30 Jun, 2013
PRESENTATION: Two coils are placed close to each other in such a way that the changing magnetic flux of one of them goes through the other inducing a current in it. It can be seen that the current induced in the second coil exists only while the current of the first changes and consequently the initial magnetic field is kept.
- An Inexpensive Source of High Voltage, Carlos Saraiva, Phys. Teach. 50, 224 (2012)
- Using a roll of household wire for magnetic induction demonstrations, Blane Baker, Phys. Teach. 49, 392 (2011)
INTRODUCTION: Two coils (or solenoids) are placed in series so that they can share the same axis of symmetry. If a current is made to go through both coils two consequences can be observed: either both coils attract each other, or both repel each other. These effects are the consequence of the generation by the coils of their own magnetic field, which can be calculated on the basis of the application of Ampère’s Law, which will lead to the forming of two magnetic poles, one positive and another negative, in each of them.
OBJECTIVE: to test the effects of attraction and repulsion between the3 coils experimentally and offer an explanation of these phenomena.
MATERIALS: 2 coils (identical if possible), two power sources, four cables, 2 bases, 2 nuts, a stick, wire.
SETUP: First the bases are placed so that they can hold the stick horizontally with the help of two nuts. Then 4 wires (or more) will be tied to that stick and their lower ends will be tied to the coils so that these can be kept suspended horizontally and at the same height (or in series and sharing the same symmetry axis). Later on the power supplies will be connected to the electric main and the voltage will be regulated (in this case 30 V) so that the coils can’t become overheated. Then the coils will be connected to the power supply by means of the cables. According to the direction of the currents either an attraction or a repulsion will be observed between both coils.
EXPLANATION: When a coil of N spires and L length is connected to the power supply, a current intensity I will circulate through the coil and will generate a magnetic field whose direction depends of the direction of the current. It can be calculated on the basis of Ampère’s law and will render the following expression: B= μ0·I·N/L . Due to the existence of this magnetic field at the ends of the coil two poles are formed, one negative and the other positive, in this way the coil behaves like a magnet. Therefore, when two coils are connected to the current each of them will generate its own magnetic field and inherently its positive and negative poles. If both approach, we will be able to see two effects depending on which poles are closer.
- If the closer poles have opposite signs (the directions of the currents are the same) both coils will suffer attraction forces.
- If the closer poles are of the same sign (the directions of the currents are opposite) both coils will suffer repulsion forces.
CONCEPTS: magnetic field and force, Apere’s law, current intensity, magnet, pole.