Lab9_phy2_report.docx

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Lab 9. Faraday's Law & Lenz's Law

In 1831, Michael Faraday made a discovery with enormous technological consequences. He discovered that for an electric current in one circuit to induce a current in a second circuit, the current in the first circuit must be changing with time. The induced current is caused by the changing magnetic flux through the area bounded by the second circuit, which in turn is generated by the changing current in the first circuit. This changing flux induces an emf in the second circuit and, consequently, an induced current. The technological consequence of all this is our whole system of electrical power generation. Faraday’s Law summarizes this quantitatively, stating that the magnitude of the induced emf in a conducting loop is equal to the rate at which the magnetic flux through the area bounded by the loop is changing with time, i.e.

The negative sign is related to a sign convention for the direction of the induced current. Alternatively, this direction is given by Lenz’s Law, which states that an induced emf or current tends to oppose, or cancel out, the changing flux that caused it. So if the flux is increasing, the magnetic field of the induced current will tend to decrease the flux. In this lab, you will be performing a series of mainly qualitative experiments that will strengthen your understanding of Faraday’s and Lenz’s Laws.

Objectives

· Change the flux through a circuit by moving a magnet.

· Change the flux through a circuit by changing the current in another circuit.

· Perform a series of steps to test the validity of Lenz's Law.

Preliminary Question (1,2)

Procedure (2,3,6,8,14)

Analysis (1)

50

Lab 9. Faraday's Law & Lenz's Law In 1831, Michael Faraday made a discovery with enormous technological consequences. He discovered that for an electric current in one circuit to induce a current in a second circuit, the current in the first circuit must be changing with time. The induced current is caused by the changing magnetic flux through the area bounded by the second circuit, which in turn is generated by the changing current in the first circuit. This changing flux induces an emf in the second circuit and, consequently, an induced current. The technological consequence of all this is our whole system of electrical power generation. Fa ada La summarizes this quantitatively, stating that the magnitude of the induced emf in a conducting loop is equal to the rate at which the magnetic flux through the area bounded by the loop is changing with time, i.e.

t = or, more correctly, .

dt d

=

The negative sign is related to a sign convention for the direction of the induced current. Alternatively, hi di ec i i gi e b Le La , hich a e ha a induced emf or current tends to oppose, or cancel out, the changing flux that caused it. So if the flux is increasing, the magnetic field of the induced current will tend to decrease the flux. In this lab, you will be performing a series of mainly qualitative experiments that will strengthen your understanding of Fa ada a d Le La . OBJECTIVES

Change the flux through a circuit by moving a magnet. Change the flux through a circuit by changing the current in another circuit. Perform a series of steps to test the validity of Lenz's Law.

MATERIALS

nested pair of solenoids center-reading galvanometer power supply connecting wires bar magnet current probe computer

compass Labquest Mini

PRELIMINARY QUESTIONS 1. What is the difference between magnetic field and magnetic flux?

2. Consider the magnetic field lines surrounding a disc magnet as pictured at right. The magnet moves from right to left through the wire loop. Describe how the magnetic flux through the surface defined by the loop changes (if at all) as the magnet 1) approaches the loop, 2) passes through the loop, and 3) recedes from the loop. Sketch a graph of magnetic flux through the surface vs. time as the magnet undergoes this motion.