Magnetism

Standard 6.4: How can we use the Right Hand Rule #2 to predict how moving charges are affected by magnetic fields (the direction of the magnetic force on the moving charge)?

Right Hand Rule #1

Right Hand Rule #2

   
  In physics class, we discovered the Right Hand Rule #2 which allows scientists to discover the direction of a magnet's force on a current. Whenever a moving charge comes in contact with another magnet's magnetic field, it experiences a force. "F" is a force on a moving charge as a result of the magnetic field and in this rule, the palm of a hand is represented as the force. In terms of using the Right Hand Rule #2, it would be correct to say that I place my thumb in the left direction because a current is moving left. I curl my fingers away from me going towards the paper on top of the wire, and my fingers curl towards me under the wire. This represents the magnetic field. When I open my fingers, my palm is  facing toward the paper, proving that the magnetic field is going that way. The Right Hand Rule #2 identifies all variables needed to figure out the characteristics of a magnetic field. 

Voltage

The real question is... how is it possible that we can play games, take notes, surf the web, and take videos on our Ipads? How does the Ipad function? Well the Ipad contains a Lithium-ion battery that produces the energy for the Ipad to run on. But how does this battery charge? There's some key information needed in order to understand:

• What is a charge?

• a cell with less elections than protons (positively charged) or a cell with more electrons than protons (negatively charged)

• Who do charges transfer?

• touch: friction
• **like charges repel, opposite charges attract**
• when charges transfer, only electrons move

• Affinity

• cells are either "electron loving" or "electron giving"
• meaning that positively charged cells with less electrons tend to give their electrons away or negatively charged cells with more electrons tend to keep their electrons 
• different materials are naturally positive like human hair, wool, and air
• different materials are also naturally negative like rubber, tape, and polyester

• Voltage

• electric potential that is attractive and repulsive; a field that surrounds charged objects
• voltage is to electricity as height is to gravitational potential energy
• electric potential energy (Ue) is the charge times the voltage.. this is what gives the Ipad juice 
• the higher the voltage, the more charge, the greater the Ue

Real World Connection:

       A Lithium-ion battery is supposed to last 10 hours of use before needing to charge. The battery has a high power density that gives you a long battery life in a light package. When the Ipad is plugged into a charger which is plugged into a wall outlet, electrons are flowing. We use this "mountain" analogy to describe voltage and the charge's path. Objects with a positive charge give their electrons away and "run down the mountain." Objects with a negative charge "run up the mountain" to gain more electrons. The cycle continues when charging the Ipad. The negative charges in the battery run uphill and the positive charges run downhill.

http://www.apple.com/batteries/

Ipad Battery

Projectile Motion Reflection

     In this week's lab, I learned about projectile motion: how it works and how to analyze it. A projectile is an object in which only the force of gravity is acting upon it. Its general path of motion is a parabolic trajectory.

      Using the Video Physics App on our Ipads, we analyzed the path of motion of a basketball when shot in the air. After taking the video, we plotted points along the ball's path through the air. We then used these points to graph the x and y components of the ball's path.

X-Component

      The top graph shows the x position over time. The change in position over the change in time is also known as the slope. The slope is constant in this case. The bottom graph shows the x velocity over time. Because acceleration is the change in velocity over the change in time and we know that the velocity is also constant, there is no acceleration and no net force.

Y-Component

       The top graph shows the y position over time. The change in position over the change in time is the slope and velocity. The graph shows that the slope is not constant; therefore, the object is accelerating. The bottom graph shows the y velocity over time. The decreasing line crosses the x-axis, and at that moment the object has a velocity of 0, which means it is at its peak. The rest of the time it is decreasing.

Forces in 2D and Circular Motion

Big Questions

1. What does it mean to analyze forces in 2D?

          Analyzing forces in 2D means that there are two dimensions that need to be measured, an x-component and a y-component, which make up a force at a certain angle. Once we break the two dimensions up, we can use SOH CAH TOA to find the magnitude of Fx and Fy. Then we can find Fnetx and Fnety to calculate the total force. The photo shows an example:

2. How do forces cause objects to move in a circle?

          Centripetal force is a center-pointing force that causes objects to move in a circle. The force and velocity act perpendicular to each other. The object does accelerate; however, it is not the magnitude that changes, but the direction. The magnitude remains at a constant speed, but the direction keeps changing. Without this centripetal force, an object would continue to move in a straight line, tangent to the circle. In the hover disk lab we did this week, we spun the disk connected to a rope in a circle at a constant speed. When we let go, the disk continued moving, but in a straight line since there was no centripetal force.

3. What does it mean to be an orbit? How do satellites orbit planets? How do planets orbit the sun?

          In the hover disk lab, the disk orbits the person, just as satellites orbit planets and planets orbit the sun. When something is in orbit, it is constantly moving around another object in a circle. When satellites orbit planets, the centripetal force acting on them is gravity, pulling them to the center. A satellite in space is actually moving at a free-fall toward the planet. The satellite moves in a sideways direction and falls into orbit. The planets orbit the sun in the same way.

Newton's Three Laws of Motion

Over the past 2 weeks, we have been deriving Newton's three laws of motion by performing the hover disk lab and the fan cart lab.

Newton's First Law

• An object at rest or traveling at a constant speed will continue to do so, unless a net force acts on it.

• Any object moving at a constant speed or at rest, has no net force acting on it. (Fnet = 0). The hover disk lab allowed us to see how this law works. When the hover disk is at rest or moving at a constant speed, it will not have a change in velocity, or accelerate, because there is no net force acting on it.

