MEDE3500 Lab Guide Lab session: Electromagnetic Field

Transcription

1 MEDE3500 Lab Guide Lab session: Electromagnetic Field Department of Electrical and Electronic Engineering The University of Hong Kong Location: CYC-102/CB-102 Course Lecturer: Dr. Philip W. T. Pong Demonstrators: Chao Zheng Xu Li Wenchao Miao Lab report submission: the lab report must be submitted through the Moodle system within two weeks from the laboratory session. Late submission will not be marked.

2 Part I Demonstration: Magnetic Levitation The demonstrators will demonstrate the magnetic levitation to you and explain to you the physics behind it. You will also have a chance to try it yourself. How long did you manage to sustain the levitation? Objective: Part II Experiment: The Ampère's Circuital Law The objective of this experiment is to gain both qualitilive and quantitative understanding of the Ampere s Circuital Law. Preparation: Before coming to the Lab, deduce the magnetic field generated by current line with the Ampère's Circuital Law ( B dl 0I ). Apparatus and Required Parts: Wire Compass Smartphone as a Gaussmeter * DC power supply Series/parallel light bulb board * Please refer to the appendix for the instructions 1

3 Introduction The compass needle stays in alignment with the earth magnetic field when there is no other external magnetic field. The emanated magnetic field from the current applies a torque on the compass needle and makes it rotate. On the other hand, the earth magnetic field also applies a torque on the compass needle. If the emanated magnetic field is comparable with the earth magnetic field, the compass needle will finally be balanced at some deflection angle. The final deflection angle of the compass needle from its original position directly depends on the magnitude and direction of the emanated magnetic field. In this experiment the deflection angle is used to qualitively investigate the emanated magnetic field. The magnitude of magnetic field can be quantitively measured with a Gaussmeter. In this experiment, the magnetic field generated by the circuit line can be determined by calculating the difference between the measured magnetic field value with and without the current. The Ampère's Circuital Law determines the relation between electric current and the magnetic field emanated from the current. By varying parameters of the Ampère's Circuital Law, the emanated magnetic field can be changed. In this experiment, we will examine the influence of each parameter on the magnetic field and try to explain the changes of the magnetic field by the Ampère's Circuital Law. Procedure: 1. Qualitative characterization Connect a wire to the DC power supply with 5 bulbs in a parallel configuration. After fixing the compass with Blu-Tack or double-sided adhesive tape on the table, lay the wire right on top of the compass needle. (1) Effect of the magnitude of electric current Caution: The current and voltage of the DC power supply should NOT exceed 1.5A and 9V respectively in this section. As shown in the picture, lay the wire on top of the compass in alignment with the compass needle. Apply different currents to the wire and observe the change of the compass needle. Try to deflect the compass needle by 10, 15, 20, 25, and 30, and record the corresponding applied current for each deflection angle. Plot the current vs. deflection angle curve. Base on this experimental result, explain the relation between the emanated magnetic field and the electric current with the related physical law. 2

4 (2) Effect of the separation Lay the wire on top of the compass as in Experiment (1). Gradually turn up the current of the DC power supply and observe the deflection angle of the compass needle. Fix the applied current when the deflection angle of the compass needle reaches 30. Vertically lift up the wire and observe the change of the deflection angle. Then lay the wire back onto the compass as the beginning. Laterally move the wire sideway from the compass needle. Write down your observation on the change of the deflection angle. (3) Effect of the field direction Lay the wire on top of the compass as in Experiment (1). Apply appropriate current to the wire to make the compass needle deflect 30 clockwise. Fix the applied current. As shown in the picture, rotate anticlockwise the wire (γ) from 0 to 90 and observe the deflection angle of the compass needle (θ). Record the corresponding needle-deflection angle (θ) for each wirerotation angle (γ). Plot the θ versus γ curve. Based on this experimental result, explain the relation between the emanated magnetic field and the electric current with the related physical law. 2. Quantitative characterization Refer to the appendix for setting up your smartphone as a Gaussmeter. Connect a wire to the DC power supply with 5 bulbs in a parallel configuration. Pull the wire into straight line 3

