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=**Wiki # 1 - Projectiles**=

If an object is shot into the air with gravity being the only force acting on it, it is called a projectile.

Projectiles may be thought of to have two separate components, vertical and horizontal, and they act independently of one another. When a projectile moves, it travels in a parabolic path (similar to a rainbow for those of you who are more imaginative).

The horizontal velocity does not change, it will stay the same because there is no opposing force. So in a place with no gravity on object will go on and on. But, on earth, we have gravity acting at -9.8m/s^2 that pulls objects down. Vertical velocity changes because of gravity.

Remember that all forces going to the same direction has to have the same sign.

Inertia is the tendency an object to remain unchanged.

A projectile will try to remain the same due to inertia, but gravity will cause it to change. =**Wiki # 2 - Forces and Newton's Laws**=

Forces is a great part of dynamics, and Sir Isaac Newton was one of the founders of dynamics. His three laws of physics can be summarized to the following:

1. The Law of Inertia: An object at rest will stay at rest, while an object in motion will stay in motion.

How does this work?

Imagine you're now a ball on a newly waxed floor, so friction is minimum. You're on this floor for one year, ten years, or even a hundred years, and you haven't moved an inch. Why? Because no one has exerted a force on you yet. So after one hundred years, someone discovers you, the ancient ball, and decides to give you a push. Now off you go, rolling across that floor, and not likely to stop, because there is no other force acting on you. In other words, you can only move if a force is exerted on you, and you can only stop if another force that's going in the opposite direction is exerted.

2. Newton's second law of motion states: The acceleration of a body is directly proportional to the net force exerted, but is inversely proportion to its mass.

Now going back to our previous ball example:

So after one hundred years, someone discovers you and gives you a good kick. He exerts a force of 10 N, and your mass is 2 kg. Since F=ma, your acceleration would be: 10=2(a), and your accerleration is 5 m/s^2. Now imagine that you sudden got really heavy and expanded, your mass is now 5 kg, and that same person exerts the same force of 10 N onto you, your acceleration would now be: 10=5(a), and that will give you 2 m/s^2. As you can see, if the same amount of force is exerted on objects with different masses, the object with the higher mass would have a smaller acceleration and vice versa. The bigger you are, the smaller your acceleration.

3. Newton's Third Law: The Law of Action-Reaction. This law states, when one body exerts a force on a seond body, and secondy body exerts back a force that's equal in magnitude, but opposite in direction.

So, we are at the part where someone decides to give you, the 100 year old ancient ball, a good kick:

This guy decides to kick you so hard that he exerts a force of 500 N! As he kicks you, he misses and kicks a wall. Oops, but now look at the guy, "OWWWWW!" Now the guy thinks he broke his toe, which he probably did, according to the Law of Action-Reaction. This unlucky guy here exerted a force of 500 N onto a wall, and what happened was the same 500 N of force was exerted back to him from the wall, and therefore causing great pain to this unlucky individual. The moral of this lesson is, look before you kick.

That's it for now with forces, I'll tell you more later. Hopefully that guy heals his toe.

ps. Check out "Newton's Law Motion Song" made by Paul Chung

http://www.youtube.com/watch?v=fuHWQBaifpk

ps.ps. A good photo of Newton's thrid law is:

http://www.dynamicmarching.com/custom/action_reaction_2.JPG

=**Wiki #3 - Forces**=

A 7-kg box is released on a 45° incline and acclerate down the incline at 2.3m/s2. What is the coefficient of friction?

First, find Fp Fp = mg sin(theta) = 7kg*9.8m/s2*sin(45) = 48.5N

Second, find Ff Fnet = Fp - Ff ma = Fp - Ff 7kg*2.3m/s2 = 48.5N - Ff Ff = 16.1N - 48.5N = -32.4N

Third, find the coefficient of friction Ff = mgu 32.4N =7kg*9.8m/s2*u u = 0.47

The coefficient of friction is 0.47

=**﻿Wiki # 4 - The Conservation of Energy**=

As far as we're concerned, there are two types of energy, potential energy and kinetic energy. Kinetic energy, as the name suggests, comes from an object in motion. Potential energy, on the other hand, can be gained by doing things such as raising an object high up in the air.

