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Some basic facts about magnets:
Some materials like iron, cobalt, and nickel are classified as ferromagnetic. These materials have tiny regions in them called “magnetic domains” that can be visualized as miniature magnets. The atoms in each magnetic domain have their electron spins aligned along a common axis. In a bar of iron that is not a permanent magnet, the arrows below represent the fact that the domains point in random directions (note that this is only a representation; there are so many domains in the bar and they are so small that it is impossible to draw them all).
Under certain conditions, the domains of an induced magnet may continue to point in the direction of the external magnetic field even after that field is removed. This phenomenon is known as magnetic remanence, a property used to create permanent magnets. There are 4 common ways that magnetic remanence is obtained:
How do magnetic forces of attraction and repulsion arise? Magnetic forces occur when electric charges move with a velocity that is not parallel to external magnetic fields. In contrast to magnetism, charges experience electric forces in electric fields whether they are moving or not. Consider two bar magnets applying magnetic forces to one another. The first bar magnet establishes an external magnetic field. When the second bar magnet with electrons spinning about a common axis is placed in this field, it may experience an attractive or repulsive force.
A common but somewhat outdated technology involves storing information on magnetic tape (e.g. cassette tape recordings of music). Information is stored permanently on the tape using what physical phenomenon?
The correct answer is C. Magnetic tape, which usually has the ferromagnetic molecule chromium-dioxide throughout it, is heated and then swept under the recording head. This recording head imprints magnetic information in the molecular domains. When the tape is cooled, magnetic remanence insures that the domains retain their new alignment.
Previously we learned that electric field lines are lines of force that exist in the vicinity of charged objects. The directions of these lines of electric force are determined by the direction of force on a positive test charge. In this lesson, we will study lines of magnetic force.
A classic grade-school experiment is to place a bar magnet under a piece of paper and to sprinkle iron filings over the paper. The iron filings align themselves like compass needles to show the magnetic field around the magnet. To determine the direction of a magnetic field, we imagine placing a compass in that field. The north needle of the compass points in the direction of the magnetic field and is used to map out the magnetic field lines. For example, the magnetic field for a bar magnet is shown below:
Figure 1: Magnetic Field of Bar Magnet shown by iron filings on paper
On the figure above, the magnetic field lines point away from the north end of the magnet. This is because the north needle of a compass is repelled from the north end of the magnet. Likewise, the north needle is attracted to the south end of the magnet.
Notice how the magnetic field lines continue into the bar magnet. The field lines within the magnet demonstrate how the magnetic domains within the iron are aligned along the field. Magnetic field lines always form closed loops because it is impossible to have a single magnetic pole (monopole). In other words, if magnetic field lines leave a north magnetic pole, they have to loop back to the south pole of the same magnet.
Previously we learned that magnetic forces are exerted on moving charges. In this lesson, well see that moving electrical charges produce magnetic fields.
In 1820, Hans Christian Oersted made an observation that marked the beginning of the field of electromagnetism. When current flowed through a wire, he noticed that a nearby compass needle moved. This indicated to him that current-carrying wires produced magnetic fields. This exercise highlighted the strong connection between electricity and magnetism.
Oersted’s discovery was used to create electromagnets. When current is sent through a coil of wire, the coil behaves like a magnet, as the following diagram shows:
Oersted’s famous experiment with a current-carrying wire and a compass marked the beginning of the study of
The correct answer is B. Oersted’s observation showed the connection between electricity and magnetism.
Oersted’s experiment demonstrates that moving electrical charges produce magnetic fields, but is the reverse true? That is, do magnetic fields produce electrical currents? The answer is yes, but only under certain conditions.
Consider a coil of wire attached to a current meter. When a magnet sits inside the coiled wire, no current will be detected. However, if the magnet is moving back and forth in the coil, a current will be detected. Work must be done to move the magnet against the magnetic force, and this transfers energy to the circuit.
An induced electromotive force (EMF) across the ends of the loop occurs when any of these three changes occur. Faraday’s law states that the induced EMF equals the time rate of change of magnetic flux within a loop. The quicker the flux changes, the greater the induced EMF.
Electric generators use the ideas embedded in Faraday’s law to operate. An external source of mechanical energy does work to rotate a coil in the presence of a magnetic field. This rotation results in a flux change through the coil and electricity is generated.
Which of the following will NOT produce an induced EMF across the ends of a coil?
The correct answer is B. According to Faraday’s law, the magnetic flux must change within a coil to induce an EMF.
Previously we learned that moving electrical charges produce magnetic fields and that changing magnetic field generates electricity. In this lesson, we will study how electricity and magnetism produce electromagnetic radiation. We will also study the spectrum of electromagnetic waves.
How is light produced? When electrons drop energy levels in an atom, energy is released as light. Because electrons are charged, they produce an electric field in their vicinity. When the electrons move, they also produce magnetic fields. The changing electric and magnetic fields around moving electrons is radiation that we call light. As we shall see, not all of this radiation is visible to the human eye.
How are radio waves produced? Imagine a transmitting antenna for a radio station. If electrons are accelerated up and down the wires in the antenna, changing electric and magnetic fields will be produced in a similar manner to the electrons changing energy levels in atoms. This radiation moves out into the air at very high speeds. In fact, this radiation doesn’t even need air to move. Radio waves can be transferred through the vacuum of deep space.
Visible light and radio waves are both electromagnetic waves. In fact, there are many other types electromagnetic radiation depending on the frequency of the vibration that produced it. Here is a list of the electromagnetic spectrum in order, from low frequency (long wavelength) to high frequency (short wavelength), as well as some common properties of each:
You may have noticed on the spectrum above that as the frequency of the radiation increases, the wave becomes more dangerous. This is because the higher frequency radiation is more energetic and has the ability to alter DNA.
What do all the types of electromagnetic radiation have in common? They are all waves that can move through a vacuum. Unlike sound waves and water waves, electromagnetic waves do not need a physical medium to travel. In a vacuum, they all travel at a speed of 300 million meters per second (3.00 · 10 8m/s). The only way to slow down this radiation is to change the medium. For example, when light strikes a diamond, its speed slows down to 1.24 · 10 8m/s.
Which of the following is fundamentally different from the others?
The correct answer is B. Sound waves require a medium to travel. All the other types of waves are electromagnetic radiation that travels about one million times faster than sound.