Need something lightweight and mobile, but still have a keyboard? Try a netbook or
chromebook. They usually are lightweight and have great battery life.
If that's not your style you can always try a tablet, which range from very simple to being a tablet PC. They tend to be more lightweight than netbooks and even allow you to take notes the old fashioned way. May be easier to read digital texts as well.
Or you could go with a regular laptop. Compared to the above two options it's going to fall flat on its face, you may think of them as mobile, but unless you do a lot of traveling, you'll find that the "mobile" allure isn't always very alluring. If you do lug it to classes every day it's going to be a large and constant nuisance, since presumably you'll have gotten a large screen since it serves as your dorm computer as well. I'll give you a list of reasons why getting a laptop for college is a bad idea:
PortabilityMost laptops with larger screens and decent cooling are going to not only be heavier, but more bulky and unwieldy. It's really just going to be like an extra book that you drag around, which can quickly grow annoying. Not to mention when it's parked on your desk it's going to be hooked up to at least a mouse, perhaps some headphones, and a wired internet connection (optional, but may be faster than the wireless the uni offers). That's just the basics, you could also have it hooked up to an additional larger monitor and a keyboard. Now when you want to bring it somewhere you have to unhook up all of that stuff (let's face it, in college you're going to be lazy).
PowerIf you think that your laptop is going the be the only thing you need to drag along you're horribly mistaken. Unless it has an ungodly battery life you're going to be bringing your power cord. Which is just one more thing to carry.
DistractionOh sure sitting there in lecture and writing notes on your laptop is just a grand idea isn't it? The hell it is unless you have some super zen like discipline. Too many college students have already fallen victim to booting up their computers and jumping on the internet or playing games instead of paying attention to the lecture.
Typing NotesTyping notes in lectures may work for a philosophy or literature class. That's the extent of it, really. The harder into the science you get and the more you'll find out that your word processor is downright terrible at recording mathematical formulas and dimensional analyses. It's downright impossible in calculus classes when the professor starts drawing things. Unless of course, you're a master at creating interactive Mathematica functions. It's a damn hassle and far easier just to write it out on a sheet of paper (or alternatively, a tablet).
CostThe mac daddy of it all, cold hard cash. The aforementioned technical problems (weight, battery life) can be remedied easily by spending more money. Spend enough and you can even get a machine that has a respectable battery life and can play Skyrim at high settings with respectable frame rates. But even if you did have the money to drop on such a machine, I doubt you would want to subject it to trekking all over campus -- it also makes you a target for theft. Unless money just doesn't matter to you in the slightest, don't bother.
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There are plenty of reasons to get a nice big sturdy desktop, preferably with a case that locks. It's cheaper to the same amount of power, and you can run the latest games no problem. And it may be wise to secure it to your desk as well -- regardless of how vigilant you are of locking the door when you leave, you can't be assured that your roommate is and theft can be a problem (all the more reason not to get a laptop).
More options include a higher quality smartphone as a makeshift graphing calculator or just use an actual calculator (
Wolfram Alpha Mobile). In my Chemistry experience I've never needed more than a basic TI-30X, but it wouldn't necessarily hurt to have a fancier graphing calculator (although I've never found a use for one). I advocate learning how to do your math without a calculator unless it's for just raw calculation (which the TI-30X is more than sufficient for).
Convenience is the ultimate key here. A smartphone, for example, is something you never forget, can easily pick up, and can slide in your pocket no problem. A tablet is a bit heftier but still is easy to grab and just go with. A laptop is not.
None.