Now, the law of conservation of mechanical energy states: "The total energy of a system is neither increased nor decreased in any process. Energy can be transformed from one form to another, or from one body to another, but total energy remains the same." To think of this in a really big picture, we can think that when the universe started all those billion years ago, it came with this immense amount of energy. All these years, this immense energy travels around, changes form from heat to sound to motion, and in the end, it's still the same energy from the start of universe. If this sounds too abstract or too big to imagine, think of this: you're in a group passing a ball around, the ball gains potential energy. Now oops, someone drops the ball and hits the ground, some of that potential energy gets transformed into sound, and now as it rolls away, the ball now has kinetic energy. Since energy is conserved no matter what, this kinetic and sound energy should add up to the potential energy from the beginning.

Formula-wise, we have this:

KE + PE = KE' + PE'

The sum of initial kinetic and potential energy is equal to the sum of final kinetic and potential energy

Now have fun with conservation of energy :)

=**Wiki #5 - Momentum**=

What is Momentum? Momentum, p, is defined as the porduct of the mass of the object and its velocity. It could be also written as p=mv. It is a vector because it is the product of a scalar, mass, and a vector, velocity. It has the unit of kilogram-metres per second (kgm/s).

Momentum is the same as the newton's second law. Fnet = ma Fnet= m(Δv/Δt) Fnet = m(Vf-Vo)/Δt Fnet= Δp/Δt The rate of change of momentum of a body is equal to the net force applied to it. The conservation of momentum states that the momentum before the collision is equal to the momentum after the collision. m1v1 + m2v2 = m1v'1+ m2v'2 The total momentum of a closed system stays constant.

What is Impulse? Impulse is the product of net force and interaction time. It is also equivalent to the change in momentum because impulse =ΔF*Δt ΔF= Δp/Δt Δp= ΔF*Δt impulse= Δp It has the unit of the newton-second (Ns). On a net force and time graph, the area gives the magnitude of the impulse.

Elastic collision: the total KE before the collision is the same as the toal KE after the collision the KE is conserved in elastic collisions Inelastic collision: the KE is lost or change into other forms of energy

If you like to watch a video instead of reading, introduction to momentum : [] 2 dimensional momentum problem : []

=**Wiki # 6 - Torque**= What is torque? According to dictionary.com, torque is " something that produces or tends to produce torsion or rotation; the moment of a force or system of forces tending to cause rotation." But of course we're going to go further into that.

Torque is measured in Newton-metres (N*m). What happens when there is torque is that a force is exerted at a certain distance away from a pivot. The greater the distance away from the pivot, the greater the torque (assuming the force exerted is constant). So now you have a force, and you use this force a certain distance away from a pivot point, and when you multiply these two things together, TADA, you have torque.

If you're getting confused now, you're probably not alone. Let's look at this another way: Let's say you're at the Amazing Race, and one of the challenge is you have to push open this gigantic door that leads you to the next clue, and you, getting really excited and now in the lead, obviously wants to push open this door. You start pushing at the point closest from the hinge of the door, you push.. push... and PUSH... and still can't open the door. Then at this point, the person who's second catches up and pushes the same door. Except this guy pushes at the point furthest away from the hinge, and with a gentle push of his hand, the guy opens the door, and now he's in the lead. "WHAT HAPPENED??" You asked yourself, well, it all has to do with torque. You decided to push at the place closest to the hinge, which is a pivot. Remember torque is the amount of force multiplied by the distance away from the pivot, and since your distance away from the pivot is so small, your torque is also very small. Then the guy comes along and pushes at point farthest from the hinge, his distance from the pivot is greater, and with the same amount of force exerted, he gets a greater torque and is able to open the door.

Keep in mind of what we've learned from dictionary.com, torque is something that produces rotation, so if we have torque on an object, then it's going to start spinning. How do we stop this spinning? Well, we can balance the torque on both sides and create an equilibrium. We can balance torque by using different masses, placed at different distances away from the pivot. The pivot is usually around the middle of the object, where the centre of gravity is, but depending on where the mass is concentrated, the pivot point can change. Go try this for yourself with things such as a metre stick, or a baseball bat.

=** Wiki #7- Centripetal Acceleration and Force **=

Have you ever experienced uniform circular motion? If you have been on a merry-go-round, you certainly have. :D Uniform circular motion is said when an object is travelling in a circle at constant speed. The magnitude of the velocity remains the same, but the direction of the velocity changes as the object travels in the circle. The equation for centripetal acceleration is ac= v^2 / r

Where is ac the centripetal acceleration, v is the velocity in m/s and r is the radius of the circle. The acceleration centripetal depends on v and r. - The greater the speed v is, the faster the velocity changes direction - The larger the radius is, the less rapidly the velocity changes direction

There are 2 terms you should know. They are frequency and period. Frequency(f) is the number of cycles per second. Period(T) is the time it took for one complete cycle. They could be written as T= 1/f.

