the dc motor

Physics 1051 Laboratory #6 The DC Motor

The DC Motor


Contents

Part I: Objective Part II: Introduction Magnetic Force Right Hand Rule Force on a Loop Magnetic Dipole Moment Torque Part II: Predictions Force on Current Carrying Wire A Force on Current Carrying Loop B Force on Current Carrying Wire C Force on Current Carrying Loop D

Part IV: Apparatus and Setup Apparatus Setup Details The Wire The Coil The Stick The Support Arms The Circuit Part V: The Experiment Get Your Motor Running Which End is Up? Current and Rotation Relative Vector Directions Part VI: Summary


Part I: Objective

In this experiment you will:

Determine the direction of the magnetic moment vector for a current carrying loop. Determine the direction of torque on a current carrying loop in a magnetic field. Build a DC motor and determine the orientation of an unknown magnet.


Part II: Introduction Magnetic Force

A current carrying wire in a magnetic field will experience a force perpendicular to both the field and the current. For example, a wire carrying current I out of the page placed in a magnetic field B to the right will experience a force F directed up as shown.

The force is given by

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! B

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! F B

!FB = I

!L×!B.


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! I

Part II: Introduction Right Hand Rule

You may find the direction of the force by applying the right hand rule:

!  Point your fingers in the direction of the

current !  Bend your fingers (90º) towards the

magnetic field (you may have to twist and turn your hand!).

!  Your thumb indicates the direction of force. If the current and magnetic field are parallel, the force is zero!

This method applies to all cross products.

©2006 Brooks/Cole - Thomson. Fig. 22-4, p. 731, from Serway and Jewett, Principles of Physics. This material may not be copied or distributed to any third party.


Part II: Introduction Force on a Loop

The right hand rule may be applied to each segment of a loop to determine the force on each segment.

The image at right is a simple DC motor.

The two magnetic forces FB and –FB produce a torque on the loop, tending to rotate it about its central axis. The other two sides of the loop do not experience a force since the current and magnetic field are parallel.

The force vectors may be added to determine the net force.

N S

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! B

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! F B

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! F B

_


Part II: Introduction Magnetic Dipole Moment

A current carrying coil has a magnetic dipole moment . This vector points in the direction normal to the plane of the coil and has magnitude µ= NIA where N is number of turns, I is current, and A is the coil area.

The figure at right shows a right-hand rule for finding the direction of :

Point or curl the fingers of your right hand in the direction of current at any point on the loop. Your extended thumb then points in the direction of the magnetic dipole moment.

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µ

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µ

©2006 Brooks/Cole - Thomson. Fig. 22-21, p. 742, from Serway and Jewett, Principles of Physics. This material may not be copied or distributed to any third party.


Part II: Introduction Torque

A current carrying coil experiences a torque that is related to both the magnitude and direction of the and vectors. This vector relation is written as

Torque direction may be found in one of two ways:

1.  If you know the directions of µ and B, you may use the right hand rule. Point your fingers in the direction of µ. Bend your fingers towards B. Your thumb indicates the direction of torque. If µ and B are parallel, the torque is zero!

2.  If you know the direction of rotation, curl your fingers in the directions of rotation. Your thumb points towards torque.

This torque is the basis for the operation of a DC motor.

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τ = µ × B€

µ

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B


Lab Report

Lab Report 1: Write the objective of your experiment. Lab Report 2: Write the relevant theory of this experiment.

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Part III: Predictions A: Force on Current Carrying Wire

All throughout the boxed region below, there is a uniform magnetic field pointing into the page (as indicated by the cross). A wire segment carrying a current in the direction shown is placed inside the region.

Lab Report 3: Indicate the direction of the force on the wire segment, using either arrows or the dot or cross symbols. If the force is zero, write “zero”.

B I

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Part III: Predictions B: Force on Current Carrying Loop

Lab Report 4: On each of the four sides of the loop, indicate the direction of the magnetic force (if there is one) or write “zero.”

Lab Report 5: Is there a net force acting on the loop as a whole? If so, state its direction. If not, explain.

Lab Report 6: Indicate the direction of the magnetic moment vector of the loop.

Lab Report 7: Indicate the direction of the torque vector of the loop or write “zero” if the net torque is zero.

Lab Report 8: Assuming the loop is free to move, will it rotate? Explain.

B I

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A square wire loop carrying a steady clockwise current is placed in the

region. (Current in each of the four sides is equal.)


Part III: Predictions C: Force on Current Carrying Wire

In this region, a uniform magnetic field is present that points toward the bottom of the page. A wire segment carrying a current in the direction shown is placed in the region.

