or small.  Clever, eh?

You must be explain how a flashing neon bulb can be controlled using a capacitor & resistor arranged in series.  Here is a short video introduction to help with that.

There are people working to replace heavy battery packs with modern, high capacitance devices called supercapacitors.  These supercapacitors have superior energy storage compared to the normal electrolytic capacitors you will have used in class.  This video goes one step further and shows the fun you could have with an ultracapacitorDo not try this at home!

Of course, you can always make your own capacitor with paper and electrically conductive paint.

Finally, you looked at capacitors in ac circuits.  You need to know that a capacitor will allow an ac current to flow. The current in such a circuit will increase as the current increases.  Mr Mallon’s site has a revision activity about capacitors in ac circuits.

Now download the pdf below. It contains notes to help with your prelim revision and some extra capacitor problems.

Thanks to Fife Science for the original pdf from Martin Cunningham.

Using Excel for your LO3 and Investigation

We’ll spend the next two lessons in the library learning how to use Excel.  Download the instructions using the link below.  If you have your own LO3 data, feel free to work with those values instead of the numbers I have provided.

By the end of this activity you will be able to;

• manipulate raw data using formulae in cells
• plot a graph of your results
• add a line of best fit

AH Physics – using excel

Here is the file you will need for the graphing activity in today’s lesson.

capacitors – charge, energy and graphs

We’ve just completed the topic on capacitors in dc circuits, finishing off with a detailed study of the graphs obtained for current & voltage against time when a capacitor is charged or discharged through a series resistor.  There are some additional notes and practice questions at the end of this post but please watch the clips first.

This introduction to capacitors from the nice people at Make Magazine is a good starting point.

The S-cool revision site has some helpful notes and illustrations on capacitor behaviour; try page 1 (how capacitors work) and page 2 (charging and discharging).

Here is a video that covers some of the areas we discussed in class. Ignore the maths at the end of each section of the film, you won’t need it.  Notice how the man in the film uses a lightbulb, rather than an ammeter, to show when the current is large or small.  Clever, eh?

One use of capacitors you should know about is the flashing lamp.  We’ll cover this application next week.

I compared normal electrolytic capacitors to a 10F supercapacitor, and we observed its superior performance in terms of energy storage.  This video goes one step further and shows the fun you could have with an ultracapacitor. Do not try this at home!

Of course, you can always make your own capacitor with paper and electrically conductive paint.

Now download the pdf below. It contains notes to help with your prelim revision and some extra capacitor problems.

Thanks to Fife Science for the original pdf from Martin Cunningham.

how to measure internal resistance

Warning: I am expecting you to do more than just read this text.  Please plot the graph and find the properties of the cell.

You’ve just completed an experiment in class (it is listed as “Method 2″ on page 8 your printed notes) where you built a simple series circuit using a cell, a resistance box and an ammeter.  A voltmeter was connected across the resistance box and you recorded the voltage across (TPD) & current through the resistor as you changed the resistance from 0.5? to 1.5? in steps of 0.1?.

The video below shows the same type of experiment, but uses a potato and two different metals in place of normal cell.  Watch the video and note the values of I and V each time the resistance is changed – remember you can pause the video or go back if you miss any.

Now plot a graph with current along the x-axis and TPD along the y-axis.  If you don’t have any sheets of graph paper handy, there is a sheet available to download using the button at the end of this post.  Or you could try printing out a sheet from a graph paper site, use Excel or download the free LibreOffice.org Calc spreadsheet.

Draw a best-fit straight line for the points on your graph and find the gradient of the line.  When calculating gradient, remember to convert the current units from microamps (uA) to amps (A).

The gradient of your straight line will be a negative number. The gradient is equal to -r, where is the internal resistance of the potato cell used in the video.

You can obtain other important information from this graph;

• Extend your best fit line so that it touches the y-axis.  The value of the TPD where the line touches the y-axis is equal to the EMF of the cell. (Explanation: on the y-axis, I is zero so TPD = EMF)
• Now extend the best-fit line so that it touches the x-axis, the current at that point is the short-circuit current – this is the maximum current that the potato cell can provide when the variable resistor is removed from the circuit altogether and replaced with just a wire.

AH – help using Excel

This tutorial will help you with adding a line of best fit to your LO3 graph.  If you use the linest function, you can extract useful information on gradient and intercept of the line.  There are also screenshots to show how error bars can be added to individual points.

Excel Tutorial: Using LINEST function, Plotting a graph, Adding Error Bars