Now, we can come up with one more equation for velocity, it is v= 2**πr/ T** and a= v2/r= (2**πr/T)2/r= 4π2r2/T2r=** 4**π2r/T2**

Now, we know what centripetal acceleration is, then what is centripetal force? It is a force that makes a body follow a curved path. Since it is a force, what do you think the equation for it? Yes it is F=ma but you have to remember that a is v squared divided by radius. A lot of people think that the direction of the force is the direction of the velocity, but it is WRONG. The direction of the force is toward the center of the circle. In order to keep the ball moving in a circle, you pull inwardly on the string.

Here is a summary of what I just said on centripetal acceleration and force. taken from []

= Wiki #8 - Electric Force and Electric Field = Do you know 2 forces existing without any physical contact? Correct! They are gravitational force and electrical force. They are the forces that act at distance. Today, I'm going to talk about the electrical force. The eletric force is also known as Coulomb's Force. Can you guess why?? Good job again :D It is because Charles Coulomb conducted an experiment and came up with an equation! He conducted an experiment, called Cavendish experiment.

Through the experiment, he found if he doubled the magnitude of charge on either object, the force was doubled as well. Also, if he doubled the magnitude of charge on both objects, the force increased to four time to the original value. If he doubled the distance between the two objects, the force decreased to four times to the original value. Therefore the equation is K is proportionality constant, 9.0 x 10^9 Unit for charge is C (coulomb) one electron has 1.602 x 10^-19 Unit for K is Nm^2/ C^2 Vector (has magnitude and direction)

This equation gives the magnitudeof the electric force that either object exerts on the other.

Now let's try a problem Two small, equally conducting spheres are charged, touched together, then separated until the centres are 70cm apart. If they now repel each other with a force of 1.5x 10^-5N, how much charge do they have? F= (KQq)/ (r^2) 1.5 x 10^-5 = (9.0x10^9)(x^2)/(0.7^2) x= 2.8577... x 10^-8 x= 2.9 x 10^-8C

Let's look at electric field. Electric field .... - extends otward from every charge - 2nd charge near 1st charge feels force due to electric field. - the electric field at the location of the 2nd charge is considered to interact directly with this charge to produce the force. Here is a diagram that shows these three characteristics

And the equation of the electric field is where E = electric field q = magnitude of the test charge Fe= force exerted on the test charge -unit: N/C (newton per coulomb) -vector (has magnitude and direction)

Electric field at any point can be measured, using this equation:

Michael Faraday first thought of the idea of elecric field, using lines of force to show how charges behave.

Let's try a problem now Calculate the magnitude and direction of the electric field at a point which is 10cm to the right of a 5uC charge. E = KQ/r^2 = (9.0x10^9)(5x10^-6)/(0.1^2) = 4.5x10^6 N/C right

Electric Potential and Potential Difference

Electric Potential, V, is.... - potential energy per unit charge - scalar (has only magnitude) - unit: V(volt) or (J/C)

A positve charge has high PE near positvely charged plate. As it travels toward the negatively charged plate, PE decreases as a result KE increases due to the law of conservation of energy.

Potential Dffierence - also known as Voltage - unit: V(volt) - change in potential energy = PEfianl- PEinitial - The positve charged object has a tendency to move from a high potential to a low potential, and the negatively charged object does the reverse.

Let's try a problem The diagram represents two electrons, e 1 and e 2, located between two oppositely charged parallel plates. Compare the magnitude of the force exerted by the electric field on e 1 to the magnitude of the force exerted by the electric field on e 2.


 * Answer:** The force is the same because the electric field is the same for both charges, as the electric field is constant between two parallel plates.

Yay! This is the end!

The images were taken from ... http://www.google.ca/imgres?imgurl=http://discover.edventures.com/images/termlib/c/cavendish_experiment/support.// //gif&imgrefurl=http://discover.edventures.com/functions/termlib.php%3Faction%3D%26termid%3D476%26alpha%3Dc%26searchString&usg=__j9jgPWE7ARHXXEXxB7aG7munAUk=&h=175&w=225&sz=41&hl=ko&start=0&zoom=0&tbnid=__// //__O0EQFCqPf81_FM:&tbnh=84&tbnw=108&ei=FvdeTfjOEozEsAON8JHNCA&prev=/images%3Fq%3Dcavendish%2Bexperiment%2Bcharles%2Bcoulomb%26um%3D1%26hl%3Dko%26biw%3D1003%26bih%3D569%26tbs%3Disch:1&um=1&itbs=1&iact=rc&dur=188&oei=BvdeTe_VMIu4sAPCtc3ACA&page=1&ndsp=15&ved=1t:429,r:9,s:0&tx=__// //__96&ty=62__//

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=**﻿Wiki #9- Circuits**=

Today, we are going to answer the following: 1. What is a current? 2. What is a resistor?

