Homework Help: A few questions

Thread moved to Homework Help, Intro Physics.

Welcome to the PF, Zeus12345. Homework and coursework questions must be posted in the appropriate Homework Help forum, not in the general forums. And when you post homework or coursework questions, you need to show some of your own work and thoughts, in order for us to help you the best. Also, when you start a new thread in the Homework Help forums, you will be provided a Homework Help Template, which you should use to help us provide you with assistance.

Now on your present questions, I can provide a little feedback to see if it helps.

-1- Remember that electrons are what actually flow in the circuit, but we refer to positive "current" flowing out the + terminal of the battery and around the external circuit. But since negatively charged electrons are physically what is moving, they actually flow out the - terminal of the voltage source and around the external circuit, and return to the + terminal of the voltage source. But normally we just refer to + current, so if there were + charges moving, then yes, they would have zero potential energy at the - terminal of the voltage source, their potential energy gets pumped up as they move through the voltage source to the + termainal, and then all that potential energy is given up as heat (via collisions with the atoms as the charge moves through the wires, resistors, etc. of the external circuit) as the charge makes its way back to the - terminal, where the whole process happens again.

-2- Energy is required to create the current. Energy is lost to heat when a DC current flows in a wire. Energy may also be lost to EM radiation when an AC current flows in a wire, if the AC frequency is high enough for the wire to start acting like an antenna.

-3- What can you tell us about how transformers work? What are the equations for voltage and current and impedance transfer ratios? What are self inductance and mutual inductance?

 

It's quite apparent that you're not lazy, and you are asking good questions in your learning. With coupled inductors (like a transformer), there is self-inductance and mutual-inductance. The mutual inductance (or coupling coefficient) represents how tightly the two windings are coupled magnetically. If all of the flux from one coil goes through the 2nd coil, then the coupling coefficient is 1.0, which would be an ideal transformer. Real transformers with cores made of an appropriate magnetic material (for the frequencies involved) come pretty close to having a coupling coefficient of 1.0.

When the coupling coefficient is high, and when there is a load on the secondary, then you are correct that the output current induces a back-flux in the core, which gives you a back emf in the primary. This effectively lowers the input impedance of the primary, and requires more primary current to support the input voltage. Since you know how the turns ratio affects the voltage and current ratios, what can you say about the impedance ratio? That is, if you have an output secondary load of R, and primary:secondary turns ratio of n, what is the effective load seen when looking into the primary?

So now if we put the two concepts of reflected load impedance and coupling coefficient together, I think we have the answer to your question. If the coupling coefficient is high (close to 1), then the primary and secondary are tightly coupled, and the primary current generates a flux that generates a secondary current through the load impedance, and this current generates a back-flux that requires more primary current to support the primary and secondary voltages. The closer the coupling coefficient is to 1.0, then the more this system acts like an ideal transformer. But as the coupling coefficient decreases (like the adjacent coils you mentioned without a common core to confine the flux to the two coils), then there is less flux linkage between the two coils, and there is less current induced in the secondary coil by the primary flux. So less current induced in the secondary coil means less back flux to generate the back emf, and the transformed load impedance is higher than it would be with tightly coupled coils.

Does that make sense? Good questions!