Physics Pre lab 212P 1 Exploring Static Electricity Name Section Date Read this answer the questions before coming to lab Summary of relevant concepts Fundamental properties of electric charge a There exist two types of electric charge positive and negative b Electric charge is conserved c Electric charge is quantized in units of 1 6 x 10 19 C d There exists an electric force between point charges which obeys the following rules It acts along the line joining the two points and is repulsive for like charges and attractive for unlike charges The magnitude of the force is given by Coulomb s Law F k q1q2 9 2 2 2 where k 8 99 x 10 N m C R The atomic picture of matter All matter is made of atoms containing a nucleus of neutrons no charge protons charge 1 6 x10 19 C surrounded by a cloud of electrons charge 1 6 x 10 19 C Note that the number of electrons and protons in an atom are the SAME In solid matter the atomic nuclei might vibrate but do not otherwise move the electrons on the other hand can move A useful microscopic picture of a solid is given below Gray electron clouds Black nuclei relative size greatly exaggerated Insulators are materials in which electrons are only free to move around the neighborhood of their respective atomic nuclei note that the electron cloud around each atom in the material can be distorted if an external charge is brought nearby So in the picture above imagine that the clouds of electrons can change shape Conductors are materials in which many electrons can freely move over long distances You ll learn later that these conduction electrons come from the outer regions of the atom So in the picture shown above you could imagine the gray electron clouds merging into a sea of mobile electrons Pre lab Questions In Physics 211 you learned that all masses exert a gravitational force on each other obeying Newton s Universal Law of Gravitation Q1 List the similarities and the differences between the force of gravity and the electric force Consider the example of the Hydrogen atom a simplified but very useful model for this atom consists of a single electron going in a circular orbit around a single proton The parameters of the Hydrogen atom are as follows Electron mass 9 1 x 10 31 kg electron charge 1 6 x 10 19 C Proton mass 1 7 x 10 27 kg proton charge 1 6 x 10 19 C Radius of electron orbit 1 x 10 10 m Q2 What is the magnitude of the electrical force on the electron from the proton Is it attractive or repulsive Q3 What is the magnitude of the gravitational force on the electron from the proton Is it attractive or repulsive Q4 What is the ratio of the gravitational force on the electron to electrical force Q5 If gravitational forces are so much weaker than electrical forces why do we directly experience the effects of gravity more readily in Nature than the effects of electric forces Q6 A rule of thumb calculation no calculators The Coulomb force between an electron and a proton in an atom is given to be 10 nN n nano or 10 9 If the distance between these charges is tripled what now is the magnitude of the electrical force between them Q7 How much is a coulomb The concepts you encountered in Physics 211 were somewhat familiar from everyday life e g we all have a physical feeling for the meaning of a meter kilogram and second But how much is a coulomb Imagine that you and a friend each held 1 C of charge at a distance of 1m from each other Calculate the force of repulsion that you would experience Compare this force with your weight Q8 Physics 211 Review Static Equilibrium Vector Addition Trigonometry The figure below shows a mass M suspended from a vertical string A horizontal force F is applied to the mass as shown so that it is deflected by an angle from the vertical The entire system is in static equilibrium Draw the free body diagram for the forces acting on the mass and determine an expression that relates F to the mass M and the angle F