Electricity

Law of Electrostatics 

1. Objects with like charges repel
2. Objects with opposite charges attract 
3. Neutral objects are attracted to charged objects 

Electric Charge

• Occurs if an atom does not have an equal number of protons and electrons
• Neutral objects: have an equal number of p+ and e-
• Negatively charge objects: have more e- than p+
• Positively charged objects: have more p+ than e-

Ways of Charging

Charging by conduction

• Electric charge can be given to a neutral object by touching it with a charged object 
• An object that has been charged by contact always receives the same charge as the charge that is on the object that charges it

Charging by Induction

• The charged object is brought NEAR a neutral one (no contact)
• the neutral object becomes temporarily charged (electrons move around)
• The charges in the neutral object rearrange themselves according to the law of electrostatics
• the neutral object has the OPPOSITE charge as the object that induced it.

Charging by Friction

• When two different neutral materials are rubbed together
• electrons (e-) are transferred from one object to another
• one object will lose e- to another which will gain e-
• different materials have different attractions for elections 

Factors That Affect Resistance

• Type of material: the ability of a material to conduct electricity is determined by how freely electrons can move within the material
• Cross-Sectional Area: the diameter of the cross-section gives you a sense of how thick the wire is. Thicker wires have less internal resistance than thinner ones. 
• Length: As you increase the length of a wire, its internal resistance increases. This happens because electrons have to travel through more material
• Temperature: Resistance increases when electrons bump into atoms as they move through a material. When wires get warmer, the atoms that make up the wire gain energy and move faster. Resistance increases with temperature.
• Electrical Resistance: the ability of a material to oppose the flow of electric current; measured in ohms. 
• Resistor: a device that reduces the flow of electric current
• Ohmmeter: a device that is used to measure resistance

Examples:

• Lightweight carbon resistors are used in electronics
• Heavier ceramic resistors are used in larger circuits
• Dimmer switches and volume controls are other types of resistors called variable resistors

• allows you to change the resistance in a circuit. 

Current Electricity

• The controlled flow of electrons through a conductor
• refers to the electric charges (electrons) that flow through a conductor in a controlled way

Forms of Current Electricity

• Direct Current (DC)

• electrons flow in one direction only
• produced by electric cells

• Alternating Current (AC)

• Electrons move back and forth, changing their direction 
• produced by generators

Static Electricity

• An imbalance of electric charge on the surface of an object
• Charges are at rest (ie. they do not move)
• The strength of the electric force is related to 

• Amount of charge
• Distance between charged objects

Applications of Static Electricity

• Conductor: an object that lets electrons move easily through it

• Eg. metal wires

• Insulator: a material that does not allow the movement of electrons through it 

• Eg. plastic

• Grounding: connecting an object to a large body, like Earth, that is capable of removing any electric charge on the object

• Negative object: excess elections flow the object to the Earth until it is neutral
• Positive object: electrons flow from the Earth to the object until it is neutral

Ohms Law

• Adding loads in series increases resistance. Each load acts like a speed bump
• Based on Ohms law, if resistance increases, so do voltage and current decreases. 

Electric Cells

• A portable device that converts chemical energy into electrical energy
• Consists of 2 electrodes in a conducting solution (called an electrolyte)
• Electrodes are conductors - one is positively charged and the other is negatively charged
• When electrodes are connected by wires in a circuit, electrons flow - they are repelled b the negative electrode and attract the positive electrode. 

Definitions:

• Electrical Energy: the energy provided by the flow of electrons in an electric circuit
• Electric Cell: a device that converts chemical energy into electrical energy
• Primary Cell: an electric cell that may only be used once
• Secondary Cell: an electric cell that can be recharged

Types of Primary Cells

• Primary cells fall into 2 categories: dry cells and wet cells. Chemical reactions use up materials in primary cells as electrons flow from them, thus the cell can be permanently discharged. 
• Wet cells consist of metal conductors placed in contact with a conducting liquid
• Dry cells work with wet cells, only a moist paste is used rather than a liquid

Equations

Measurements in Electrical Circuits