PEER REVIEW EXPERIMENT

FINAL PROPOSAL

FORCE AND TORQUE IN A CURRENT LOOP

CAPELLAN / FERNANDEZ / SACRO

FDE2

8 August 2008

Force and Torque in a Current Loop

Objectives

At the end of this experiment, the group aims to be able to:

Create a fully functional (simple and improvised) DC motor. Identify the factors that determine how a coiled wire is made to rotate in a dc

motor. Determine the direction of the magnetic field, direction of current flow, and/or

force on a conductor.

Introduction

A machine that converts electrical energy into mechanical energy is called a motor. Electric motors have greatly changed the way people live. Almost every mechanical movement that we see around us is caused by an AC (alternating current) or DC (direct current) electric motor.

The design of the galvanometer that we use in class is very similar to the design of an electric motor. If we modify the design of a galvanometer slightly, so that deflection makes a complete rather than a partial rotation, we have produced an electric motor.

There are “invisible forces” that causes the “rotating effect” within a motor. Essentially, one would need a current source and a coiled wire to have the current pass through a magnetic field source. The true inner workings of an electric motor are quite complex but with the aid of simple electricity and magnetism concepts, we will examine a small electromagnetic motor and figure out how and why it works.

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Theory

The principal difference between a galvanometer and a motor is that for the latter, the current is made to change direction every time the coil makes a half rotation. After being forced to turn one rotation, the coil continues in motion just in time for the current to reverse, whereupon instead of the coil reversing direction, it is forced to continue another half rotation in the same direction. This happens in cyclic fashion to produce continuous rotation, which has been harnessed to run clocks, operate gadgets, and lift heavy loads.

Figure 1. Diagram of a current loop in a wire.

Figure 1 shows that the sum of the forces (total or net force) in this current loop is zero because each of the forces cancel out in pairs. (i.e., forces +F and –F on opposite sides of wire add up to produce zero net force)

The magnitude of each single force F and their direction is given by:

(Equation 1. Force on a Current Loop)

where:F = Force q = chargev = speedB = magnetic field

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The magnetic dipole moment or simply, magnetic moment, causes the rotation on a current loop once exposed to a magnetic field. This gives us a second equation:

(Equation 2. Magnetic Dipole Moment)

where:mew = magnetic dipole momentI = currentA = area

Magnetic dipole moment is essential to why this current loop will rotate once exposed to a magnetic field. A magnetic dipole moment quantifies the contribution of the system's internal magnetism to the external dipolar magnetic field produced by the system. Now let us consider the torque and how this will induce a turning effect on the coil:

Figure 2. Schematic Diagram of a Simple DC Motor

Figure 2 illustrates the directions of the torque, current, and magnetic dipole. Since we have the presence of a dipole on a magnetic field, a net torque will be induced.

This torque is described by:

(Equation 3. Net Torque on a Current Loop)

where:T = torquemew = magnetic dipole momentB = magnetic field

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Materials1. DC Battery size D2. Electrical Tape

3. Magnetic Wire

4. A strong enough magnet

5. 2 large Safety pins

6. Sandpaper

7. Scissors, long nose pliers, and mechanical pliers

Procedure

1. Coil magnetic wire 15 times around the battery allowing excess wire on both ends to be used to tie the ends to secure the coil.

2. Once an estimate of the needed length is taken, cut the appropriate length and remove enamel insulation around the wire by scratching it off using the long nose.

3. Coil up the wire and secure the coilage by tying a knot at the last end. Make sure both ends are aligned. This will serve as a rod to support coil turning.

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4. Sand off the head and the bottom of the safety pin. Make sure that it is clean for these parts will be the contact terminals for the current to flow (from the battery to the coil).

5. Connect the safety pin head to the terminals and secure using electrical tape.

6. Insert the makeshift wire rod in the safety pin hole.

7. Introduce a magnetic field to the system.

Note: Depending on the strength of the current and magnet, the coil will either oscillate for rotate. If it oscillates, try light flicking the coil first and immediately introduce the magnetic field.

Improvised DC Motor Set-up

References

Hewitt, P.G., Conceptual Physics, 9th ed., Addison – Wesley / Prentice Hall USA (2004)

Freedman, R. and Young, H., University Physics, 11th ed., Addison – Wesley Publishing Co. USA (2004)

http://wow.osu.edu/experiments/magnetism/elecmagmotor.html http://www.instructables.com/id/Simple-Electric-Motor

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