Electric Charge and Static Electricity 20.1

Electric Charge Electric charge is a property that causes subatomic particles (protons and electrons) to attract or repel each other. There are two types of electric charge: Positive (proton) Negative (electron)

Electric Charge An atom is neutral with a positive nucleus and is surrounded by a negatively charged cloud of electrons. An excess or shortage of electrons produces a net electric charge (ion). The SI unit of electric charge is the coulomb (C)

Recap of the Atom

Electric Forces Like charges repel, and opposite charges attract. The attraction or repulsion between electrically charged objects is electric force. EX: balloon and hair

Electric Forces The electric force between two objects is directly proportional to the net charge on each object and inversely proportional to the square of the distance between them. aka the larger the charge, the larger the electric force. The larger the distance, the smaller the electric force.

Electric Forces Inside an atom, electric forces are much stronger than gravitational forces.

Electric Fields The effect an electric charge has on other charges in the space around it is the charge’s electric field. The strength of an electric field depends on the amount of charge that produces the field and on the distance from the charge.

Electric Field The lines representing the field are closer together near the charge, where the field is stronger.

Field of a Positive and Negative Charge

Static Electricity and Charging Static electricity is the study of the behavior of electric charges, including how charge is transferred between objects. Charge can be transferred by friction, contact (conduction), and induction.

Static Electricity and Charging The law of conservation of charge states that the total charge is the same before and after the transfer occurs.

Charging by Friction Rubbing two objects together causes friction, which causes a transfer of electrons.

Charging by Contact Whenever two objects touch (not rubbed together) and charge is transferred.

Charging by Induction A transfer of charge without contact between materials. EX: walking across carpet and then touching a door knob.

Static Discharge Static discharge occurs when a pathway through which charges can move forms suddenly. Charges will not travel through air from your hand to the doorknob. The air becomes charges suddenly when the gap between your finger and the doorknob is small. This air provides a path for electrons to flow from your hand to the doorknob.

Electric Current and Ohm’s Law 20.2

Electric Current The continuous flow of electric charge is an electric current. The SI unit of electric current is the ampere (A), or amp, which equals 1 coulomb per second. The two types of current are direct current and alternating current.

Electric Current Direct current (DC) is where charge only flows in one direction. Alternating current (AC) is a flow of electric charge that regularly reverses its direction. Current is the direction in which positive charges flow.

Direct Current

Conductors and Insulators An electrical conductor is a material through which charge can flow easily. EX: copper and silver A material through which charge cannot flow easily is called an electrical insulator. EX: wood, plastic, rubber, and air

Resistance Resistance (R) is opposition to the flow of charges in a material. The SI unit of resistance is the ohm.

Resistance A material’s thickness, length, and temperature affect its resistance. A superconductor is a material that has almost zero resistance when it is cooled to low temperatures.

Voltage In order for charge to flow in a conducting wire, the wire must be connected in a complete loop that includes a source of electrical energy. Potential difference is the difference in electrical potential energy between two places in an electric field.

Voltage Potential difference is also called voltage (V). Three common voltage sources are batteries, solar cells, and generators. A battery is a device that converts chemical energy to electrical energy.

Ohm’s Law Ohm’s Law states that the voltage (V) in a circuit equals the product of the current (I) and the resistance (R): V = I x R Increasing the voltage increases the current. Keeping the same voltage and increasing the resistance decreases the current.

Ohm’s Law Practice What is the voltage if the resistance is 3 ohms and the current is 3 amps? V = I x R