observation of force and torque on a current loop using a simplified electric motor

8/14/2019 Observation of Force and Torque on a Current Loop using a Simplified Electric Motor

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Observation of Force and Torque in a Current Loop

Using a Simplified Electric Motor

Plaridel, Neopet, Chuvaneshca

Physics 72.1 FDE2

National Institute of Physics, University of the Philippines

Diliman, Quezon City 1101, Philippines

I. Abstract

The group replicated a DC motor to be able

to observe the force and torque present on a

current loop. The motor set-up used

permanent bar magnet on a DC Battery (size

D) coiled fifteen times with a magnetic wire.

This paper discusses our observations on how

electricity and magnetism concepts apply on

an actual motor through the aid of a simplified

motor design.

At the end of the activity, the group was able

to induce a net torque effect in the coil. The

coil either oscillates or rotates depending on

the strength of the current and/or the magnet.

With the aid of known theories on the presence

of force and torque on a current loop, the

group was able to observe how a motor works.

II. IntroductionA machine that converts electrical energy

into mechanical energy is called a motor. The

design of the galvanometer that was used on

class is very similar to the design of an electric

motor. If the design of a galvanometer was

modified slightly, so that deflection makes a

complete rather than a partial rotation, an

electric motor is produced. 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.

The group aims to be able to create an

improvised and fully functional DC motor.

The principal goal is to be able to identify the

factors that determine how the coiled wire

from our improvised set-up rotates in a DC

motor. The direction of the magnetic field and

the current flow on a conductor will also be

observed and analyzed with the aid of theknown governing concepts of electricity and

magnetism.

III. Methodology

To be able to construct a simplified model of

an electric motor, the following materials are

needed: a DC Battery (size D), electrical tape,

copper/magnetic wire, a permanent bar magnet

(and a small circular magnet if preferred), two

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large safety pins, sandpaper (for smoothening

out the edges of the safety pins), scissors, long

nose pliers, and mechanical pliers.

Figure 1. Coiling the wire on the battery

The wire was coiled fifteen times around the

battery and secured to prevent relapse.

(Winding the coil more than fifteen times on

the battery will affect the magnitude of the

magnetic field strength.) Excess wire was

placed and stripped at the start and end points

of the coil to serve as rod or shaft to suspend

the coil on the support pillar provided by the

safety pins. (An alternative set-up replaces the

wire with a small circular magnet.) Once the

current was established, the circuit was

exposed to the magnetic field.

Figure 2

An Alternative Improvised DC Motor Set-up

with a Magnet placed on the battery

IV. Results and Discussion

As noted, 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.

(1)

Equation 1 defines the magnitude of a force

present on a circular loop, where F is the

magnitude offorce present on a current loop, q

is the amount ofcharge, v is thespeedandB is

the strength ofmagnetic field.

For our DC motor set-up, the sum of the

forces present is zero. This is because each of

the forces present on the opposite sides of the

wire cancels out thus leaving no net force on

the system.

In theory, a magnetic dipole moment, or

simply a magnetic moment, causes the rotation

on a current loop once exposed to a magnetic

field.

(2)

Equation 2 expresses the magnetic moment

as the product of the current (I) and the area

(A).

The concept of a magnetic dipole moment is

essential in understanding why a current loop

rotates 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.

(3)

Equation 3 describes the torque induced on a

circular loop as the magnetic dipole moment

(greek letter mu) cross the magnetic field

strength (B).

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For this experiment, rotating the wire fifteen

times over the batterys central body produced

enough dipole moment strength to induce a net

torque once the magnetic field was introduced.

The initial results, however, produced a coil

that only oscillates back and forth. This is

because for an electric motor to undergo

continuous rotation, a commutator is needed.

The magnetic moment was changing

directions and thus the fixed magnetic field

causes the coil to turn back and forth. This is

the nature of a moving magnetic moment

when exposed to a fixed magnetic field.

A member of the group touched the set-up

and gave it an initial movement/rotation

(flick). When this initial flick (that would

give momentum to the coil) is exposed to the

magnetic field, the relatively quick changing

of the magnetic moment allows the magnetic

field to induce a constant rotation since the

coil already have an induced momentum.

V. Conclusion

The group noticed that the rate of rotation

increases as the distance between the magnet

and the coil decreases. This experiment had

also confirmed that by inducing a flick or

initial movement on the DC set-up, a

continuous rotation will be generated by the

motor. This is because the momentum of the

coil neglects the inappropriate direction of the

magnetic moment that it would have to pass

through.

VI. References

[1] Dobkowska, M., Gupta, A., Majcher, A.,

Wojewoda, K., Simple Models of An Electric

Motor. August 6, 2008

[2] Hewitt, P.G., Conceptual Physics, 9th ed.,

Chapter 5, Addison Wesley / Prentice Hall

USA (2004)

[3] Young, H. & Freedman, R. UniversityPhysics,11th ed., Chapter 27, Addison Wesley,

San Francisco California, (2004).

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observation of force and torque on a current loop using a simplified electric motor

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