Charging objects by contact

Learning goals I will be able to explain how to charge objects by contact I will be able to conduct an investigation into the transfer of electric charges by friction, conduction, and induction and produce labelled diagrams to explain the results

Recall… Law of electric charges Law of Attraction An object that is charged will attract an object that is oppositely charged or neutral Law of Repulsion Two objects that have the same type of charge will repel each other

Methods of charging Charging Objects by Induction Temporary – Induced Charged Separation Permanently Charging Objects by Contact Friction Conduction

Example: Balloon Rubbing against your hair

Here’s my silly demo. The balloon will rub against the head and then pull away with the “hair” standing on end. You might need to explain what it’s supposed to be. The next few slides show what is happening to the charges. These should run automatically.

- + - + + - - + - - + + + - + - - + - +

- - + - + + - - + - - + + + - + - + - +

- - + - + + - - + - + + + - + - + - +

- - + - + + - - + - + + + - + - + +

- - + - + + - - + - + + + + - + +

- - + - + + - - + - + + Re iterate that the head an balloon each started off with the same number of positive and negative charges but the rubbing moved charges onto the balloon and off the head. The balloon is now negatively charged and the head positively charged so that because opposites attract the hairs are attracted to the balloon And because like charges repel the airs are pushed apart from each other. + + - + +

Charging objects by contact: friction Occurs when two different neutral materials are rubbed together or come in contact (touch), they produce a net static charge Since the two objects are made of different materials, their atoms will hold onto their electrons with different strengths As they pass over each other, the electrons with weaker bonds are “ripped” off and transferred onto the other material

Example: Balloon Rubbing against your hair The rubber balloon has a stronger attraction to electrons than the hair, and therefore pulls the electrons away resulting in a negative charge while the hair becomes positively charged.

How do we know which material will lose the electrons and which one will gain them? ELECTRON AFFINITY The tendency of a material to gain and hold onto electrons Simply put, it refers to the amount of love that a material has for electrons.

Electrostatic series A list of materials arranged by their likelihood to attract electrons Used to predict resulting charges from contact The lower the material is on the series, the weaker its hold on electrons

Electrostatic series Relative Charge Negative (-) Positive (+) Material Rubber Ebonite Polyethylene (plastic) Cotton Silk Wool Glass Acetate Cat’s fur Charge Tendency HIGH Attraction for Electrons LOW attraction for electrons

You Try! Relative Charge Negative (-) Positive (+) Material Rubber Ebonite Polyethylene (plastic) Cotton Silk Wool Glass Acetate Cat’s fur Charge Tendency HIGH Attraction for Electrons LOW attraction for electrons Your rub a piece of cat fur against plastic. What charge does each material have now? Plastic is higher on the list than cat fur, therefore it has a greater attraction for electrons than cat fur. Plastic will gain electrons from the cat fur and become negatively charged. The cat fur will lose electrons to the ebonite and become positively charged.

Charging objects by Contact: conduction Occurs when two objects with different amounts of electric charge come in contact (touch) and electrons move from one object to the other. This can happen between: Neutral Object & Charged Object Two Charged Objects

Conduction: Neutral Object & Charged Object Between a neutral object and a charged object, the neutral object takes the charge of the charged object upon contact. As a result of the contact, some of the excess electrons on the bar which are repelling each other, move onto the sphere.

negative positive Negatively Charged Neutral Object becomes Positively Charged Neutral Object becomes negative positive When a negatively charged object touches a neutral object, the object becomes negatively charged When a positively charged object touches a neutral object, the object becomes positively charged

Conduction: Two charged Objects Electrons always move from the object with a larger negative charge (less positive) to the object with the smaller negative charge (more positive). This produces a more even distribution of electric charge between the two objects Metal Rod X: Metal Rod Y: Overall Charge: +3 Overall Charge: +1

Conduction: Two charged Objects Electrons always move from the object with a larger negative charge (less positive) to the object with the smaller negative charge (more positive). This produces a more even distribution of electric charge between the two objects Metal Rod X: Metal Rod Y: Overall Charge: +2 Overall Charge: +2

You Try! If the following two objects are charged by conduction, what are their final charges? (A) (B) Overall Charge: +1 Overall Charge: -1 Overall Charge: Overall Charge: Overall Charge: -5 Overall Charge: -3 Overall Charge: Overall Charge:

You Try! If the following two objects are charged by conduction, what are their final charges? (A) (B) Overall Charge: +1 Overall Charge: -1 Overall Charge: Overall Charge: Overall Charge: -5 Overall Charge: -3 Overall Charge: -4 Overall Charge: -4

Insulators vs. conductors These results depend on the type of material used: Conductors: electrons move freely from atom to atom in this material Examples: Metal, tap water Insulators: electrons do not move freely between atoms in this material Examples: wood, wool, rubber, wax Semi-conductor (“Fair Conductor”): electrons can move fairly well in these non-metal materials Examples: Human body, graphite, silicon Silicon Transistors

Investigation: Charge it! Follow all lab safety protocols Groups of 4 One person from each group will gather all materials needed Investigation is divided into: Part A: Charging by Friction Part B: Charging by Conduction Part C: Charging by Induction Read the procedure of each section thoroughly before commencing. Record your Observations. Once all procedures have been completed, continue with the Discussion and Conclusion questions. Due Wednesday May 27