Before we start talking, there are a few symbols we need to remember. The battery and resistor symbols are the most frequently used today!

On a battery symbol, the long bar represents the postive, high potential and the short bar represents the negative, low potential.

Resistors could be simply resistors, or they could be light bulbs, heating elements or other resistive devices.

1. What is a current? Current - Definition: the rate at which charges are flowing - Symbol: I - Unit: A (Amperes) I = Q / t

Let's try a problem now! Two cross- sectional areas are located 50cm apart. Every 2.0 seconds, 10C of charge flow through each of these areas. The current in the wire is ___A. a. 5.0 b. 0.50 c. 20 d. 1.0

I = Q / t = (10C) / (2s) = 5 A The answer is A.

2. What is a resistor? George Ohm sutdied the relationship betweeen the voltage across a device compared to the current. He discovered that there was a linear relationship. As voltage and current increase, resistance also increase. The slope is defined to be the resistance, R.

Resistor - Symbol: R - Unit: Ohm -R= V/I

Let's practice with another problem.

What is the resistance of a microwave if 110V produced a current of 2.1A? V = I * R 110V = (4.3A) * R R = 26

This concludes our introduction to circuits today.

=**#10 - Electric Circuits (EMF, adding resistances and Kirchhoff's Law)**= Electromotive force is the greatest potential difference that can be produced by some source of electrical current. However, since "electromotive force" is not technically a force, we are going to call it EMF. EMF is the amount of work that can be done per unit of charge in order to create a potential difference in an electrical circuit. Voltaic cells, solar cells, electrical generators are some example sources of EMF. Although rhw power in a circuit is usually treated as being ideal, in reality, all sources of EMF have internal resistance which affects the voltage supplied to the circuit. The effect voltage acorss an EMF source is called terminal voltage. Given the formula, V = E - Ir, V stands for terminal voltage of the EMF source, E stands for EMF, I stands for current going through a battery and r stands for internal resistance through a battery. Now, let's try a problem! What is the terminal voltage of the battery in the circuit shown below? Rt = 4.5 ohms V = IR 6 = (I)(4.5) I = 1.3333... A V = E - Ir = 6 - (1.33333...)(0.5) = 5.3333...V Terminal voltage is 5.33V.

Adding resistances There might be more than one resistor in a circuit, which means there is more than one resistance. There are different ways to find the total resistance for parallel and series circuits. In order to find the equivalence resistance in series circuits, you add each resistances up like the following: Rt = R1 + R2 + .......

In order to find the equivalence resistance in parallel circuits, you have to take the reciprocal of each reistance, add them up and take the reciprocal of the sum like the follwoing: 1/Rt = 1/(R1) + 1/(R2) + ......

Determining the current, voltage and reistance proerties of a circuit is called circuit analysis. In order to do so, you need to know Kirchhoff's Law. Kirchhoff's first rule is the junction rule. That is the sum of currents into into a current is equal to the sum of the currents out of the current.



Kirchhoff's second rule is the loop rule. That is the sum of the change in potential around any loop in a circuit must equal to zero.



Try a problem now. Find the current flowing through resistor R2 in the circuit shown below: V = IR 21 = 5 (R) Rt = 4.2 ohms because it is a parrallel circuit, 1/(Rt) = 1/(R1) + 1/(R2) 1/4.2 = 1/14 + 1/R2 R2 = 6 ohms V = IR 21V = I (6) I = 3.5 A

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=﻿Wiki # 11 - Intro to Electromagnetism and Magnetic Fields=

The discovery of magnetism and magnets started in a place called Magnesia, where rocks in this region would attract each other. These rocks are now called magnets. Magnets have both a north pole and a south pole, and if you split a magnet into two pieces, you'd get a new north and south pole. Scientists have tried very hard to isolate the poles are create a monopole, so far it hasn't been very successful. So what's so awesome about magnets? Well, all materials are affected by magnets when there's one near by. However, most of these effects are too small to be detected. Materials that readily show large attractive forces to magnets, such as iron, are known to be ferromagnetic. And paramagnetic materials, such as aluminum, are materials that are very weakly attracted to magnets. Or if you really hate magnets and repel them, like gold and silver, then you're known as diamagnetic. There are different kinds of magnets too. Permanent magnets stay magnetized for a long time, and electromagnets are magnetized when there's a current in them. The strength of magnets depends on the alignment of the domains, the more aligned domains it has, the more magnetized the material is.

