Physical Science Homework Set 4
Every permanent magnet ever observed has had at least how many poles?
2. If you break a bar magnet in half, each half still behaves as a complete magnet.
What is the magnetic force on a charged particle that is not moving but stationary in a magnetic field?
A stationary charged particle in an electric field experiences a force proportional to its charge. A moving charge has not just an electric field but also a magnetic field, and in general the electric and magnetic fields are not completely separate phenomena.
What is a cathode ray tube?
A vacuum tube in which a hot cathode emits electrons that are accelerated as a beam through a relatively high voltage anode, further focused or deflected electrostatically or electromagnetically, and allowed to fall on a phosphorescent screen.
How is alternating current different from direct current?
Electricity or 'current' is nothing more than moving electrons along a conductor, like a wire, that have been harnessed for energy. Therefore, the difference between AC and DC has to do with the direction in which the electrons flow. In DC, the electrons flow steadily in a single direction, or "forward." In AC, electrons keep switching directions, sometimes going "forwards" and then going "backwards."
What is the right hand rule for a coil, and what does it tell you?
Grab the wire in your right hand with the thumb pointing in the direction of current flow (DC or instantaneous if AC). The curl of the other four fingers indicates the direction of the magnetic field around the wire.
What is the (three finger) right hand rule for the Lorentz force, and what does it tell you?
Hold out your left hand with forefinger, second finger and thumb at right angle to one another. If the fore finger represents the direction of the field and the second finger that of the current, then thumb gives the direction of the force. F= BIL It tells us that a force acts on the conductor, in a direction, perpendicular both to the direction of the electric current and the magnetic field.
How can one determine in which way the induced current flows?
If the B field is increasing, the induced field acts in opposition to it. If it is decreasing, the induced field acts in the direction of the applied field to try to keep it constant.
What is alternating current?
In electricity, alternating current (AC) occurs when charge carriers in a conductor or semiconductor periodically reverse their direction of movement.
What is Lenz's law?
The direction of the induced current will be to minimize the change in magnetic flux. Lenz's Law is all about conservation of energy. It guarantees that induced currents get their energy from the effect creating the change. The force acting against the conductor being moved earlier is actually an invocation of Lenz's Law. ( As the conductor moves down, the flux increases, so the induced field opposes this which leads to the direction of the Induced current - which in turn shows the direction of the force back on the current.)
Draw a diagram depicting a negatively charged object moving to the right, perpendicular to a magnetic field pointing into the sheet of paper. What is the direction of the Lorentz force acting on the object?
The force is going up.
What is the reason for the direction of the Lenz's law reaction force (explain what gives rise to the force)?
The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. In the examples below, if the B field is increasing, the induced field acts in opposition to it. If it is decreasing, the induced field acts in the direction of the applied field to try to keep it constant.
Sketch the magnetic field of a coil. Do not forget to also draw the field INSIDE of the coil. Label its North and South poles. How does it compare to a bar magnet?
The magnetic field of a coil is the same as the magnetic field of a bar magnet.
How does a magnetic north pole respond to a magnetic field?
The north pole of a magnet will tend to line up with the magnetic field, so a suspended compass needle will rotate until it lines up with the magnetic field.
If two oppositely charged particles are moving with the same velocity through a magnetic field, how will the magnetic forces on them compare?
There will be zero force.
What happens when you let a magnet fall down within a hollow copper tube? Give a detailed explanation for the observed phenomenon. (You should list two arguments, one involving conservation of energy, and the other discussing the involved forces and what gives rise to them.)
This is because the copper tube "sees" a changing magnetic field from the falling magnet. This changing magnetic field induces a current in the copper tube.The induced current in the copper tube creates its own magnetic field that opposes the magnetic field that created it. If a magnet falls through a copper tube, it induces currents in the copper and these currents exert a force on the magnet which tends to slow it down.
How many types of magnetic poles are there? What are their names?
Two (north and south pole).
What is the right hand rule for a straight conductor and what does it tell you?
Using your right-hand: Curl your fingers into a half-circle around the wire, they point in the direction of the magnetic field, B Point your thumb in the direction of the conventional current. It determines the direction of the magnetic field around a current-carrying wire and vice-versa
How can a compass be used to identify magnetic poles?
In response to the Earth's magnetic field, the compass will point toward the geographic North Pole of the Earth because it is in fact a magnetic south pole.
Explain in detail how a magnet interacts with an iron nail. Why is there an attraction, even though the iron nail does not appear to have any magnetic properties unless it is brought into contact with a magnet?
Iron atoms have unpaired electrons and these electrons have a spin which interacts with a magnetic field. Temporary magnet: a magnetic object that can become a magnet when it is touching a permanent magnet but loses its magnetic properties when it is no longer touching the permanent magnet.
What is Faraday's law of electromagnetic induction?
It serves as a succinct summary of the ways a voltage (or emf) may be generated by a changing magnetic environment near a conductor. Magnets can create currents.
What are the rules for the forces between magnetic poles?
Like poles repel each other; opposite poles attract.
If a magnet has only two poles, what type of poles must they be?
North and South. A north magnetic pole never exists without the presence of a south pole, and vice versa.
What is a magnetic pole?
One of the regions on a magnet that produces magnetic forces.
Is the magnetic force on a current perpendicular or parallel to the magnetic field?
Perpendicular.
What is the Lorentz force (formula and explanation)?
the Lorentz force is the combination of electric and magnetic force on a point charge due to electromagnetic fields. If a particle of charge q moves with velocity v in the presence of an electric field E and a magnetic field B, then it will experience a force
How does a magnetic south pole respond to a magnetic field?
the South indicator of the compass will point toward the North pole of a magnet
A current is flowing downwards through a magnetic field that is pointing left. Predict the direction of the force on the current.
the force is going into the paper
A wire is lying (horizontally) on a table in front of you. There is a magnetic field of strength 20 mT pointing vertically into the surface of the table. The length on which wire and field intersect is 5cm. You measure a force of 0.05 N pushing the wire away from you. What is the magnitude and the direction of the current in the wire?
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From the formula of the Lorentz force on a single charged particle, derive the Lorentz force on a current carrying wire running through a magnetic field.
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Make a sketch of the wire swing apparatus that we used in class. What experiments did we perform and what did you observe?
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Part A) a proton (Q= 1.6* 10-19 C) moves to the right in a magnetic field of strength B=1T. Let's assume that the field is pointing into the sheet of paper. What's the magnitude of the Lorentz force acting on the proton? What is the direction of the force? Draw a sketch Part B) add the path that the proton will take through the magnetic field. What will happen to the path if the proton moves faster?
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Sketch the magnetic field of a straight conductor
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a 0.1 meter long wire experiences a force of 2*10-4 N in a magnetic field of 0.004 Tesla. How much current is flowing in this wire?
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What must occur for a magnet to induce electric currents in a coil?
Change/motion. The change could be produced by changing the magnetic field strength, moving a magnet toward or away from the coil, moving the coil into or out of the magnetic field, rotating the coil relative to the magnet, etc.
Which pole of a permanent magnet is attracted to geographic north?
Your magnet's North pole is attracted to the Earth's North pole. Technically, the Earth is labeled backwards -- the geographic North pole is actually like the magnetic South pole of a magnet.
What conversion of energy takes place when electromagnetic induction occurs?
mechanical energy into electrical energy
