PHYS 259 W2024 - Lab 1 - Charges and Forces

PHYS 259, Labatorial 1, Winter 2024 University of Calgary Department of Physics and Astronomy PHYS 259, Winter 2024 Labatorial 1: Electric Charges and the Forces between them Electrostatics is the study of the forces between objects that carry an electric charge. Electric charges can either be positive or neg- ative, and these charge can neither be created nor destroyed, but simply flow from one object to another. Any two charged ob- jects exert an electric force on each other equal in magnitude and opposite in direction; opposite charges attract, like charges repel. The magnitude of the electric force between two point charges is given by Coulomb’s law, F C ( r ) = 1 4 π 0 | q 1 q 2 | r 2 . (Image from fisitech.wordpress.com) An important application of the electric force is the electrostatic scrubber, shown in the figure on the right. It is used to remove dust, smoke, bacteria and other microm- eter or submicrometer size particles from exhaust gases or indoor air: charged air- borne particles are attracted by the oppo- sitely charged scrubbing droplets. (Image from www.imp.gda.pl, where you can learn more about this procedure.) Goals: To understand how a macroscopic object acquires a net charge, and what that means on a microscopic level. To practice how to find the resulting force on a charge by using vector addition. Preparation: Halliday, Resnick, and Walker, “Fundamentals of Physics” 12th edition, Wiley: 21.1–21.3. Pre-Lab Assignment: Complete Checkpoint 1 (page 2-3) prior to arriving at the lab. Your TA will check for completeness. Any student who does not complete the pre-lab assignment will not receive credit for the first checkpoint. Equipment: Scotch tape; ruler; the applet: https://www.compadre.org/physlets/electromagnetism/illustration23 2.cfmu. 1

PHYS 259, Labatorial 1, Winter 2024 1 “Static Electricity” Question 1 : As a precaution, the levers on gasoline pumps in many provinces in Canada must be manually operated at all times. One of the reasons for this is especially relevant in the winter time because the fabrics used for heavy clothing and coats makes it easy to build up a static electric charge. Explain why it could be dangerous to allow someone to have the gas continue pumping while they are doing something else; imagine what might happen when they come back and touch the pump. 2 Electric Charge and Charge Carriers Question 2 : What is an electric charge? Try to write down a definition. Question 3 : When an object acquires an electric charge, is the charge created and then placed on the object, or is the charge transferred from one object to another? Explain. 2

PHYS 259, Labatorial 1, Winter 2024 B: A: CHECKPOINT 1: Before moving on to the next part, discuss your results as a group, then have your TA evaluate your answers. 3 Electric forces between two point charges Question 8 : The figure below shows two point charges of - 1 μC and +1 μC . Draw the force vectors on each charge, making sure to accurately represent the relative lengths of the force vectors. How would the direction and magnitude of the force vectors change if the distance between the two charges were doubled? _ + Question 9 : This time, consider two point charges of - 1 μC and +2 μC . Draw the force vectors on each charge. Is the force on the - 1 μC charge greater than the force on the +2 μC charge? Why or why not? _ + + 4

PHYS 259, Labatorial 1, Winter 2024 Question 10 : For the configuration of charges in Question 9, what would the distance between the charges have to be in order to double the magnitude of the force? 4 Superposition of electric forces in one dimension Question 11 : The figure shows four configurations of three charges in one dimension. For each configura- tion, draw the forces acting on the middle charge, then use a different colour to draw the net force on the middle charge. Note: the relative magnitudes must be correct. x x x + + + + + + + _ _ x + _ _ Question 12 : A charge q 1 = +2 . 0 nC is located at the origin, and a second charge q 2 = +4 . 0 nC is located at position x = d . a) At what position x will a q = - 1 nC charge experience zero net force? 5

PHYS 259, Labatorial 1, Winter 2024 Question 15 : Is the force calculated in the previous question repulsive or attractive? How do you know? Question 16 : If there is such a strong repulsive force between the protons, there must be an even stronger attractive force between them to keep the nucleus together. Is this the gravitational force between the protons? Support your conclusion with a calculation. Question 17 : Calculate the ratio of the Coulomb force to the gravitational force, F C /F g , between an electron and a proton inside a hydrogen atom. What is the ratio if they are 100 km apart? B: A: Last Checkpoint! Before being dismissed from the lab, discuss your results as a group, then have your TA evaluate your answers. Be sure to sign the grade sheet before you leave. 7