In 1820, Hans Christian Oersted discovered a connection between electricity and magnetism. An electric current produces a magnetic field, the field lines form concentric circles perpendicular to, and centered on the current. The direction of the field around these circles may be determined by the right hand rule (sorry for lefties out there). The thumb of the right hand points as the direction of the current, and the curled fingers point in the direction of the magnetic field. When working with electromagnets, a solenoid, composed of many loops of wire, can be used to create a large magnetic field. And there's another right hand rule, known as the solenoid rule, that can be used. Curl your fingers in the way of the current, and your right thumb will always be pointing towards the north pole.

A magnetic field has no apparent effect on a stationary charged particle, but if it's moving, it may experience a force. An electric field exerts a force on a charge, a magnetic field exerts a force on a current. The force exerted by the eletric field is in the direction of the field, and the force that the magnetic field exerts on the current is perpendicular to the field, which is also perpendicular to the current. This brings us to the third and final right hand rule, the right hand motor rule. In this rule, you point your thumb in the direction of the positively charged particles, your fingers point in the direction of the magnetic field. Now your palm faces the directions that the positive charges would be pushed by the magnetic field, in the case of an electron, the back of your hand would indicate the direction of force.

Now for some equations: two equations can be used to represent force. F = BQv and F = ILB, where B represents magnetic field strength, if the particle is moving at an angle with the field, then the equation is F = QvB sin(theta) and F = ILB sin(theta). Because the magnetic force is always perpendicular to velocity, eventually the charge will undergo circular motion, and therefore QvBsin(theta) = mv^2/r.

Whoa, that was really long, but we're not done yet, so let's have an intermission.

Back from our intermission, we'll continue and move onto magnetic field from a current in a straight wire. We have a couple more equations to use to figure out magnetic field. For all of these equations, you'll need something known as U0 and has the value of 4pi * 10^-7 T m/A.

Now, our first equation: B = U0(I)/2pi(d), this equation is for a long straight wire, where d is the distance away from the centre of the wire. Our second equation is B = U0(N)(I)/2r, this equation is for the magnetic field at the centre of a wire of N loops of radius r. For solenoids, we have: B = U0(N)(I)/L, this is used to find out the magnetic field inside a solenoid of length L, with N turns of wire. Our final equation is used to find the magnitude of force per unit length between wires carrying currents. Wires carrying current in the same direction attract, and wires with current running in opposite directions repel. F = (U0* I1 *I2 * L)/(2pi(d)).

Now that we've covered all the equations, this concludes our introduction to electromagnetism. But wait, there will be more later.

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=Wiki #12 - ﻿Electromagnetism Continued: Magnetic Fields, Mass Spectrometer & Motors, and Electromagnetic Induction=

We'll start off this post with some knowledge about the electron. First of all, JJ Thomson was the first person to measure the ratio of charg to the mass of particles in cathode rays. Using both magnetic and electric fields, he placed a magnetic field perpendicular to the path of electrons from a cathode ray tube, and the field deflects the beam elecrons a certain distance from its original position on a screen.

The magnetic field pushes the electron downward with F = BQv = Bev, and because the magnetic force will result in centripetal acceleration toward the centre of its circular path, and F = Bev = mv^2/r.

Thomson eliminated the need of knowing v by making the electric force equal to the magentic force, which means there is zero deflection. From this, he found the mass to charge ratio, which is 1.76 * 10^11 C/kg.

Knowing the results of Thomson's experiment, Millikan was able to measure the charge on an elecron by suspending oil drops between parallel charged plates. He calculated that the charge was 1.6 * 10^-19 C and the mass of an electron would be: 9.1 * 10^-31 kg.

Now moving onto mass spectrometers and electric motors. In the early 20th century, the most accurate way of measuring the mass of an atom is by using a mass spectrometer. Heat or an electric current would produce ions and these ions would pass through an initial slit and enter into a region made up of electric and magnetic fields. Feeling both an electrical and magnetic force, the ions pass through another slit and into another magnetic field, forcing the ions into a circular pth and hitting photographic film. A mass spectromter separate elements in a mixture, isotopes in an element, or different molecules.

