# UMass Amherst PHYSICS 132 - Lab 4 132 (3 pages)

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## Lab 4 132

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- School:
- University of Massachusetts Amherst
- Course:
- Physics 132 - Intro Physics II

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Magnetism Induction Physics 132 Thursdays 9 am ILC S110 Abstract In this experiment we looked at the basic properties of magnets and magnetism We did this by examining how electric currents can generate a magnetic field and how an electric field is generated by a magnetic field this was done using a small magnet and rod Magnetism is an important force that mediates the force between objects such as permanent magnets made of ferromagnetic materials Magnetic flux is one way to measure the changes of magnetism in a given area with a certain amount of time Questions Answers 1 Define magnetic flux in your own words What two quantities does magnetic flux depend on 1 point Magnetic flux is the product of the average magnetic field and the surface area Magnetic flux depends on its change over time and the movement of the coil and magnent in relation to each other 2 Consider The magnet flux through a coil of wire with area A 100 cm2 and n 160 turns If a magnetic field is increasing through the coil at a rate of 0 75 Tesla sec what is the induce emf around the coil 2 points Lenz s law relates emf V to the flux via V d BA dt or V change in BA change in time We know that B 75 T sec You are also told the coil has an area of 100 cm 2 and 160 turns so the total area is 160 100cm 2 1 6 m 2 So V A change in B change in time 1 6m 2 0 75 T s 1 2 Volts 3 Hold the magnet as steady as you can in the coil Is there an induce emf around the coil Why or why not 2 points As we move either the magnet or coil the reflection will change and thus emf is induced in the coil of wire whenever there is change in the magnetic flux The relationship between the direction of field direction of motion and magnitude of induced emf is E BAcos where the angle between the magnitude induction also known as B and the line perpendicular to area also known as A So as we kept the magnet as steady there will be an at least minimum emf in the coil 4 Assume the frequency generator used had a fine adjustment range of 0 1 3 With the course adjustment being in powers of 10 What setting for the two frequency knobs on the function generator would give a signal with a frequency of 4000 Hz 1 point A setting for the two frequency knobs on the function generator that would give a signal with a frequency of 4000 Hz on the range of 1 3 is a fine adjustment of 04 Hz and a course adjustment of 4 Hz using the formulas below 10 x y 4000 y 04 x 4 10 4 100000 10 4 4 4000 5 On the Oscilloscope what do the vertical and horizontal axes represent Recall that each axis was divided into little boxes There are 10 horizontal boxes and 8 vertical boxes If TIME DIV is set to 1 second what is the minimum frequency of a waveform that could be fully displayed on the screen 2 points The vertical axes represents volt per division while the horizontal axes represents time If there are 10 horizontal boxes one complete cycle would be T 10 s We can find the minimum frequency of a waveform using the formula f 1 TF 1 T f 1 10 Hz 1Hz So the minimum frequency is 1 Hz 6 If a scope has the Volts DIV set to 2 What is the maximum signal voltage you can display Assume the screen has 8 divisions 1 point In order to find the maximum signal voltage we would multiply the Volts DIV by the number of divisions the screen has which would give you 2 x 8 16 volts 7 What were the maximum currents and voltages you were able to induce with the permanent magnet How long were they present for estimate What affected the amplitude of the induced current and voltages 2 points The voltage is called an induced emf and has been induced into the conductor by a changing magnetic field due to electromagnetic induction with a negative sign in Faraday s law telling us the direction of the induced current The self induced emf will oppose the change in current in the coil and because of its direction this self induced emf is generally called a back emf The maximum currents we were able to induce with the permanent magnet would be 1000 Hz which in turn increases the amplitude These currents were only present for a short amount of time because the reaction happened quickly 8 In part 4 3 3 Induction between two Coils there is no magnetic like previous part 4 3 2 Magnetic and Coil but there is an induced voltage emf in the second coil as measured by the Oscilloscope Where does the changing magnet flux that induces the voltage come from How is the magnetic flux propagated to the second coil 2 point Under these conditions there will be no current flowing in the primary coil This means that the second coil will be reversed This will cause an overall slowing of speed If the speed slows down then we also will not induce as much voltage If that induced voltage drops then so to does the opposition to the flow of current All of this happens very quickly The net result is basically an opposition to change Make a fast change in current in a coil and you get a strong opposition to that change Reduce the rate of change and you reduce the amount of opposition to that change 9 Plot the voltage amplitude vs frequency for your data taken with the two coils What can you say about the relation between the two Why might you expect this 3 points We can see that based on the data points the graph is mostly linear with a bit of a divot but it is clear that as frequency increases amplitude also increases The reason this might be the case is because there is more space vertically and less horizontally Having less space means there is a higher frequency Conclusion From this experiment we have found that whenever there is relative motion between a conductor and a magnetic field the flux linkage with a coil changes and this change in flux induces a voltage across a coil Our experiment was pretty straight forward which leads me to believe that no errors were created Human error of course always needs to be accounted for including how we held the magnets but our results seem to be what was expected besides of the first number we recorded for the value of a frequency of 200 Hz the TA said it was abnormal but consistent among other lab groups so we are not sure what caused this difference

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