MASSACHUSETTS INSTITUTE OF TECHNOLOGY Department of Physics 8.02 Spring 2013 Exam 1 Practice Problems Solutions Part I:Short Questions and Concept Questions Problem 1:Spark Plug Pictured at right is a typical spark plug(for scale,the thread diameter is about 10 mm). 10 mm About what voltage does your car ignition system need to generate to make a spark,if the breakdown field in a gas/air mixture is about 10 times higher than in air?For those of you unfamiliar with spark plugs,the spark is generated in the gap at the left of the top picture(top of the bottom picture).The white on the right is a ceramic which acts as an insulator between the high voltage center and the grounded outer(threaded)part. Answer. From the picture,if the threads have a diameter of 10 mm,then the gap is about 0.5 mm.The breakdown is 10 times higher than in air,so its 3 x 10'V/m.So to get breakdown you need a potential difference across the gap of △V=Ed=(3×10V-m-)5×104m)=1.5×104V 11 MASSACHUSETTS INSTITUTE OF TECHNOLOGY Department of Physics 8.02 Spring 2013 Exam 1 Practice Problems Solutions Part I: Short Questions and Concept Questions Problem 1: Spark Plug Pictured at right is a typical spark plug (for scale, the thread diameter is about 10 mm). About what voltage does your car ignition system need to generate to make a spark, if the breakdown field in a gas/air mixture is about 10 times higher than in air? For those of you unfamiliar with spark plugs, the spark is generated in the gap at the left of the top picture (top of the bottom picture). The white on the right is a ceramic which acts as an insulator between the high voltage center and the grounded outer (threaded) part. Answer. From the picture, if the threads have a diameter of 10 mm, then the gap is about 0.5 mm. The breakdown is 10 times higher than in air, so its 3 x 107 V/m. So to get breakdown you need a potential difference across the gap of !V ! Ed = (3"107 V#m$1 )(5"10$4 m) = 1.5"104 V