UNIT 6 REVIEW

In this Unit, we learned 6 major topics:

1. Charges and Polarization (Coulomb's Law)
2. Electric Fields and Electronic Shielding
3. Current
4. Electric Potential/Electric Potential difference
5. Capacitors
6. Ohm’s law and Resistance
7. Power
8. Parallel and Series Circuits

Charges and Polarization

When there are more electrons, like the second image above, the object is negatively charged

Whe there are more protons like the third image above, it is positively charged

Like charges attract each other

And opposite charges repel each other

How do these charges travel and move? There are 3 ways:

1. Direct contact (shocked someone by touching them)
2. Friction (steal electrons from eachother)
3. Induction charge without contact (lightning) shown below

Why does our hair stand up after putting on a sweater?

• Sweater steals electrons through friction, therefore becoming negatively charged
• Therefore, each hair is becoming positively charged
• The now positively charged hairs will repel because they are like charges, therefore will stick up

Polarization

When an object is polarized, it is not charged, the charges are simply rearranged
Therefore, the net charge is still 0

Conductor vs Insulator:

• Conductors let charges move through them
• Insulators stop charges from moving

Conductors can become polar

Polarization is when charges in an object separate from each other

In the image above, the ball is now polarized because a negatively charged pole separated the charges to either side

We can see polarization in LIGHTNING

The movement of particles in the clouds causes friction which causes the cloud to polarize. Because the electrons move to the bottom of the cloud, the positrons from the ground become attracted to the cloud, therefore polarizing the ground as well. The cloud and ground want to equalize, so once the pull builds to a certain point, an electric shock will transfer charge to equalize them, which we see as lightning

How do lightning rods protect structures?
Lightning rods conduct the positive charge on the ground's surface and bring it to a high point to reach further into the air. If lightning strikes, it will take the path of least resistance to the ground. The lightning rod provides much less resistance than a wooden structure.

Coulomb's Law states that force between any 2 charges is inversely proportional to the distance

F= (k q1 q2)/d ²

where q stands for charge

For the question why does saran wrap stick to metal more than plastic....

As you can see above, the saran wrap (in the purple) has a negative charge (because as it was unrolled, friction caused it to steal electrons)
When you bring the negative wrap to the bowl, the positive charges will go toward the wrap and the negative charges will go away from it.
The bowl is still neutral, however, it is now polar.
The glass bowl will remain polarized.
However, metal in a conductor. So once the wrap comes close to the metal bowl, the negative charges will travel straight to the ground
But why does it stick? Coulomb's Law!
The distance between the attractive force is small, so it will have a large force
The distance between the negatives is much larger, therefore will have a much smaller force
Therefore, the attractive force will be greater, causing the plastic wrap to stick

Electric Fields
Electric fields are the area around the charge that can influence (push or pull) another charge.
How do we draw electric fields?

For the proton, arrows face outward, and for the electron, arrows face inward

• If a positive charge were close to the positive charge on the left, it would repel away
• If a positive charge were close to the negative charge on the right, it would be attracted inward

And notice how as you travel further from the charge, the lines get farther apart. This indicates the strength of the push/pull decreasing

Electronic Shielding
How can we keep things like microchips in computers safe from outside charges?
Electric Shielding!

When surrounded by a conductor, for example, metal box. Positive charges will accumulate all over the box. Because of electric fielding, the sensitive equipment inside will not feel a force. This is because since the metal's charges are all evenly distributed, force will remain the same AT ALL points within the container.

Current

Current is the flow of charge

Charge flows when there is a difference of potential energy between two points, and it will continue as long as there is electric difference between these two points.

It is important to remember that voltage causes current, current does not cause voltage

Current comes in two forms, AC and DC (not the 70's hard rock band)

• AC - Alternating Current  
• switches direction
• used in batteries
• DC - Direct Current
• used in wall sockets

How do batteries work and why do they eventually stop working?

Batteries are able to have current because they have a difference in electric potential or a voltage between the positive and the negative side. Overtime, the difference decreases and eventually there is no longer an electric potential difference. When there is not an electric potential difference, there is no current, which is why batteries stop working.

Why do flashlights get dimmer?

Because as stated above, as a battery wears out, it's voltage decreases.  We know that voltage causes current, so as voltage decreases, current decreases as well.

Electric Potential
A charged object has electric potential through it's location in an electric field.

Electric Potential Difference is the difference in

∆PE = ∆KE

Electric Potential = PE / q

Voltage= ∆PE / q

However, as an electron gains potential energy, it does not get kinetic energy.  Instead, it goves it our to the circuit as power.

Capacitors

In order for a camera to produce a flash, a large amount of energy is needed. This energy cannot be created in such a short period f time. Therefore, it requires a capacitor. Capacitors build up energy over a period of time and release it all in a short burst.

Speed of electrons - are electrons as fast as the speed of light?

No - Electrons moving through a conductor are moving very slow. However, they move all together like a train, so when one moves even slightly, every other electron in the conductor will move almost immediately

Ohm's Law
The amount of current that exists depends on the voltage as well as the electrical resistance.
This brings us to Ohm's Law stating that...

V = I * R

The electric potential difference between two points on a circuit (V) is equal to the product of the current between those two points (I) and the total resistance of all electrical devices present between those two points (R). 

Resistance is affected by heat and thickness of the wire.
Therefore the resistance of a wire depends on the thickness, length and material.

• Heat increases resistance
• Width (of the wire) decreases resistance

Why do lightbulbs burn out when you first turn them on?
When a lightbulb has been off, the filament has cooled.  Therefore, it has little resistance.  When a large current travels though, since there is little resistance, the filament will blow.
Electric Power
Power is voltage times current 

P = VI

And power's relationship with energy is...

Power = Energy/ Time

Series and Parallel Circuits
A circuit is a path along which electrons can flow
There are two different kinds of circuits, series and parallel
These circuits have a box supplying 12 V of voltage

Series

• voltages add
• current decreases?
• brightness decreases
• resistance adds
• when one lightbulb is removed, the whole circuit is opened, therefore stops working

Parallel

• voltages are the same
• current adds
• brightness remains constant
• resistance decreases by half

Houses use parallel circuits so you can use some outlets without having to use them all.  However, because current increases with each device added on, it can reach dangerous levels.  This is why we use fuses.

A fuse is connected to the circuit where it is still in series (before it breaks off into separate sections).  If the current reaches a certain point that is too great, the fuse will melt, therefore breaking the circuit, and cutting al devices off from the voltage source.

OVERVIEW - Units
Charge - q (Coulomb)
Voltage - V or J/c (volt or joules per charge)
Current (I) - A
Power - W (watts)
Resistance - Ω (ohm)

Ohm's Law Resource

This video was very helpful because it really focuses on the basics of Ohm's Law as far as equations go, and simple ways we use it in problem solving.  The diagrams were also very clear