1.) Magnetism; Magnetic Poles;Electromagnetism
2.) Forces on Charged Particles in an Electric Field; Motors
3.) Electromagnetic Induction and Common Appliances
4.) Generators and Energy Production
5.) Transformers and Energy Transfer from Power Company to Home
Part One: Magnetism; Magnetic Poles; Electromagnetism
What is a domain?
- A domain is a cluster of electrons all spinning in the same direction.
What is the difference between the domains being aligned versus unaligned?
- Aligned domains have a magnetic field, while the unaligned do not.
What direction do the field lines run inside a magnet?
- Inside of a magnet, the field lines run from the south pole to the north pole.
What direction do the field lines run on the outside of a magnet?
- Outside a magnet, the field lines run from the north pole to the south pole.
What is the source of all magnetism?
- The source of all magnetism is moving charges.
What is the shape of the magnetic field around the magnet?
- It is actually the shape of a cloud, it's 3D.
Draw a magnet with the field lines.
TADA!
What is the difference between North and South versus the Globe's North and South?
- The north on our globe is actually magnetic south, while our globe's south is actually magnetic north.
What is a compass?
- A compass is a magnet that is free to move.
How does a compass show the direction of the magnetic field?
- It aligns with the direction of the earth's magnetic field.
How is the magnetic field around a wire different?
- It is similar to a sphere. However, depending upon the direction of the current, the direction will go from back to front or front to back.
Examples and Practice Problems
1.) Explain why a paper clip sticks to a magnet? (Use pictures to aid your explanation)
- The domains (clusters of electrons that are all spinning in the same direction) of the paper clip are randomly aligned.
- When the permanent magnet is brought near the paperclip the domains are aligned, meaning the paperclip now has a magnetic field. This means that the paperclip is now a magnet that has a north and a south pole. The paperclip's north and south poles are attracted to the opposite poles in the permanent magnet, thus it sticks to the magnet.
2.) What will happen when the paper clip is removed? If the middle paper clip is removed? Why?
Since they are now magnets, they will continue to be magnets, even if they are detached from the original permanent magnet.
4.) Both sides of a permanent magnet can pick up a paper clip. Which is greater, the force from the magnet on the paper clip or the force from the paper clip on the magnet?
- According to Newton's Third Law that states, " for every action there is an equal and opposite reaction," the forces are equal.
5.) Draw and explain why opposite poles attract each other and like poles repel each other?
- Opposite Poles: The field lines are pointing in opposite directions, pushing the magnets away from each other.
- Like Poles: The field lines are pointing in the same direction, attracting (pulling) the magnets towards each other.
Part Two: Forces on Charged Particles in an Electric Field; Motors
What are the two essential parts of a motor?
- The current carrying wire and the magnet.
What is the fundamental reason a motor works?
- A current carrying wire feels a force in the magnetic field. This force causes a torque, which causes the current carrying wire to spin.
When trying to find the direction of the force exerted by the magnetic field on a current carrying wire, you use the right hand rule. Explain what three fingers you use and what they stand for.
- Thumb= Force
- Index Finger= Current
- Middle Finger= Magnetic Field
What is the specific way you scrape an armature and why?
- You either scrape just the top or just the bottom of both ends of the current carrying wire. This is because then the current will flow in one direction, pushing the wire away from us. If current is flowing on both the top and bottom of both ends of the current carrying wire then the wire will be pushed away from us, but immediately return back to the original position.
Examples and Practice Problems
1.) Why do cosmic rays enter at the poles and not at the equator? What phenomenon is produced by cosmic rays entering the earth?
- When moving charges are perpendicular to the magnetic field, the charges feel force. At the poles, the moving charges are parallel to the magnetic field lines so they don't feel a force and can enter the atmosphere. When at the equator, the moving charges are perpendicular to the magnetic field so they feel a force and are deflected back into space.
2.) A wire in a current is running toward the left of this page. It is in a magnetic field that is running up to the top of this page. Which way will the wire be forced? (Up, down, left, right, in, or out of the page)
- The wire will be forced into the page.
3.) Use the diagram below for the following questions
A.) Draw the current through the loop of wire and the magnetic field on top of the magnet
-Red = Magnetic Field
-Blue = Current
C.) What will supply the current to the wire?
- The battery will supply the current to the wire.
4.) If you were to add some fan blades to the wire that this motor will spin, where would you add them? How would the motor make the fan blades spin?
- You would add them to the the ends of the loops of the current carrying wire. The current carrying wire when near a magnet feels a force. This force causes a torque, which causes the current carrying wire to spin. If the blades are attached to the wire then when the torque causes the wire to spin, the fan blades will spin.
5.) Below are two position of the loop. The bottom one has been rotated. Use the right hand rule to show the direct that the slope will be forced. Note: only of these situations will actually produce a torque on the loop. One of these situation will just cause the loop to move to the side and the other will actually cause it to spin.
- According to the right hand rule, the loop will be pushed in.
Part Three: Electromagnetic Induction
What is electromagnetic induction?
-It is the act of inducing voltage by changing the magnetic field in loops of wire.
How does electromagnetic induction work?
-You have a loop of wire in which when a magnet (going through, above, or near) will change the b (magnetic field) of the loop of wire, which will induce a voltage that causes a current. This current is used for a variety of things....
Examples: metal detector at an airport, credit card machines, and traffic lights.
What is Farraday's Law?
-The voltage induced is directly proportional to the number of loops.
What are the three ways to induce voltage in a loop of wire?
-The first is to move the loop near the magnet. The second is to move the magnet near the loop. The third is to change the current in a nearby loop.