Lab Report 9: Indicate the direction of the force on the wire segment, using either arrows or the dot or cross symbols. If the force is zero, write “zero”.

B I

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Part III: Predictions D: Force on Current Carrying Loop

Lab Report 10: On each of the four sides of the loop, indicate the direction of the magnetic force (if there is one) or write “zero.”

Lab Report 11: Is there a net force acting on the loop as a whole? If so, state its direction. If not, explain.

Lab Report 12: Indicate the direction of the magnetic moment vector of the loop.

Lab Report 13: Indicate the direction of the torque vector of the loop or write “zero” if the net torque is zero.

Lab Report 14: Assuming the loop is free to move, will it rotate? Explain.

B I

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A square wire loop carrying a clockwise current is placed in the region.


Part IV: Apparatus and Setup Apparatus

You have been provided with: Wooden base DC power supply Wires and connectors 2 Magnets Wooden stick 2-3 m of thin copper wire

(insulated) Paper clips Some masking tape Sandpaper 200 g mass


Part IV: Apparatus and Setup Setup Details

The DC motor consists of loops of wire. Current must be running through the wire. The loops must be set in a magnetic field.

Remember: For motion to occur, there must be a net torque on the motor. For there to be a net torque, the magnetic moment vector and the magnetic field must be perpendicular.


Part IV: Apparatus and Setup The Wire

You have been given insulated copper wire, leads with alligator clips, and paper clips. The wire is insulated with a thin plastic coating. Current will not flow from the paper clips to the wire if it has this coating. Current will flow from the paper clips to the wire if this coating has been removed at the point of contact. You have been given a piece of sandpaper. Once the plastic insulation is removed from the appropriate part of the copper wire it is bright and shiny in colour.


Part IV: Apparatus and Setup The Coil

You may use the 200g mass as a base to keep your coil round.

To obtain a sufficient magnetic moment vector, it is recommended that you have a minimum of 15 turns in your coil.


Part IV: Apparatus and Setup The Stick

You have been given a small wooden stick which may be used to hold the loops. Be sure the loop is symmetric on the stick.

When securing the loose ends of the wire to the stick, think carefully about the current flow: Where must the wires be secured to ensure current will flow?


Part IV: Apparatus and Setup The Support Arms

The motor may be supported using the paper clips and the wooden base.


Part IV: Apparatus and Setup The Circuit

Use the power supply, leads, and alligator clips to connect a circuit with your DC motor.

Make sure the current and voltage knobs are turned all the way to the left. Switch on the power supply. First slightly increase the voltage knob, then adjust the current until you observe a voltage and current output. Adjust both the voltage and current knobs until you observe a current (maximum of 2.0 A). If there is no current, carefully check that you have a closed circuit.


Lab Report

Lab Report 15: List your apparatus and sketch your setup.

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Part V: Experiment Get Your Motor Running

You may need to give the loop a slight flick with your finger to get it spinning.

You should adjust your apparatus until the spinning is smooth and even.

If there are sparks, you may decrease the voltage a little. These sparks will not hurt you.

Note the rotational direction of your motor! You will use this information to determine the orientation of your magnet.

Don’t run your motor for more than a minute straight. The coil can get very HOT!

Before proceeding, have an instructor see your running motor and sign your lab report. This Checkpoint is worth 15% of your lab!


Part V: Experiment Which End is Up?

Lab Report 16: Based on the direction of current and rotation of your motor, determine which pole of your magnet is facing up. Explain your answer.

Before proceeding, have an instructor check your answer with a

labelled bar magnet to see if you are right.

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Part V: Experiment Current and Rotation

Lab Report 17: What would happen to your motor if you switch the direction of the current by switching the alligator clips? Why?

Test your prediction. Lab Report 18: Did you observe your prediction to be true? If not,

explain why.

Lab Report 19: What would happen to your motor if you increase the current? Why?

Test your prediction. Lab Report 20: Did you observe your prediction to be true? If not,

explain why.

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Part V: Experiment Relative Vector Directions

Think about the orientation of your coil relative to the magnet. In particular, consider the time when the current is flowing through the coil. Lab Report 21: What were the relative directions of the magnetic moment vector and the magnetic field? Provide a sketch of your apparatus with these directions clearly indicted. Why did they have to be arranged this way?

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Part VI: Summary

Lab Report 22: Outline briefly the steps of your experiment. Lab Report 23: List your experimental results and comment on how they

agreed with the expected results. Lab Report 24: Are there any experimental uncertainties associated with

this experiment? Explain.

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Wrap it up!

Check that you have completed your Lab Report.

Make sure an instructor has checked your motor and signed your report.

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the dc motor

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