Charges and Electricity Reflection

Charges and Polarization including Coulomb's law

Coulomb's Law

F=k(q1q2/d2)

Why does your hair stand up when you take off a hat?

Hat and hair charged by friction. The like charges repel each other and make the hair on your head stand up

Induction? Other ways to charge things?

Induction steals the electrons from an object and makes the object positively charged. Friction and contact are the other ways of charging.

The Difference Between AC and DC

AC stands for 'alternating current'. This means that the current will alternate directions.
DC stands for 'direct current'. This means that the current will keep the same direction throughout.

What is Voltage?

Potential Difference.

Change in PE/ charge

V=(change in PE)/q

Units
Volts (V) = Joules/Coulomb

Energy given to each Coulomb of charged, measured across two points.

What is current?

current is the flow of charge

Units
Amps (A)= Coulombs/second

Current goes through the circuit

What is Resistance?

Resistance is the resistance to the flow of charge.

Unit
Ohms (omega symbol)

Things to Remember:

The electrons that flow are already in the circuit. The actual speed of electrons is extremely slow (think snail's pace).

The Energy/Electric Field travels at neat the speed of light (NOT the electrons themselves... REMEMBER!!! Snail's pace!!)

When something is charged, it means that there is a 'imbalance of protons vs. electrons. To charge an object, charges must be transferred.'

There are 3 methods of charging:

1.) Contact

2.) Friction

3.) Induction

When something is Polarized it means that the the charges within an object have rearranged themselves and that the Net charge still equals zero. This does not mean that the object is now charged.

Electronic Shielding 

(Electric field/ Electric force inside a conductor=0)

Ohm's Law is V=IR

Coulomb's Law

F=k(q1q2/d*2)

The changes in distance affect changes in force shown by:

2d=1/4F and 1/2d=4F

Power=VI

Different filaments have different resistances. The resistance of something can increase in three ways:

1.) longer

2.) thinner

3.) both longer and thinner

What was difficult for me this unit was my organization, or more realistically the lack-there-of. my non-existent organization this unit is making my efforts in studying harder and harder. Next unit, I really need to make a designated binder to keep my quizzes, notes, and handouts in. My poor organization skills are going to kill my grade.

http://goblues.org/faculty/lawrencel/files/2012/08/ONQ-6-2-Answers1.pdf

http://goblues.org/faculty/lawrencel/files/2012/08/ONQ-6-1-Answers.pdf

Ohm's Law

This cartoon helps me remember that Ohm's law has to do with resistance. V=IR is the equation for Ohm's law. V= voltage, I= current, and R= resistance.

Units for V, I and R

The Filament in a light bulb is what gives resistance. The thinner, longer or both thinner and longer the filament is, the more resistant it will be. Resistance also increases the hotter it gets.

Voltage Resource

In this video, the person speaking makes clear some very important points about voltage.

• V= Potential Energy/q
• When you are dealing with volts, you are specifically dealing with energy
• and it is energy per charge
• 1 Volt= 1 J/C
• deals with 2 points

I really liked how he said "when we push, from A to B, we are exerting a force through a distance... We are expending energy, and the energy that we expend goes into potential energy and stored as electrical potential energy... There is more Potential Energy at point B then it had at point A." This was really helpful to hear and see him drawing it out. His comparison between gravity and electrical current was very helpful as well (when he is talking about the toaster). I wish he did more with solving problems, though. Pretty helpful, though, with getting the main ideas down.

Mousetrap Car Lab

This car made 2nd place in our class and 8th out of 18 cars with a time of 3.72 seconds.

Newton's 1st Law of Motion:

An object at rest, tends to stay at rest and an object in motion tends to stay in motion unless acted upon by an outside force.

Applied:

The mousetrap car will stay at rest unless triggered. The mousetrap in motion will continue to move unless affected by an outside force. The outside force of friction between the wheels and the floor caused the car to slow down and eventually stop. 

Newton's 2nd Law of Motion:

Force is directly proportional to acceleration, and indirectly proportional to the mass of an object.

Applied:

We purposefully made our car have less mass because the less mass, the force will cause a greater acceleration to occur. 

Newton's 3rd Law of Motion:

For every action, there is an equal and opposite reaction.

Applied:

Wheels push backward on ground, ground pushes forward the wheels. String pulls axle, axle pulls string. Axle spins wheels, wheels spin axle. 

Two types of friction present:

1. Static
2. Kinetic

Static friction is the type of friction that keeps and object from moving. This would occur when the mousetrap car would not move.

