For students taking the SAT subject test, physics can be an incredibly interesting topic. It encourages students to observe the physical world around them and inquire about the forces of nature.

An important topic in the SAT II Physics subject test is the topic of electromagnetism. Electricity and magnetism are very closely linked together as electromagnetism that has numerous applications in the world around us.

SuperProf is here to guide students taking the SAT II physics exam. We covered this topic in our UK blogs for the potential students of the Physics syllabus.

There is a reciprocal relationship between electric charges and magnetic fields. A moving charge generates a magnetic field.

Electromagnetic forces play a crucial role in space physics.

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Permanent Magnets

All electrons create minuscule magnetic fields when circling a nucleus. Unfortunately, for most materials, the fields do not line up and point in different directions.

However, iron and other ferrous alloys fall into the category of ferromagnets. These are materials where the fields stay lined up when magnetized.

There are two other types of magnetic materials- paramagnetic and diamagnetic materials.

Paramagnets are any nonferromagnetic materials that are attracted by a magnet—the atoms in them align in the direction of a magnetic field.

Diamagnets are nonferromagnetic materials repulsed by a magnet—the atoms in a diamagnet align against the direction of an external field.

Magnetic Field Lines

Permanent magnets have a north and a south pole. Magnetic field lines protrude from the north pole and go to the south pole.

To make your physics lessons more interesting, you can incorporate some experiments in them. To demonstrate magnetic fields, you can place ferrous filings around a permanent magnet. The filings will align according to the field lines.

You'd be surprised to know that the Earth itself is a magnet. Our planet's core is made of iron and exerts a magnetic force. It's the reasons why a compass needle points north, and the north and south poles experience auroras. Often, you will find you can incorporate many real-world examples like this into your physics lessons.

cardinal directions
While humans may not be able to determine the Earth's magnetic fields, a simple tool like a compass can do the work for us using the four cardinal directions. The needle always points north because of the Earth's internal magnetic core. (Source: pixabay)

Magnetic Force On Charges

As we have established, a magnetic field exerts some force on an electric charge. The unit for measuring magnetic field strength is measured in teslas.

The force on a moving charge is calculated as a cross product of the charge's velocity and the magnetic field strength. It's a good idea to have a grip on math exercises, such as cross and dot products, for any physics test.

If the velocity of the charge changes, so will the direction of the force on it. The force on a moving charge can change its trajectory from linear to non-linear.

When the velocity vector is perpendicular to the magnetic field lines, the force will also be perpendicular to both vectors. This causes the object to move in a  circular path.

If the velocity and field happen to be parallel, the force is zero. However, when the velocity and field lines are neither parallel nor perpendicular, we need to break the velocity vector into x and y components parallel and perpendicular.

What about when a magnetic field and electric field overlaps? In that case, both forces act together. They are added together to create the total force.

The direction of other forces, such as electric forces, is determined in much the same way

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Magnetic Force On An Electric Current

Electrical current is a collection of many electric charges moving in unison. Therefore, wires carrying current are also subject to a force inside a magnetic field.

Conversely, a moving current can also induce a magnetic field. However, you don't have to focus on any physics problems for this topic. SAT II Physics does not cover this topic.

All you need to remember is that the strength of the current and the magnetic field strength is proportional. Do not make prep difficult by focusing on topics out of the syllabus.

There is a simple rule to determine the direction of the magnetic field strength called the right-hand rule. Curl your right hand as if you are gripping a wire. Your thumb should stick out in the direction of the current. The direction your fingers curl should give you the magnetic field direction.

Electric and magnetic energy is bound together by Maxwell’s Equations.

Electromagnetic Induction

A moving magnet creates an electric field. This phenomenon is called electromagnetic induction. This has various applications in everyday life around you, such as dynamos, electric generators, and power production.  Electromagnetic waves are a way of transmitting energy.

Your SAT Physics exam doesn't generally include quantitative questions on this topic; they are mostly qualitative with minimum calculations.

When an electrical circuit is introduced in a magnetic field, an electromotive force is generated within it.

Faraday's Law

Faraday's law describes how an electric current generates a magnetic field and, conversely, how a magnetic field change generates a current.

A commonly used method to induce current is to change the magnetic flux through a circuit.

What is the magnetic flux? It is the product of the area and the magnetic field perpendicular to it. It is the density of flux lines passing through a certain point, to put it in easy words.

How do we change magnetic flux? It can be done in three ways.

Firstly- Change the field strength by moving a magnet mechanically.

Secondly-Move the magnet and increase or decrease the flux lines.

Thirdly- Rotate the magnet and change the number of flux lines passing through it.

Lenz's Law

This law states that the direction of the induced current opposes the magnetic field generating it.

Lenz's law indicates the direction of the current. You can use the same thumb rule to find out its path. Stick out your thumb in the opposite direction to the flux change, and where your fingers curl will show you where the current flows.

If the induced current did not oppose the change in flux, the bar would keep accelerating without any change in input of external energy. This is not possible, according to the law of conservation of energy. Hence Lenz's law is a special case for the conservation of energy.

We learned about changing the magnetic flux by increasing the current and rotating the circuit. Another method exists, and that is by changing the field while the course stays still. You can do this by moving a magnet towards a wire because a moving magnet will change flux.

Applications

You will see applications of electromagnetic induction all around you. Human life, as we know it today, cannot exist without this phenomenon. All the electrical appliances and gadgets we use are powered by electricity produced by electromagnetic induction.

You will find this in action inside hydroelectric motors, cycle dynamos, electric generators, and transformers. Let us look at two examples where electromagnetic induction is used.

The Electric Generator

You probably have seen generators or electric motors sometime in your life. They work on the principle of electromagnetic induction- by converting mechanical energy into electrical energy. A coil in the generator turns inside a magnetic field. With the changing magnetic flux, a current is created.

bicycle lights
The backlight that is featured on some bikes is known as a dynamo and is a type of DC generator. (Source: pixabay)

The Transformer

A transformer has two coils wrapped around a single iron core. Current is applied to the primary coil and then induced in the other secondary coil. The number of turns in the coil determines the current magnitude.

A step-up transformer has more turns on the secondary coil and produces a larger current than the original current. This is used when transmitting power across power lines. Electricity is transmitted in the form of electromagnetic waves.

A step-down transformer has more turns on the primary coil and produces a smaller current than the original current. This is used when power enters your house from the power lines.

If you are wondering how a transformer coil looks like, a solenoid is a good example.

using a transformer
Transformers change the potential difference of an alternating current. They can be used for imported appliances when living in a foreign country. (Source: Pixabay)

SAT students in the US can check out our articles on the particle model of matter and atomic structure.

If you're planning to sit for your physics SAT exams this year, head on over to Superprof! We've got the best tutors who can make SAT prep seem like a walk in the park.

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