This lab had the intention to show to the students how the electrical forces can work by using material that we are in contact with on daily basis.
Our physics professor Dr. Mason started the lesson rubbing cat hair and a balloon to see what happens when the glass and the balloon are in contact (first picture). By rubbing this two materials, electrons will be transferred from one to another, causing the balloon to increase its electrical charges. When the balloon was placed near the glass they sticked together as they were glued. This happens because each one presents different chargers when in contact. The balloon may have negative and the glass positive charge, or the glass may have negative and the balloon positive charge. In fact, if they both would have the same charge, positive or negative, they would not be sticked because same charge always repels each other.
In the second photo, a silk was used to see if the balloon and the glass would behave in the same way. Although the rubbing object used was different the effect was the same. This two experiences had shown that no matter how a person may rub a balloon, or other mass the can gain electrical charge, it will always generate an electrical charge in such away that when placed near a mass with an opposite charge they will stick together.
The third photo explains what happens in both experiences, on the left side of the last photo is a free body diagram which represents the forces acting on the balloon and the contact force. On Y-axis: Force due to gravity (+Fg, taking the positive down) and Friccion (-Fr, because it goes in the opposite direction of the Fg), and on X-axis: Force that the balloon does on the glass (Fb) and the force that the glass does on the balloon (-Fg, or normal force). The forces in the X-axis are equal in magnitude but opposite in direction, so when added the net force is zero. And that is the reason why the balloon and the glass in both cases can stick together.
Aclaration: Even Though the red balloon is sticked to the wall, initially it was with the glass. The intention was to show that two methods were used, and they had the same effect.
After doing the practical learning, student went to the theoretical learning which consist in try to find a definition for electrical charges. And at the end, the simple definition that was found is: Charge is how much matter interacts with other matter electrically.
After the definition, students had to perform a experience with tape. Four strips of tape were used, and each one was labeled. Two were written of top and the other two bottom. They had to induce some charge on each of it by sticked them on the table and pulled. After that, they had to three different combinations, such as, top and top, bottom and bottom, and top and bottom. The conclusions were: when the two tops were placed together, they would repeal each other because the have the same electrical charges. The same happened when the bottom were placed together. When top and bottom were placed together, they sticked since they had different electrical charges.
The first photo represents the derivation of the force used to calculate the electrical forces when one mass is approaching another one with the same electrical charge, and the second one starts to repel the first one.
The second and the third pictures are simple calculations and the answers for the questions on the lab.
Those formulas are also known as the Coulomb's Law, which mathematically says that the force of the electrical interaction between too chargers depends how apart they are from each other. Bigger the distance, smaller the force. And the most important of all, the distance cannot be zero, otherwise the force would be infinity.
No comments:
Post a Comment