Electric Dipole in an Uniform Field

Developed by David Urminsky

This set of exercises will guide students through calculating the torque on an electric dipole in a uniform electric field. It will then have the students use the small angle approximation to show that the dipole will undergo simple harmonic motion for a small perturbation from the stable equilibrium. Finally, the students will explore the concept of how small is small for the small angle approximation to provide the correct dynamics.
Subject Area Electricity & Magnetism
Level First Year
Available Implementation IPython/Jupyter Notebook
Learning Objectives
Upon completing this exercise set, students will be able to: - analytically calculate the net force and torque on an electric dipole in a uniform electric field. (**Exercise 1**) - determine stable and unstable equilibrium points for the dipole. (**Exercise 1**) - understand the link between the torque on the dipole with the dipole moment (**Exercise 1**) - demonstrate analytically that for a small angle, $\theta$, displacement from the stable equilibrium point the dipole will undergo simple harmonic motion (**Exercise 2**) - determine the angular frequency of the motion for small angle displacement. (**Exercise 2**) - calculate a numerical solution to the motion to compare with the solution with the small angle approximation(**Exercise 2**) - explore how increasing $\theta$ breaks down the small angle approximation (**Exercise 3**)
### Exercise Set 1 1. What is the total net force on the dipole? 2. What is the magnitude of the net torque on the dipole in terms of $q$, $d$ and $|\mathbf{E}|$? 3. Express the torque as a vector in terms of the vectors $\mathbf{p}$ and $\mathbf{E}$. 4. For what angles $\theta$ is the dipole * in stable equilibrium? + in unstable equilibrium? + undergoing maximum torque? ### Exercise Set 2 1. Using the small angle approximation (i.e. for $|\theta| \ll 1$ then $\sin(\theta) \approx \theta$), show that for small initial displacement, $\theta$, the electric dipole undergoes simple harmonic motion. (Hint: use your result from Exercise Set 1 and remember that $\tau = I\alpha$ where $\alpha$ is the angular acceleration and $I$ is the moment of inertia.) + What is the angular frequency of the oscillations? + Write out the formula for $\theta$ as a function of $t$. (Hint: this should be a solution to the second order differential equation above) + Edit the code below to plot a graph of the angle of displacement from equilibrium vs. time where $q=1.6\times 10^{-19}$ C, $d=5$ cm, $m = 1.67\times 10^{-27}$ kg, and $E=100$ N/C. ```python theta0=(10.0/180)*np.pi #initial displacement in radians q = #Charge on dipole in Coulombs d = #distance separating the charges (m) m = #mass of a single charge (kg) E = #magnitude of the electric field ############################################################ ######## EDIT BELOW WITH YOUR VALUE ########### ######## THIS IS THE ANGULAR FREQUENCY ########### omega = #enter your angular frequency ############################################################ ############################################################ ######## Determine the Period of the motion ######## ######## in terms of the angular frequency ######## T = #edit this for the period of motion # ############################################################ t=np.linspace(0,2*T,2000) #time interval from t = 0 to t = 2T theta = theta0*np.cos(omega*t) #solution for Simple harmonic motion ############################################################ ######## The following code plots the data ####### plt.plot(t,theta) plt.title("Simple Harmonic Motion - dipole in an Electric field") plt.xlabel("time (s)") plt.ylabel("angle (radians)") plt.grid() plt.show() ############################################################ ``` ###Exercise Set 3### 1. in the code below you can see some parameters which you can edit. Also, there is a field in which you need to put in your value of the angluar frequency. $\omega$ you obtained above. (a) Determine the time, for a dipole with $q=e$, $d=3\times 10^{-9}$ m, $m=3\times 10^{-5}$ kg in an electric field, $E=1000$ N/C, at which the small-angle solution differs from the non small-angle solution by 0.01 rads. 2. Try other parameter values as well. ```python ############################################################ ################ Start of parameters ################### theta_degrees = 30.0 #intial angle of displacement (degres) Tmax = 500.0 #upper bound for time (s) q = 1.0e-19 #Charge on dipole in Coulombs d = 3.0e-9 #distance separating the charges (m) m = 3.0e-5 #mass of a single charge (kg) E = 1000 #magnitude of the electric field N = 50000 #number of time intervals error = 0.05 ############################################################ ######## EDIT BELOW WITH YOUR VALUE ########### ######## THIS IS THE ANGULAR FREQUENCY ########### omega = np.sqrt((2.0*q*E/(d*m))) ## EDIT THIS ## ############################################################ ################ End of parameters ################### ############################################################ ############################################################ ############################################################ #Set the times to evaluate t = np.linspace(0,Tmax,N) #convert initial angle to radians theta_rads = theta_degrees * np.pi / 180.0 ############################## # compute approximate theta values on the time intervals Approx_Solution = theta_rads*np.cos(omega*np.array(t)) ############################## # Set the intial condition [initial angle, initial velocity] x0 = [theta_rads, 0.0] ############################## # numerically solve the real solution using odeint sol2 = odeint(torque,x0,t,args=(q,d,m,E)) ############################# # Plot the two solutions plt.plot(t,theta_rads*np.cos(omega*np.array(t))) plt.plot(t,sol2[:,0]) plt.grid() plt.xlabel("time (s)") plt.ylabel("angle (rads)") plt.show() ############################ # compute the difference between approx and actual on intervals diff = [np.abs((a1-b1)) for a1,b1 in zip(Approx_Solution,sol2[:,0])] ######################################### # Plot the difference vs time plt.plot(t,diff) plt.xlabel("time (s)") plt.ylabel("Difference") plt.show() ######################################### # Find when the two differ by .01 radians flag = True for i in range(len(t)-1): if abs(diff[i]> error): print("after t =", t[i],"the two solutions differ by more than .01 rads") flag = False break if flag: print("solution always within",error," rads") ``` 2. Try varying the magnitude of the electric field (e.g. $1000 \leq E \leq 5000$). What do you notice about the time it takes for the solutions to differ by .01 rads? 3. If you run the last cell, you will obtain a short animation displaying the dipole oscillating about the stable equilibrium point and below that a graph of the difference between the two different solutions and the approximation. What can you notice about when the differences are largest? Can you explain?

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Credits and Licensing

David Urminsky, "Electric Dipole in an Uniform Field," Published in the PICUP Collection, July 2017.

The instructor materials are ©2017 David Urminsky.

The exercises are released under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 license

Creative Commons Attribution-NonCommercial-ShareAlike 4.0 license