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Ch 20 Electricity

20.1 Electric Charges Electric charge is a property that causes subatomic particles such as protons and electrons to attract or repel each other. Recall that Protons have a positive charge and electrons have a negative charge. The atom is neutral when it has equal numbers of positive and negative charges. If an atom gains electrons, it becomes a negatively charged ion. If an atom loses electrons, it becomes a positively charged ion.

Electric Force Like charges repel, and opposite charges attract.
The force of attraction or repulsion between electrically charged objects is electric force. Inside an atom, electric forces are much stronger than gravitational forces.

Electric Fields An electric field is a region of space that exerts electric forces on charged particles The strength of an electric field depends on the amount of charge that produces the field and on the distance from the charge.

Static Electricity & Charging
Static electricity is the study of the behavior of electric charges. According to the law of conservation of charge, the total charge in an isolated system is constant. Charge can be transferred by friction, by contact, and by induction.

Charging by Friction Rubbing a balloon on your hair causes charging by friction. Electrons move from your hair to the balloon because atoms in rubber have a greater attraction for electrons than atoms in hair. The balloon picks up a net negative charge. Your hair loses electrons and becomes positively charged.

Charging by Contact A Van de Graaff generator builds a charge on a metal sphere. Touching the sphere transfers charge by contact. The sphere is still charged, but its net charge is reduced.

Charging by Induction Why do you get a shock from a doorknob?
The spark you feel is a static discharge. Walking on a carpet builds a negative charge on your body. The negative charge in your hand repels electrons in a metal doorknob. The doorknob is still neutral, but charge has moved within it. This is induction, a transfer of charge without contact between materials. Static discharge occurs when a pathway through which charges can move forms suddenly.

Lightning is Static Discharge Too
Lightning is a more dramatic discharge. Charge builds up in a storm cloud from friction between moving air. Negative charge in the lower part of the cloud induces a positive charge in the ground below the cloud. As the charge in the cloud increases, the force of attraction between charges in the cloud and charges in the ground increases. Eventually the air becomes charged, forming a pathway for electrons to travel from the cloud to the ground.

20.2 An electric current can only flow when there is a closed path in which electrons can move. There are 2 types of current; Direct (DC) Alternating (AC)

Current The continuous flow of electric charge is an electric current.
Charge flows only in one direction in direct current (DC). A flashlight and most other battery-operated devices use direct current. Alternating current (AC) is a flow of electric charge that regularly reverses its direction. Electric current in your home and school is mostly alternating current.

Conductors and Insulators
Electrical conductor – material through which a charge can easily flow. Example metal wire Electrical insulator – material through which a charge cannot easily flow. Example the plastic coating around metal wire Metals such as copper and silver are good conductors. Wood plastic rubber and air are good insulators

Resistance Resistance is opposition to the flow of charges in a material. The SI unit of resistance is the ohm. A material’s thickness, length, and temperature affect its resistance. Using a thick straw to drink a milkshake is easier than using a thin straw. Similarly, electrons flow more easily through a thick wire than they flow through a thin wire of the same material.

Resistance is greater in a longer wire because the charges travel farther. As temperature increases, a metal’s resistance increases because electrons collide more often. 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 – the difference in electric potential energy between two different points measured in volts (V), often called voltage

How are current, resistance and voltage related?
Increasing the voltage increases the current. Keeping the same voltage and increasing the resistance decreases the current. This relationship can be expressed in Ohm’s Law According to Ohm’s law, the voltage (V) in a circuit equals the product of the current (I) and the resistance (R).

Practice Ohm’s Law What is the voltage if the resistance is 5 ohms and the current is 3 amps?

V=15 volts V=? Given: current (I) = 3 amps Resistance (R) = 5 ohms
V=IR V=(3 amps)(5 ohms) V=15 volts

20.3 Electric Circuits Circuit – a closed path through which electrons can flow Circuit diagrams – use symbols to represent parts of a circuit, including source of electrical energy and devices that run by the electrical energy

Two Types of Circuits: Series & Parallel
Series circuits – charge has one path of flow. If one element stops working, none of the elements can operate. The amount of current is the same through every part. When any part of a series is disconnected, no current flows through the circuit.

Parallel circuit- Two or more paths through which charges can flow
Parallel circuit- Two or more paths through which charges can flow. Voltage is the same in each branch. When one branch is open current continues to flow through the other branches. If one element stops functioning in a parallel circuit, the rest of the elements still can operate. Homes are wired in parallel circuits

Electrical Power Power – the rate at which work is done or energy used. Electrical power is a measure of the rate at which electricity does work or provides energy Calculating power Power = Voltage x Current P=VI

Practice Calculating Power
An electric oven is connected to a 240-volt line, and it uses 34 amps of current. What is the power used by the oven?

Identify the variables
P=? I= 34 amperes V= 240 volts P=VI P= (240 volts)(34 amperes) P= 8200 watts

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