5 and fix it to the table. Place the smartphone right beside the wire, and put some paper under the wire to make sure the wire is at approximately the same hight with the center of the smartphone. (1) The dependence of magnetic field on the magnitude of current. Place the smartphone beside the current line. Switch on the DC power supply and keep the output OFF. Adjust the current and voltage on the power supply to 0. Measure the background magnetic field in the z axis. Slowly increase the output current to 1.2 A with a step of 0.2 A, record the measured magnetic field at each current value. The magnetic field generated by the current can be defined as the difference between the measured value and the background value. Plot the magnetic field value versus the output current. (2) The dependence of magnetic field on the spatial separation. Place the smartphone beside the current line, and measure the background magnetic field in the z axis. Set the output current to 1 A, and change the distance between the smartphone to 8 cm with a step of 1 cm. Record the measured values respectively, and subtract the background at each position. Plot the magnetic field value versus the distance. Report requirement: 1. Present your data and results in table or graphical form wherever appropriate. 2. Examine your observations and results critically according to the Ampère's Circuital Law. 3. Try to deduce the magnetic field generated by the current line with Biot-Savart Law Idl r where 2 dl is a vector whose magnitude is the length of the differential 4 r 0 ( db element of the wire and r is the full displacement vector from the wire element to the point at which the field is being computed) and compare the results with the Ampère's Circuital Law. Objective: Part III Experiment: The Faraday's Law The objective of this experiment is to investigate and verify the fundamental properties and the physical meaning of the Faraday s Law. Preparation: Before coming to the Lab, go over the Faraday s Law and try to investigate the relation between the output voltage and angular velocity of the armature for a simple generator. 4

6 Apparatus and Required Parts: Hand crank generator Bulb board Oscilloscope (Reminder: please bring USB memory or digital camera for data storage) Wires Introduction: The Faraday s Law governs the relation between the induced voltage and the changing magnetic flux. This law is applied in generating electricity in a generator. Electric current can be produced by inducing an electromotive force (EMF) by a changing magnetic flux. This experiment aims at investigating and verifying the fundamental properties of the Faraday s law of induction. We will explore how the EMF depends upon the rate of change of magnetic flux. The rate of the changing magnetic flux can be varied by rotating the handle of the hand crank generator at different speed. The produced EMF can then be measured with the oscilloscope. As such, the relation between the EMF and the rate of changing magnetic flux can be determined. Procedure: 5

7 (1) By making use of the bulb board, apply different loads on the hand crank generator. Rotate the handle at the same speed and feel the resistive force to the hand crank generator. Compare the resistive force in the following scenarios: (a) no load, (b) five light bulbs in series, (c) five light bulbs in parallel. Compare the power supplied for different loads and try to explain the relation between the resistive force and the loaded power from the perspective of energy conservation. (2) Connect the two poles of the hand crank generator with the oscilloscope. Rotate the handle at different rates (e.g. 0.3 second per cycle to 0.9 second per cycle) for one cycle and record the waveform. Use the oscilloscope to observe and record the curve shown on the screen. To better observe the waveform, the horizontal and vertical axis of the oscilloscope are advised to set as 200 ms and 5 V. Choose some representative waveforms (at least 15) and measure their waveform periods and peak voltages. Plot curve diagram to investigate the relation between the peak voltage and the period. Report requirement: 1. Present your data and results in table or graphical form wherever appropriate. 2. Deduce the EMF (in volts) produced by a simple generator with the following parameters: number of loops in the armature (N) external magnetic field strength (B) area of each loop (A) angular velocity of the armature (ω). Examine your observations and results and check if they agree with your deduced formula. Can you explain the relation between the peak voltage and the period in this experiment with your deduced formula? Bonus questions (6%) 1. Do you think it is fun to do this laboratory session? Which part do you enjoy the most? 2. Does this laboratory session help you to understand how the EM theory you learned in class can be put into use in practice? 3. Any other comments/suggestions? 6

8 Appendix: How to measure magnetic field with a smartphone OS ios Android App Install EMF Meter Install Sensor Kinetics Direction Procedure Place the smartphone along the wire, and the direction of magnetic field generated by the current is perpendicular to the phone plane (i.e. the Z axis in the App). Press the timer button on the App UI to start the measurement. Press the timer button again to stop the measurement and to record the values measured at Z axis. Record three values in each measurement and calculate the average to minimize the measurement error. Choose the Magnetometer and press Start to enable the measurement. Hold for ten seconds and read the average value from the figures. 7