Kinetic Friction is the type of friction that occurs when an object is moving. This would occur when the mousetrap car was in motion.

In regards to friction, my partner and I encountered a problem where the car would not move very far, due to a lack of friction. We fixed this by adding balloons to the rear wheels which gave the wheels more friction, but also more traction so that they would be able to move the car. 

My partner and I found a model where the car had 4 wheels, and thought that that would be a good idea. Using 4 CD's, this made for an easy configuration. The smaller the wheel, the less tangential velocity it has, so using CD's we were able to get a lager tangential velocity than we would have if we used bottle caps (as some other people did). 

Energy can neither be created nor destroyed. This means that energy can never be lost, just transformed into something else. When our car slowed down, energy was transformed into either sound or heat (another form it could take when 'energy is lost' is light, but that did not apply to our car). The car's potential energy can never be greater than what it had in the beginning, which will be the same amount of kinetic energy for the car. 

At first, our lever arm was as long as the distance between the bar of the trap to the back axle. This was a problem because once it unwound from the axle, it would stop the car. By lengthening the lever arm, we found a solution for that problem, but by increasing the lever arm, we lessened the force which meant that our car would go slower. 

By using a CD, rather than a hoop, there was less rotational inertia for our wheels, because the mass was closer to the axis of rotation than the hoop's mass would have been. 

We can't calculate the potential energy that was stored in the spring or the kinetic energy, nor the amount of work the spring did not the car, nor the force the spring exerted on the car to accelerate the car.

REFLECTION:

Our final design was very similar to our original plan. One change we made was instead of using soda-can-taps to keep the front wheels in place, we used zip-ties. The zip-ties were better to use because they were more predictable, and were easier to adjust. Another change was that instead of using balloons to keep the front wheels attached to the axle, we used electrical tape. This was better because the electrical tape was a smoother surface which enabled us to put the wheels where we wanted to and kept them in that place (but also allowed for changes if we needed to). The electrical tape did not completely solve the problem of keeping the wheels centered, but it was much better than the balloons we were using. 

One major problem we encountered was our car stopping because the lever arm was too short. We solved this by increasing the length of the sting used for our lever arm. Another problem we had was keeping the wheels where we needed them. We originally used balloons to keep the front wheels attached to the front axle, but they would not keep the wheels where we needed them, causing the car to run into the wall a lot. By replacing the balloons with electrical tape, this enabled us to more predictably adjust the wheels and keep them where we needed them to be.

If I was to do this project again, I would spend more time on the length of the lever arm. I noticed that by the time the car slowed to a stop, there was still extra string. I would have shortened the lever arm, making the force exerted greater, as well as the acceleration. 

Unit 5 Reflection

This unit, we studied Work and Power, the relationship between Work and Kinetic Energy, the Conservation of Energy, and Simple Machines.I feel really good about this unit, and my only difficulty with this unit is remembering what I have to show to get full credit on tests/quizzes, though I know how to get to the right answer. My effort has been good, though my organization needs to get better.

Real world application:

I can connect this unit with daily life very easily. While walking around campus, I notice when I either go up stairs or an inclined plain, knowing that, when going to the same place, the stairs and the inclined plain require the same amount of work, they just have different forces and distances.

Work and Power

Equation: Work=(force)(distance)
Units: Joules (J)

Equation: Power=work/time

Units: Watts

For work to happen, force and distance must be PARALLEL!!

Example problem:

If a 600N person walks up stairs that are 4 meters high, how much work is done?

Work=F*d

Work=600N*4m

Work=2400J

When is there more work? When a person goes up the stairs to third or when the elevator carries them to third?

The work required is the same. The upward force against gravity (or weight) is the same, and the distance that is parallel to that force (height) is the same, therefore work is the same. Horizontal distance does not matter because it is perpendicular to the upward force, and therefore contributes nothing to the work done.

When considering work, only the height matters.

Work happens to an object when force is exerted on that object over a distance. The equation for work is work=force*time, and it is measured in m/s or Joules. For there to be work, the force and distance must be parallel. In this example, the person's wright, or force, is vertical, and therefore for there to be work, the distance must be vertical as well. Work does not happen when the distance is perpendicular to the force, or when there is no distance.

If the force of the person is 4N, and the staircase is 15m tall, how much work will there be? 

work=force*distance

        = 4N*15m

work= 60 Joules

Power is how quickly work is done. The equation for power is Power=(work)/(time) and is measured in Watts.

If it takes the person 10 seconds to reach the top, how much power is exerted?

