Magnets and Magnetism
Moving charges are the source of all magnetism. On a closer level, consider the fact that all object are made of atoms and that these atoms contain electron clusters. We know that moving electrons cause current, so this should begin to sound familiar. In an unmagnetized object, these electron clusters are all spinning in random directions. A group of electron clusters spinning in the same direction is called a domain. When an object is magnetic, all of its domains will be spinning in the same direction.
In a magnet, field lines explain why like sides repel and opposing sides attract. This is magnetic flux.
The domains in a magnet are all pointing in one direction, when you hold a compass (which is simply a magnet that is free to move) over a magnet, it will move based on the alignment of the magnet's domains. They go from south to north inside the magnet and from north to south outside the magnet.
Two magnets are attracted to each other because their domains are all spinning in the same direction.
So: how does a paper clip stick to magnet?
The domains in a paper clip are random, so it has no poles. A domain is a cluster of electrons that are spinning in the same direction. The magnet has a magnetic field, which means that it has a north and a south pole. When the magnet is close to the paper clip, the domains of the paper clip align to match the magnetic field of the magnet. The paper clip now has a north and south pole and the north pole of the paper clip is attracted to the south pole of the magnet. Thus, the paper clip sticks to the magnet.
Here is a great video explaining magnetism with helpful visuals:
Electromagnetism
An electromagnet is a magnet that needs a current-carrying wire that has a magnetic field. The domains of a magnetized object can align with that field, and then have a magnetic field of its own.
In order to create an electromagnet, we need a current. That's where electromagnetic induction comes in. Electromagnetic induction is when the magnetic field of a loop of wire is changed by moving a magnet in or over loops of wire without an additional voltage source. The relative motion between the magnet and the loops induces voltage, which causes current. This means that it is a method of transforming mechanical energy to electrical energy.
The relationship between loops of wire and voltage is directly proportional. The greater the number of loops, the greater the voltage. Additionally, slow motion means low voltage and quick motion means high voltage.
Examples of how electromagnetic induction is useful in daily life are: traffic light triggers, hybrid cars converting braking energy to electric energy, credit card machines, and security sensors.
So: how does a credit card machine work?
Credit cards work by electromagnetic induction. This is when an electric current is produced in a wire by moving a magnet near a loop of wire or vice versa. This induces voltage by changing the magnetic field through relative motion between the magnet and the wire, which causes current. Each credit card has a specific pattern of magnets in a strip on the back. Each credit card machine has a loop of wire where you swipe the card. When you swipe your credit card, the voltage induced is specific to your card so the individual current flow that is caused identifies your card to the machine.
Forces on Charged Particles
As charges move around a magnetic field, the charges that are moving perpendicularly to the magnetic field feel the force from the field.
While this seems abstract, there is a natural phenomenon that is caused specifically by this physics concept: the aurora borealis (northern lights). Simply put, cosmic rays are charged particles that enter the earth's magnetic field from outside of the atmosphere. When these cosmic rays are moving parallel to the earth's magnetic field, they cannot enter the atmosphere. This means that the charged particles are deflected along the equator, which runs parallel to the earth. However at the north and south poles, the charged particles can enter the earth's atmosphere which causes the light show often referred to as the northern lights. Aurora borealis is the name for this phenomenon when it occurs in the northern hemisphere, it can often be seen in places like Canada, Norway, Russia, Finland, Scotland, Iceland, Greenland, Denmark, and some parts of Alaska in the United States of America. In the southern hemisphere, it is called aurora australis and can be seen in Australia, Tasmania, New Zealand and Antarctica.
A while back, I posted about the motor I built using paper clips, a magnet, a 9V-battery, rubber bands, and a copper wire. That exercise is a prime example of how the two essential parts of a wire and a magnet. The key principle that a motor relies on is that the current-carrying wire (remember: moving charges!) will feel a force from the magnetic field which will cause a torque. Remember from unit four that torque is what causes rotation? Well, that is why the current-carrying wire spins. This is a great example of a conversion of electrical energy to mechanical energy.
To figure out which way the magnetic field will make the wire spin, we can use something called the right hand rule. As we can see from the diagram below, we can use our right hand to visualize this concept. The thumb represents the force, the index finger represents the current and the middle finger represents the magnetic field.
As an example, if you know that the magnetic field is going forward, the force will go to the left. Try it!
Energy Production
The opposite of a motor, a generator converts the mechanical energy of a spin to electrical energy. A generator uses electromagnetic induction by moving magnets around a wire or a wire around magnets to produce energy. All generators are the same. You are most likely already familiar with the three most common ways that companies generate energy: steam, water, and wind.
Energy Transfer
A transformer is a pair of wire coils that converts alternating current from an outlet to direct current for a device. A transformer has either a high number of coils to a low number, or vice versa. Transformers require alternating current to function because the constant change in direction in the current causes a change in the magnetic field. This changing magnetic field in the primary (the first coil) will induce a voltage in the secondary (the second coil).
Ding! Ding! Ding! What does that sound like? Electromagnetic induction! It's everywhere this unit.
There are two types of transformers we need to know about: a step-up transformer and a step-down transformer. A step-up transformer increases the amount of voltage going to the device. This could be seen with a large device like a washing machine, which needs more voltage than the typical American outlet provides (120V). A step-down transformer is used for a smaller device like a cellphone, which would likely overheat if it used 120V.
We can use something called Faraday's Law for calculations involving transformers. It looks like this:
# of loops in the primary/ voltage of primary = # of loops in the secondary/ voltage of secondary
We also need to know that the power of the primary and the power of the secondary will always be equal. This means that if the primary has a small current and a high voltage, the secondary must have a high current and a low voltage. This will be the only time that current and voltage are inversely proportional.
Remember: We can also use I = v/r to find current.
Something interesting about transformers is that they are all around us, but we barely notice them. Ever wondered what that huge block on your laptop charger is? It's a transformer. Ever wondered what those gray boxes on power lines are? They're transformers, too. Transformers are put on power lines in order to decrease the amount of current flow in each line. This is in order to conserve energy. Remember that energy is released as light, heat, and sound? If transformers were not on power lines, more current than necessary would run through them and would be wasted and released as heat. I think the use of transformers is a pretty neat solution.
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