Saved by Magnetism!

A side view of my MagSafe charger plugged into my laptop.

Last weekend my sister left her MacBook Pro connected to its charger on our kitchen table. While I was sitting on my couch observing the physics-filled world, I heard my mom exclaim, “Why does she always leave her laptop here? One day someone is going to trip over that cord and the laptop is going to fall to the ground!” I corrected my mom about this statement for two reasons: one, because I am aware of Apple’s MagSafe technology that is related to the magnetism unit I just learned, and two, because I  am a die-hard Apple supporter and will do anything to praise their products. Building off of the iDevice charger blog post that I made last week, I thought that I would continue with a discussion about the design of MacBook Pro chargers.

FYI: The above photo is an up-close view of Apple’s new MagSafe 2 charger. The middle node notifies the charger that it is connected to the laptop, the inner-middle nodes transfer power to the laptop, and the outer nodes are ground connections so that you do not get shocked when you touch the charger.

So, what does Apple’s MagSafe charger technology have to do with physics anyway? In two words: a lot. For this blog post, we will focus primarily on the magnetic design features of the MagSafe chargers. When you bring the end of the MagSafe charger near the charging port of the MacBook Pro, the charger becomes attracted to the charging port, snaps into place, and begins to transfer charge to the laptop. What causes the attraction between the charger and the laptop boils down to the basics of magnetism. Magnets have two ends called north and south poles. Similar to electric charge, likes repel and opposites attract. That is, north and south poles will attract each other while like poles will repel each other. Given this information, what can we conclude about the poles of the charger and the laptop charging port? Because the two attract each other, the poles must be opposite: one must be north and the other must be south. The idea to create a magnetic charger was extremely profound. Through Apple’s practical application of magnetism, they were able to create a charger that would save humans (from tripping) and Macbook Pros (from falling) alike.

This discussion of chargers leads me to another profound application of physics on Apple’s part. Apple’s MacBook Pro chargers output only 18.5 volts, yet the voltage supplied in a typical American power outlet is 120 volts. How does such a large voltage drop occur? Amazingly enough, Apple’s MacBook Pro chargers have tiny transformers inside of them that can transform a high voltage/low current situation into a low voltage/high current situation. Transformers achieve this phenomenon through alternating current and varying magnetic fields, which allows them to adjust voltage and current accordingly to suit the device’s power needs.

Who knew that magnets had better uses other than being toys for children who try to stick their same poles together? If you did not already know of some of their practical uses, I hope I have now educated you. If I have not, feel free to continue sticking like poles of magnets together to see how that works out for you.

The Physics of iDevice Chargers!

iPhone charger (left) and iPad charger (right).

A few days ago before I went to bed, I could not find my iPad charger for the life of me. Although I did not have an iPad charger handy, I found an extra iPhone charger on the floor of my room. Perfect, I thought, surely there is not that much of a difference between the two chargers. To my surprise, I was wrong. When I woke up the following morning I found that my iPad was only fifty percent charged. Why is it that my iPad can charge in about a night on the iPad charger but not on the iPhone charger? The simple answer comes down to basic current electricity.

Source	Voltage (V=IR)	Current (I=V/R)	Power (P=IV)
USB Port	5 volts	0.5 amps	2.5 watts
iPhone Charger	5 volts	1.0 amps	5 watts
iPad Charger	5 volts	2.0 amps	10 watts

The table above shows the values of voltage, current, and power for three different types of chargers that can be used to charge an iPad. Although voltage may be the same, current differs, which is the main value we are concerned with when it comes to charging our precious iDevices. From a quick glance at the table, we see that the current of the iPad charger is twice the current of the iPhone charger. After this observation, the reason why my iPad took so long to charge with the iPhone charger becomes clear.

Let us assume that I can normally charge my iPad from 0%-100% in about five hours using the 2amp iPad charger. If we decrease the current and power by half by connecting the iPad to the iPhone charger, the iPad takes about twice the amount of time to fully charge (about 10 hours) than it would take on the iPad charger. Even worse, if I decided to charge my iPad through my computer’s USB port, it would take about four times the amount of time to charge (about 20 hours) than it would take on the iPad charger because USB current is only 1/4 that of the iPad charger.

If the iPad charger is the clear winner in terms of charging time, why not use the iPad charger for all of your iDevices? Theoretically, you could. Apple uses the same voltage on almost all of their iDevice chargers so that people can use them interchangeably. So, why not just buy all iPad chargers? In short, it is not necessary. Why waste money on electricity to charge your iPhone with a 10amp iPad charger instead of a 5watt iPhone charger when your iPhone would finish charging at relatively equal times with both? Supplying more current to the iPhone will not make it charge faster. Although the iPad charger has twice as much current as the iPhone charger, the iPhone battery circuit only takes the amount of current that it needs (around 1amp) so that it does not overheat. In many devices nowadays, oversupplying current is not a problem. Undersupplying current for rechargeable devices, however, can dramatically increase charging time.

Now that you have basic knowledge about iPad and iPhone chargers, you may want to think twice before using them interchangeably.