{"id":353,"date":"2010-11-16T09:34:05","date_gmt":"2010-11-15T20:34:05","guid":{"rendered":"https:\/\/blog.waikato.ac.nz\/physicsstop\/2010\/11\/16\/negative_resistance\/"},"modified":"2010-11-16T09:34:05","modified_gmt":"2010-11-15T20:34:05","slug":"negative-resistance","status":"publish","type":"post","link":"https:\/\/blog.waikato.ac.nz\/physicsstop\/2010\/11\/16\/negative-resistance\/","title":{"rendered":"Negative Resistance"},"content":{"rendered":"<p>I was having a conversation last week with a student about negative resistances (in an electronics context). These are just as they sound &#8211; to send a current from terminal A to terminal B you have to apply a higher potential to terminal B than terminal A.&nbsp; Sounds backwards?&nbsp; Yes &#8211; it is. That&#8217;s why the resistance is negative.<\/p>\n<p>It&#8217;s not possible to get a negative resistance with purely passive components. We can see that from thermodynamics. A normal (positive) resistor puts out heat to the surroundings &#8211; voltage times current gives us the power dissipated.&nbsp; A negative resistor would need to suck in heat and turn it to electrical energy. For example, if you connected a negative resistor to a battery, the current would flow backwards through the battery &#8211; i.e. charge it up. The energy would come from the surroundings, being sucked in by the negative resistor.&nbsp; It would be a nice solution to global warming, but unfortunately the second law of thermodynamics says you can&#8217;t do it.<\/p>\n<p>So how can you get a negative resistance?&nbsp; You can use active components &#8211; e.g. operational amplifiers, that have a power source. There are a few circuits that effectively produce a negative resistance, <a target=\"_blank\" href=\"http:\/\/en.wikipedia.org\/wiki\/File:General_negative_impedance_circuit.svg\">for example this one<\/a>.&nbsp; To put a positive current into the circuit, you have to apply a negative potential difference.&nbsp;&nbsp; If I put a battery across the terminals, it would recharge. Where does the energy come from, if it can&#8217;t come from the surroundings?&nbsp; The operational amplifier needs its own power supply&nbsp; &#8211; that is, it is not a passive component. The second law of thermodynamics can rest easy.<\/p>\n<p>There are lots of neat applications &#8211; perhaps the most obvious is simply&nbsp;&#8216;cancelling&#8217; unwanted positive resistances.&nbsp; Related to negative resistance, but not the same thing, is negative differential resistance. This is something that can be made from a purely passive component, for example in a <a target=\"_blank\" href=\"http:\/\/en.wikipedia.org\/wiki\/Tunnel_diode\">tunnel diode<\/a>. Here, if you plot a graph of the current through the diode (y-axis) against the voltage across the diode (x-axis) there is&nbsp;a region where the <b>gradient <\/b>is negative. That is, increasing the voltage causes less current to flow.&nbsp; However, this doesn&#8217;t break the second law of thermodynamics since the current is still positive for a positive voltage. In calculus terms, dV\/dI is negative (in a small region), though V\/I remains positive.&nbsp; Tunnel diodes can be used in oscillators.<\/p>\n<p>There are lots of cool things you can do with Operational Amplifiers, negative resistances are just one. Have a go at making a negative resistance, and see what you can do with it.&nbsp;&nbsp;&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>I was having a conversation last week with a student about negative resistances (in an electronics context). These are just as they sound &#8211; to send a current from terminal A to terminal B you have to apply a higher potential to terminal B than terminal A.&nbsp; Sounds backwards?&nbsp; Yes &#8211; it is. That&#8217;s why [&hellip;]<\/p>\n","protected":false},"author":20,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[7,9,14],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v22.5 - https:\/\/yoast.com\/wordpress\/plugins\/seo\/ -->\n<title>Negative Resistance : Physics Stop<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/blog.waikato.ac.nz\/physicsstop\/2010\/11\/16\/negative-resistance\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Negative Resistance : Physics Stop\" \/>\n<meta property=\"og:description\" content=\"I was having a conversation last week with a student about negative resistances (in an electronics context). These are just as they sound &#8211; to send a current from terminal A to terminal B you have to apply a higher potential to terminal B than terminal A.&nbsp; Sounds backwards?&nbsp; Yes &#8211; it is. 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