Power=(work)/(time)

=60J/10s

Power=6 Watts

Work and Kinetic Energy relationship

Formulas:

Kinetic Energy= (1/2)mv² 
Change in KE=KE_final-KE_initial
Change in KE=work
Work=fd

Why do airbags keep us safe?

KE=(1/2)mv² 

Change in KE=KE_final-KE_initial

You go from moving to not moving regardless of what you hit, therefore the change in Kinetic Energy is the same regardless of what you hit

Change in KE=work

Since the change in Kinetic Energy is the same, the work is the same regardless of what you hit.

work=fd

Airbag work=f*d

Dash work=f*d

Since the work is constant regardless of what you hit, the airbags increase the distance it takes to stop you, and thus decrease the force. Small force means less injury.

Conservation of Energy

Formulas:

Potential Energy=mgh

Change in PE= Change in KE

How does potential energy change if someone jumps off of a cliff?

As they fall, PE goes down and KE goes up by the exact same amount. The sum of PE and KE at any point will be equal to the PE they had when they were at rest at the top. Energy must be conserved.

Top: let PE=1000J, KE=0

¼ of the way down: PE=750J, KE=250J

½ of the way down: PE=500J, KE=500J

¾ of the way down: PE=250J, KE=750J

Just before impact: PE=0J, KE=1000J

When PE goes down KE goes up by the exact same amount.

Energy cannot be created or destroyed, but it can be transferred to other forms such as heat, light, and sound.

************PE is equal to the work required to raise an object to a certain height*************

PE=mgh

Work=F*d

Work=mg*h (mg is the amount of upward force required, h is the distance (height) it is lifted)

Machines

Purpose of a ramp: A ramp increases the distance over which a force is applied, thereby decreasing the amount of force required to do the same amount of work (lifting an object to a certain height).

Why do bolt cutters have such long handles but such short blades?

The work done on the handles equals the work done by the blades (Work in=Work out). The long handles allow for a large distance over which the force is applied. The short blades do the same work, but over a very short distance. This shorter distance means the force the blades apply is much greater than the force on the handles.

In this picture, I drew two mice using a pulley system to lift up pieces of cheese. The mouse on the left has almost a whole block of cheese, and the mouse on the right has the sliver that is cut out of the block. The mouse on the left has a longer rope and more pulleys while the mouse on the right has a shorter rope and has one pulley.

The machines in this picture are the pulleys. What a machine does is reduce the force it takes to do a job. Notice that it reduces force and NOT work. A machine does this by increasing the distance it takes for complete the job, making the force smaller. We know that this happens because workin=workout. this means that force-in*distance-in=force-out*distance-out.

Since the left mouse's rope is longer, that mouse can pull with less force to get the same sized cheese (it is not the same sized in the picture) up than the mouse on the left has to. The big block of cheese signifies that since there is more distance to cover, the left mouse will be able to pull up a larger piece of cheese than the right mouse.

 ******FOR MORE MACHINE NOTES... WATCH THE VIDEO AT THE TOP*************


 Efficiency=Work out/Work in

Machine Recourse

In this picture, I drew two mice using a pulley system to lift up pieces of cheese. The mouse on the left has almost a whole block of cheese, and the mouse on the right has the sliver that is cut out of the block. The mouse on the left has a longer rope and more pulleys while the mouse on the right has a shorter rope and has one pulley.

The machines in this picture are the pulleys. What a machine does is reduce the force it takes to do a job. Notice that it reduces force and NOT work. A machine does this by increasing the distance it takes for complete the job, making the force smaller. We know that this happens because workin=workout. this means that force-in*distance-in=force-out*distance-out.

Since the left mouse's rope is longer, that mouse can pull with less force to get the same sized cheese (it is not the same sized in the picture) up than the mouse on the left has to. The big block of cheese signifies that since there is more distance to cover, the left mouse will be able to pull up a larger piece of cheese than the right mouse.

Work/Power Recourse

Work happens to an object when force is exerted on that object over a distance. The equation for work is work=force*time, and it is measured in m/s or Joules. For there to be work, the force and distance must be parallel. In this example, the person's wright, or force, is vertical, and therefore for there to be work, the distance must be vertical as well. Work does not happen when the distance is perpendicular to the force, or when there is no distance.

If the force of the person is 4N, and the staircase is 15m tall, how much work will there be? 

work=force*distance

        = 4N*15m

work= 60 Joules

Power is how quickly work is done. The equation for power is Power=(work)/(time) and is measured in Watts.

If it takes the person 10 seconds to reach the top, how much power is exerted?

Power=(work)/(time)

=60J/10s

Power=6 Watts