• Although there is no net force acting on the object because the object is at rest, there are still many forces acting on everything. In the interaction diagram, you can see that there is a gravitational force acting down on person 1, person 2, the earth, and the hover disk. But what allows them to not fall through the ground is the normal force acting up on them.

• Here is an example where the object remains at a constant speed. Here we see that there is so net force on the object; however, there are still gravitational and normal forces acting on oerson 1, person 2, the disk, and the earth.

Newton's Second Law

• Force = mass (acceleration)

• From the fan cart lab, we derived this equation. We measured the acceleration of a 0.3 kg fan cart when turned on high with a constant force of 0.15 newtons. We then added weights of different masses to the fan cart to find different accelerations. From this we found that acceleration is the change in velocity divided by the change in time.

Collected Data

0.3 kg cart: 0.450 m/sec^2

0.4 kg cart: 0.380 m/sec^2

0.6 kg cart: 0.208 m/sec^2

1.1 kg cart: 0.146 m/sec^2

1.4 kg cart: 0.123 m/sec^2

1.7 kg cart: 0.105 m/sec^2

• To derive this equation, we found that as the mass increases, the acceleration decreases, meaning that mass and acceleration are inversely proportional. We also had to take into account that the force remains constant; therefore, we came up with the equation F = ma.

Newton's Third Law

• When two objects interact, they exert equal and opposite forces on each other.

• These forces are:

• equal in magnitude

• opposite in direction

• same type of force

• The hover disk lab helped us see how this law is true. When the hover disk is moving with a net force and is stopped by person 1's hand, the disc and the hand with exert and equal but opposite force on each other, which will be a normal force. 

Real World Connection

Here is a website that tells how Newton's laws of motion relate to a dancer.

http://physicsofballet.homestead.com/newtonlaws.html

Impulse Lab

Introduction
In this week's lab we crashed an empty red cart into a force-probe with aluminum (to slow down time) attached to a ring stand. We found the velocity of the cart before and after the collision. We then found the change in momentum, also known as impulse. We did this to measure the relationship between impulse, force, and time during a collision.

cart with aluminum

force prob with aluminum

Key Info

• impulse: a change in momentum (another way of representing the conservation of momentum)
• impulse is measured in Joules

• momentum = P

• impulse equation: J = Pfinal - Pinitial

• mass of car: 0.25 kg

• velocity is measured in meters per second (m/s)

Big Question

What is the relationship between impulse, force, and time during a collision?

After we collided the car with the aluminum ring, we found the velocity of the cart in the collision before and after using the sonic range finders. Then we found the momentum before and the momentum after to find the impulse. Then we recorded the area, also known as the integral, under the Force vs. Time graph. 



Here is what our data looked like:

From here, we did the calculations and found that the area of a force vs. time graph is impulse. So now the equation for impulse can be written as Impulse = Force (Time) or J = F(t).

Analyzing Data

Concluding Ideas
       Impulse and Are are the same! After determining the area of a Force vs. Time graph and calculating the impulse, the answers come out to be equal. Forces are equal and opposite; therefore, the impulse remains constant in a collision by increasing the time and decreasing the force. In the collision, the metal sheets increased the time and decreased the force.

Real Life Connection
        Boxing uses this same principle of minimizing the effect of a force by extending the time of the collision. When a boxer recognizes that he will be hit in the head by his opponent, the boxer often relaxes his neck and allows his head to move backward upon impact. This known as "riding the punch." A boxer rides the punch in order to extend the time of impact of the glove with their head. Extending the time results in decreasing the force and then minimizing the effect of the force in the collision. Now that is physics in action!
  
Here's a clip from the final fighting scene in the movie Rocky Balboa 

Collisions Lab

The BIG Question
   
    What is the difference between the amount of energy lost in an elastic collision vs. inelastic collision?
    What is a better conserved quantity- momentum or energy?

Background

• scalar quantities: magnitude (mass, energy, temperature)
• vector quantities: magnitude and direction ( + rightward, - leftward)
• types of collisions:
• elastic- objects bounce off each other
• inelastic: objects stick together
• momentum: p = m (v)
• mass (velocity)
• mass is measured in kilograms (kg)
• velocity is measured in meters per second (m/s)

• In this week's lab, our goal was to find the difference between the amount of energy lost in an elastic collision and inelastic collision. To test this, we put two cars of the same mass on opposite sides of a track, one red and one blue. We set two range finders on the ends of the track, which measure the velocity of the cars. These range finders were plugged into the electronic force probe, which was then plugged into the computer. The application on the computer graphed the velocity of both cars before, during, and after the collision.

• For the inelastic collision, we pushed the red car to the right toward the blue car, which then stuck together with velcro. Both cars continued to move together to the right. 

• For the elastic collision, we pushed the red car with a spring toward the blue car to the right. In this collision, the red car followed the blue car at a much slower pace once they hit, while the blue car continued to move to the right.

• We recorded our data and found the total momentum (p = mv) and the total kinetic energy (KE = 1/2mv^2) of both collisions.

• Then we found the percent difference of both the amount of energy and momentum that left or entered the system for both elastic and inelastic collisions.

• Percent difference: 

• Kinetic energy: (Total after - Total before)/(Total after + Total before / 2) (100)

• Momentum: (Total after - Total before)/(Total after + Total before / 2) (100) 

• We found that momentum is a better conserved quantity rather than energy because more energy is given off during a collision. Momentum has a smaller percent difference.

• We also found that inelastic collisions lose more energy than elastic collisions

Real Life Connection: 

      The game of pool uses the exact same concepts of momentum in collisions. When the white ball hits another colored ball, the hit and the colored ball keeps moving. The goal of the game is to get all the colored balls in the holes without knocking the white ball in with them. The velocity of the balls plays a key part in strategizing.