Charges, Polarization, and Coulomb's Law
There are two types of charges: protons (positive) and neutrons (negative). Like charges repel and opposite charges attract. Objects are usually neutral (not charged), meaning they have an equal amount of protons and neutrons. Objects can become charges (have and imbalance of electrons and protons) through three different ways: contact, friction, and induction.
An example of how something becomes charged through friction:
When a sweater rubs against your head, making your hair stand up…
The sweater rubs against your head, stealing electrons through friction. This means there are now more protons than electrons in your hair, making it positively charged. Since like charges repel, and there are more protons than electrons, your strands repel one another, causing them to stand up.
Induction:
Induction is a form of charging something without touching. This can be seen in lightning, which I will explain later.
Contact:
When you touch something or someone, like charges repel and opposites attract (what you feel when you shock someone).
Polarization is when opposite charges separate from each other to opposite areas of an object. The object is still neutral. This is why a balloon sticks to a wall after you rub it against your head.
The balloon is charged by friction when it rubs against your head, making it negative. When the balloon touches the wall, the wall polarizes. The positive charges are attracted to the negative balloon and the negative charges repel away from the balloon. We won't be able to understand this unless we know Coulomb's law (F=kq1q2/d^2). This means that force equals charge over distance. The smaller the distance between charges, the more force they fell. Coulomb's law tells us that the attractive force is greater than the repulsive force, because the opposite charges are closer to one another. Since there is a greater force between the opposite charges than the like charges, the balloon sticks to the wall.
Induction and polarization are the reasons lightning occurs. Air circulation causes clouds to polarize, with positive charges one top and negative charges on bottom. This causes the ground to do the same in response. The opposite charges between the cloud and the ground equalize, and energy is produced in the forms of light, heat, and sound (lightning).
An electric field is an area around a charge that can influence (push or pull) another charge.
A drawing of an electric field has arrows, which indicate the direction in which positive charges would move and how powerful the electric field is (closer lines = stronger electric field).
Electric shields protect the charges inside of electric fields from being influenced by outside forces. This is done when charges distribute evenly about the charge, so the charge inside the area will feel no force, no matter its location. A prime example of electric shielding can be seen in electronics with metal cases. Metal cases serve as electric shields, so the contents will feel no force.
Electric Potential, Electric Potential Difference, Capacitors
Electric Potential Difference, electromotive force, and voltage all mean the same thing. This means the difference in potential energy between two points. The greater the difference in charge between two objects, the greater the voltage. Voltage powers things. It is a measure of how much energy you can get out of one coulomb of charge (J/C or volts). You have probably seen appliances that say 100 v. This means that for every coulomb of charge produced, there are 120 joules in each coulomb.
Formula: V= PE/C v or J/C
A capacitor is two oppositely charged metal plates attached to a power source. The charges transfer and energy builds up between the two plates. Capacitors are the things in cameras the cause a flash. The reason cameras take time between each flash is because the capacitor has to build up charge after each time it is used, before it can flash again.
Ohm's Law, Types of Current, and Power
Current (I) is the flow of electrons in an electric circuit. It is measured in amperes (A). The equation for current is I = V/R (current equals voltage over resistance, also known as Ohm's law). Resistance is the hindrance of the flow of charges. When resistance is high, current is low, and vice versa. Resistance can be increased by making the current path longer and thinner, which makes it harder for electrons to move.
There are 2 types of current: AC (alternating current) and DC (direct current). Direct current is the flow of charges in one direction, while alternating current is when electrons move back and forth about relatively fixed positions. AC can be found in generators, while DC is found in batteries.
Power is voltage times current (Power = VI) and is measured in watts. Energy = (power)(time). We can use these equations to calculate how much it would cost to run a 60 watt light bulb connected to a 120 volt source continuously for 1 month if it costs 10 cents per kilowatt hour.
First, you must convert watts into kilowatts (move decimal 3 places to the left). This gives us .060 kw. There are 720 hours in a month (our time unit).
energy = (power)(time)
= .060 (720)
= 43.2 kwh
Now we know there are 43.2 kWh when you run a 60 watt lightbulb for a month. When we multiply the number of kWh times the cost per kWh, we get $4.32.
Parallel and Series Circuits
A circuit is any path along which electrons can flow. There are two different types of circuits: series and parallel. Series is a single pathway for electron flow and parallel has branches, each a separate path for electron flow.
Series Parallel
In a series circuit, the sum of resistance in a circuit is the sum of individual resistance along the pathway. If one device fails, the circuit is broken, stopping current and causing the other devices to stop working.
In a parallel circuit each device operates independently and connects to the same two points of the circuit. A break in one path will not affect the other branches in a parallel circuit. The total current is the sum of the current in the parallel branches. As the number of branches in a parallel circuit increase, the resistance of the circuit decreases. This can cause overheating and fire. Fuses protect from fire by melting when the current in a parallel circuit is too large. It is connected to the beginning of the circuit, so when the fuse melts, all of the devices will cut off.
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