30 Jun, 2013
PRESENTATION: The phenomena of mutual induction and self induction. In coils through which variable currents circulate in due time are the consequence of Farady-Lenz’s law with important technological applications.
- More on Faraday’s and Lenz’s laws ‐ Qualitative demonstrations, Roberto Hessel, Phys. Teach. 49, 184 (2011)
- Hands-on Experiments on Faraday’s Law, Eduardo E. Rodríguez, Phys. Teach. 43, 228 (2005)
INTRODUCTION: A transformer, in its simplest form, is formed by two stationary coils, coupled by a reciprocal magnetic flow. The coils are said to be mutually coupled because the flow that goes through one coil also goes through the other (in an ideal transformer), therefore the tension that the multitester must receive is:
V2=f (N2/N1) V1
Being N the number of spirals of the coils, which in our case will be even and, therefore, the tensions will be similar for a value f which represents the fraction of the field line (magnetic flow) kept inside the nucleus of the transformer.
OBJECTIVE: To test if a transformer connected to a power supply is able to self-induce a magnetic field and, consequently, a potential difference.
MATERIALS: 2 coils, multimeter, AC generator, DC generator, iron bars, cables.
SETUP: A structure is built with iron bars. Two of them are introduced in the coils and the others hold them to close the structure and form the nucleus of the transformer. The alternating current generator and a coil (primary coil) and the other coil (secondary coil) are connected in series with the multitester.
For the second experiment, we keep the structure and change the AC generator.
EXPLANATION: If an alternating current is applied on a coil, this will produce a variable magnetic field, and therefore a magnetic flow. The iron bars which hold the coils keep the lines of the magnetic field, therefore, the same flow will go through the second coil. When we work with alternating current, the flow is induced and provokes the potential difference. When we work with continuous current we change the flow by moving the coil and providing for variation in the tension.
CONCEPTS: voltage, coils, ferromagnetism, transformer, magnetic field, magnetic flow, temporal magnet (iron bar).
- G. Enriquez Harper, El libro práctico de los generadores, transformadores y motores eléctricos, Editorial Limusa, 2004.
- R. Serway, Física, Mac Graw Hill, 2010.
- P. Tipler, Física para la Ciencia y la tecnología, Reverté, 2012.
- STUDENTS 2012-2013: Carlos Araiz, Andrea Mouriño, Rodrigo Novoa, Víctor Pérez.
LINK pdf STUDENTS (in Spanish):