Examples and Practice Problems
1.) Why does a traffic light at an intersection change when a car approaches it?
- The traffic light at an intersection changes when a car approaches because of electromagnetic induction. In the concrete, there are loops of wire that when the car rolls over, creates a change in the b that induces a v that causes a current. This current sends a signal to the lights to change.
Part Four: Generators and Energy Production
What is a generator?
- A rotating coil in a stationary magnetic field.
Does a magnet moving in a loop of wire create or induce voltage?
- It induces a voltage.
How does a generator differ from a motor?
- The roles of input and output are reversed. In a motor, the electric energy is the input and mechanical energy is the output. In a generator, mechanical energy is the input and electric energy is the output.
What type of energy do generators convert and what is it converted into?
-They convert mechanical energy into electrical energy.
What is the underlying principle that both a motor and a generator operate according to?
-Another way to word it from the earlier section, is that moving electrons experience a force that is a moving perpendicular to both their velocity and the magnetic field they traverse.
What is the motor effect? (see picture)
- When charge move along the wire, there is a perpendicular upward force on the charge. Since there isn't a conducting path upwards, the force on the charge pulls the wire upward.
What is the Generator effect? (see picture)
- It is when a wire with no initial current is moved downward, the charge in the wire experiences a deflecting force perpendicular to its motion. Since there is a conducting path in this direction, the charge moves, conducting a current.
Examples and Practice Problems
1.) "A length of wire is bent into a closed loop and a magnet is plugged into it, inducing a voltage and consequently, a current in the wire. A second length of wire, twice as long, is bent into two loops of wire, and a magnet is similarly plugged into it. Twice the voltage is induced, but the current is the same as that produced in the single loop. Why? " (Chapter 25 Conceptual Physics)
- According to Farraday's Law, "the number of loops/turns is directly proportional to the voltage induced." This means that the second wire will more loops will have a greater voltage, a greater voltage in this instance means less current. This is because in generators, the power must remain the same, Power Primary = Power Secondary. Therefore, as the voltage increases the current must decrease to compensate. According to Ohm's Law, " I = V/ R," meaning current and resistance are indirectly proportional. This means as current decreases, the resistance will increase. That is why the current remains the same.
Part Five: Transformers and Energy Transfer from Power Company to Home
What are the two types of transformers? What is the difference between them?
- The two types of transformers are Step-Up and Step- Down. The difference between them is that in the Step-Up transformer, the primary coil will have less loops/turns than the secondary while the Step-Down's primary coil will have more loops/turns than its secondary.
Why is power sent at such high voltages in power lines from the power company to your house?
Power is sent at such high voltages so they can have low current.
Do transformers rely on AC (alternating current) or DC (direct current)?
- Transformers rely on AC current.
Why can a hum usually be hear when a transformer is operating?
- A hum can usually be heard because the current running back and forth in the wire and is vibrating the air around it and all sound is just vibrations.
What is the formula for finding the power of the primary versus the secondary coil?
Primary Power = Power Secondary
IV = IV
What is the formula for finding the number of loops/turns of a coil?
Why do you want the current in power lines to be low?
- You want the current to be low in power lines otherwise you'd lose energy to heat if it was high.
Examples and Practice Problems
1.) Why does a transformer rely on alternating current? In you answer, talk about what a transformer is, one place we would find one, and how it works.
- The primary coil relies on AC (alternating current) meaning the b is always changing. This causes the b of the secondary coil to change which induces a v that causes an I. A transformer is a device that increases or decreases the voltage going into a device. They can be found in appliances that need more or less voltage than the wall supplies. A transformer consists of two coils of wire, a primary and a secondary. The types types of transformers are step up and step down. In a step up transformer, the primary coil will have less loops/turns than the secondary. In a step down transformer, the primary coil will have more loops/turns than the secondary. This is in accordance to Faraday's Law which states" the number of loops is directly proportional to the voltage induced."
A.) what type of transformer is required?
- A Step-Down transformer is required.
B.) If the primary coil has 80 turns, how many will the secondary have?
- The secondary will have 22.85 loops/turns.
C.) You go to a friend's house and they have a computer has 20V and runs on a 2A current, what will the current drawn from the wall socket, which has 120V, be?
3.) " Your friend says that according to ohm's law, high voltage produces high current. Then your friend asks, " So how can power be transmitted at a high voltage and low current in a power line?" What is your response" ( Chapter 25, Conceptual Physics)
- Power can be transmitted at a high voltage, but a low current because the power must remain equal in the primary and secondary coils. This means that if the voltage is high, then the current must compensate for the voltage's increase by decreasing.
4.) How does the current in the secondary coil of a transformer compare with the current in the primary when the secondary voltage is twice the primary voltage?
- The secondary coil's current is half the current of the primary coil's.
5.) " In what sense can a transformer be considered an electrical lever? What does it multiply? What does it not multiply?" (Chapter 25 Conceptual Physics)
- It can be considered an electrical lever in the sense that...
-Work in = Work out and F in x d in = F out x d out
- Power of Primary= Power of Secondary and I x V =I x V
- It multiplies the voltage and it doesn't multiply the energy.
6.) Two separate but similar coils of wire are mounted close to each other, as shown below. The first coil is connected to a battery and has a direct current flowing through it. The second coil is connected to a galvanometer.
a.) How does the Galvanometer respond when the switch in the first circuit is closed?
It changes.
b.) After being closed, how does the meter respond when the current is steady?
It doesn't move.
c.) How does the meter respond when the switch is opened?
It changes.
AKA: When you turn a DC current on and off you can induce a voltage in a secondary, but when it is running you cannot.
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