Most electric devices work by either producing heat or motion. And for devices that produce motion, it does this by the force of a magnetic field exerted on a current.

An electromagnet, is just simply a solenoid, many loops of wire are wrapped around soft iron, and you can make it stronger or weaker and turn it on and off.

Now getting onto electric motors. The motor basically has 3 parts, the first part is the armature, which is the part of the motor that turns, its motion is caused by a torque exerted by the magnetic field. The second part is the split-ring commutator, a split metal cylinder found on the armature. It makes the current in the 2 halves of the coil change direction so that the magnetic force is always turning the coil in the same direction. Besides these 2 parts, there's also bars that come in contact with the split-ring commutator called brushes.

Now this is how it works: current comes in from the left at one brush and goes to the left half of the commutator, then to the coil on the armature. The current direction in the left end of the armature creates a magnetic field, making it act like a north pole and the right end becomes a south pole. Since opposite poles attract, the external field supplied by the magnets exert a torque on the armature, causing it to turn. Once making a half-turn, the commutator will cause the direction of the current running through the armature to reverse.

Our last topic of the day: electromagnetic induction. It was Joseph Henry and Michael Faraday who discovered that mgnetism could induce an emf. Faraday realized that by turning a magnetic field on and off, you can create a current, also known as an induced current, and when there's a change in magentic field, an induced emf is produced. There are 3 ways to increase the strength of the current. The faster the magnet moves into the coil, the greater the current will be. The more coils there are, the greater the current. And the stronger the magnet, the greater the current.

One formula for this section: emf = BQvL/Q = BvL

That's it for today, more coming up.

=Wiki #13 - **Electric Generators and Lenz's Law**= What is electric generator? What does it do?

You are probably familiar with motor, right? Well, electric generator is the opposite of the motor. Motor transforms electrical energy into mechanical energy and electric generator transforms mechanical energy into electrical energy. Electric generator is an application of principle of electromagnetic induction.

It is composed of loops of wire, armature, two permanent magnetic bars, slip ring and brushes. I am sorry that I can't add a picture of the generator! There's a problem with this website T.T Anyways, loops of wire on the armature are rotated through the magnetic field and this results in change in magnetic flux and an induced emf.

The armature can create the maximum force when it passes through the magnetic field at 90 degrees and create the minimum force when it passes through the field at 0 or 180 degrees. Why is that? Well, do you remember the right hand rule? The velocity is the thumb and the field is the fingers and the force is the palm. This force which is going into the page, causes the charges to move in that direction. It means that it is the same direction as the current will flow.

However, when the armature is cutting through the magnetic field, there is no maximum or minimum at that moment.

The formula to find induced emf is: emf = 2NBLvsin(theta) =(2NBL2pie(Radius)sin(theta))/T N stands for number loops, B for magnetic field and L for length of the wire.

Next, what is Lenz's Law? Before you learn what Lenz's law is, there are a few things you need to know.

There are two variables that the magnitude of emf induced depend on: - time : the faster the magnetic field changes, the greater the induced emf is - magnitude of flux : the rate of change of magnetic field passing through the loop of area 

We can use lines to represent the strength of magnetic flux just like we use lines to represent the strength of electric field lines. The total number of lines passing through the coil is proportional to its strength. Considering the definition of the magnetic flux, we can come up with this equation The unit of the magnetic flux is wb (weber) or Tm^2  Faraday discovered that emf is equal to change in magnetic flux over change in time with N turns.And, this is the Faraday's law of induction. Although he was almost right, his theory needed some tweaking up.

Years later, Lenz put a negative sign in front of Faraday's law of induction and rename the Faraday's law as Lenz's law. The negative sign is important because it indicates the direction of emf. This is the definition of Lenz's law: the polarity of the induced emf will always be such that it will produce a current whose magnetic field opposes the changing flux that produces the emf. For example, when the magnetic field points upward, the induced current field points downward. This makes sense due to the law of conservation of energy. Let's try a problem now! A coil consisting of 50 loops of radius 4.0x10^-2m is placed with its plane perpendicular to a magnetic field that is increasing at a rate of 0.20T/s. What is the magnitude of the emf induced in the coil? emf = -(50)(0.2T/s)(4x10^-2)(pie)= 0.050V. __[]__