What is the basic principle of electric pen ? Explain how it can be used for writing.
A. Electric charge on ions is used for writing with an electric pen.
(i) Take a filter paper soaked in potassium iodide solution to which a pinch of starch is added.
(ii) Place the filter paper on a metal sheet.
(iii) Connect the negative terminal of battery to the metal sheet.
(iv) Write on the paper with the end of the wire connected to the positive terminal.
(v) The writing appears on the paper. This happens because the when current is passed, the K+ ions are attracted to the metal sheet. The iodide ions (I) react with starch to turn blue black.