\subsection*{EC-6b~~Induction - Dropping Magnet}

\noindent OBJECTIVE: \latex{\vspace*{.02in}}   \\ \pind{
To show that a moving magnet induces an {\it emf}
within a coil of 
wire.  \vspace*{.02in}
} 


\noindent INTRODUCTION: \latex{\vspace*{.05in}}   \\
\pind{ After discovering the phenomenon of induction using two coils wrapped around
an iron ring, Faraday was able to show that plunging a magnet into a coil of 
wire also generated a momentary induced current. Faraday did not use the 
concept of {\em{Changing Flux}} instead he thought of the {\em{`lines of 
force'}}, and 
concluded that when these lines move across a wire an {\it emf} is induced.
Can you see the correspondence of this concept with the concept of flux?
Think about this.}
\begin{figure}[h] \centering 
\latex{\includegraphics[height=5.5cm]{figs/l104/fec6b-1.eps}}
\html{\includegraphics[height=5.5cm]{figs/l104/fec6a-1.eps}} \\
\vspace*{-.08in}
\caption{A moving magnet - lines move across a wire}
\end{figure}

\noindent EXPERIMENT: \latex{\vspace*{.05in}}   \\
\pind{

You will  be repeating Faraday's experiment by observing
 the {\it emf} induced in a coil by a fast moving magnet, and you will check that
the {\it emf} you observe obeys the `right hand rule'. If you have the time you can
see the effect of changing the speed and the polarity of the magnet.}


\hspace*{.3in} \framebox[5.2in]{\parbox[b]{5in}{\smallskip {\em YOU NEED TO
KNOW:} Lenz's Law and the use of the right hand rule.
A varying magnetic field induces a current 
that opposes the magnetic field change.   \smallskip
}}\smallskip 

\noindent EQUIPMENT: \latex{\vspace*{-.03in}}  \begin{enumerate}
\epind{
\item[$\Rightarrow$]\vspace*{-.08in} PC and PASCO interface. 
\item[$\Rightarrow$]\vspace*{-.08in} A voltage sensor is connected to the coil and to port A of the 
interface. Be sure the 
red wire is connected to the top of the coil. In this way a positive voltage
corresponds to a current in the direction of the arrow on the top of the
coil.
\item[$\Rightarrow$]\vspace*{-.08in} A stand holding a plastic tube and a coil. 
\item[$\Rightarrow$]\vspace*{-.08in} A long bar magnet, this magnet has a small cut at one end, that is the North pole.
\item[$\Rightarrow$]\vspace*{-.08in} A pair of red and green bar magnets} 
\end{enumerate}

\hspace*{.3in} \framebox[5.2in]{\parbox[b]{5in}{\smallskip PRECAUTION:
The long bar magnet is very fragile!  
DO NOT DROP IT ON THE FLOOR!.}}\medskip 

\noindent  PROCEDURE I: (15 min) \latex{\vspace*{-.03in}}  \begin{enumerate}
\epind{
\item[1.] \vspace*{-.05in} Click on the Launch EC-6B icon below (web version) to initiate
the PASCO software window.
\item[2.]\vspace*{-.05in} Set  the distance $d_1 \sim 15 \ cm$ between the 
top of the plastic tube and
the top of the coil. Record this in your lab notebook.
Be sure that the red terminal of  the voltage sensor is plugged
on the top plug of the coil.
% Check the 
%polarity of this end using the  coloured bar magnets.
%\vfill \eject 
}
\end{enumerate}

\vspace*{-.05in}\hspace*{.03in}
\parbox{0.5\linewidth}{
\begin{enumerate}
\item[3.]\vspace*{-.08in} The long bar magnet has a narrow cut at one end, this is the 
North pole of the magnet.
\item[4.]\vspace*{-.08in} Hold the long bar magnet at the top of the tube; the 
end with the cut should be down, and just one centimeter or so 
inside the tube.
\item[5.]\vspace*{-.08in} DCLK on RECORD and drop the magnet immediately after.
%\vfill \eject
\item[6.]\vspace*{-.08in} to see the whole graph, and measure the heights
 $h_1$ and $h_2$ 
of the two peaks using the cursor cross-hairs (5).
\item[7.]\vspace*{-.08in} Slide the tube upwards, remove the rubber stopper and the magnet,
replace the rubber stopper at the bottom of the plastic 
tube, and slide the tube down again.
\item[8.]\vspace*{-.08in} Print the graph on printer.
\end{enumerate}
}\hspace*{.03in}
\parbox{0.5\linewidth}{
%\begin{figure}[h] \centering 
\latex{\includegraphics[height=8.5cm]{figs/l104/fec6b-2.eps}}
\html{\includegraphics[height=8cm]{figs/l104/fec6b-2.eps}}
\hspace*{0.6in}Figure 2. The setup
%\vspace*{-.08in}
%\caption{The setup}
%\end{figure}
}
\medskip

\noindent QUESTIONS: (10 min) 
\begin{enumerate}
\epind{
\item[Q1]\vspace*{-.02in} The graph you obtained has a fall, a rise, a very short flat part,
then a rise and finally a fall. Explain what is happening at these various 
times.
\item[Q2]\vspace*{-.02in} Explain why the two voltage peaks you measured in 1.6 are not the same size. Can you
give an approximately quantitative justification for the ratio $h_1/h_2$?
\item[Q3]\vspace*{-.02in} Examine the direction of the winding in the coil; verify that 
the directions of the peaks is what you would expect using the right 
hand rule (curled fingers = current;~~~~thumb = magnetic field).
}
\end{enumerate}

\noindent PROCEDURE II: (10 min) 
\begin{enumerate}
\epind{
\item[]\vspace*{-.08in} Repeat procedure I with the magnet inverted (the South Pole at
the bottom). 
}
\end{enumerate}

\indent QUESTION: 
\begin{enumerate}
\epind{
\item[Q1]\vspace*{-.07in} Explain in what way(s) and why the new graph is different.
 }
\end{enumerate}

\noindent PROCEDURE III: (20 min) 
\begin{enumerate}
\epind{
\item[1.]\vspace*{-.08in} Move the coil further down on the tube making the
new distance $d_2$ at least about two and a half 
times larger than $d_1$. Record this new
distance $d_2$.  
\item[2.]\vspace*{-.08in} Repeat procedure I. Measure and record 
the heights $h^\prime _1$
and $h^\prime _2$ of the new peaks.
 }
\end{enumerate}

\indent QUESTIONS: 
\begin{enumerate}
\epind{
\item[Q1]\vspace*{-.08in} Explain the reason for the difference between this graph, and the 
one obtained in procedure I.
\item [Q2]\vspace*{-.08in} Can you give an approximate quantitative explanation for the
ratio $h^\prime _1 / h_1$? HINT: remember Physics 103 formula for free fall
$v=\sqrt{2gx}$.
}
\end{enumerate}



