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I keep a thin paper booklet with a magnetically encoded stripe in the same thin protective case as my 2020 MacBook Air (1.1 GHz Quad-Core Intel Core i5), and when it's in my backpack it's pressed to the bottom side of the laptop.

Three times now I've had to have the stripe rewritten or replaced, and it suddenly occurred to me that it may be related to the laptop.

The laptop power is off (laptop shut down thought the battery still discharges slowly but the fan isn't running to my knowledge.

Question: Does my 2020 MacBook Air have an external magnetic field that might be erasing a magnetic stripe pressed against the bottom? If so, where is it? Back, left/right side, front, middle?

I first thought of MagSafe, but of course this model doesn't have it.

"bonus points" This is pure physics curiosity; if there is a magnetic field, does Apple have a spec for how strong it is? Earth's field is of order a half gauss, I'm just curious how strong it might be.

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    idk whether these particular Macbooks also have sensor magnets to recognise when the lid is closed. Drag a paperclip over the surface, keyboard & screen perimeter to find any.
    – Tetsujin
    Commented Oct 6, 2022 at 9:52
  • @Tetsujin ah good thinking, I will look into it. I can also try to use my iPhone's compass app as a hack magnetometer. The Earth's field is only 0.5 gauss so if there are even very weak static fields from permanent magnets they will cause the compass to deflect. In fact, I have some real, physical magnetic compasses as well. This is going to be fun!
    – uhoh
    Commented Oct 6, 2022 at 10:34
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    For your info, there is no fan in MBA 2020 with M1 processors. Also magnetic stripes are difficult to erase it not directly putting a very strong magnetic field on it.
    – Ptit Xav
    Commented Oct 6, 2022 at 13:52
  • @PtitXav Oh that's good to keep in mind! I've added "1.1 GHz Quad-Core Intel Core i5" to the question for completeness. Note that we're talking about intimate contact for weeks or months, perhaps involving some back-and-forth rubbing motion as I walk ~5 km a day plus take it with me hiking on the weekends. Can I ask how strong "very strong" is? Note that this is a flexible printed stripe on a paper booklet, not the same as the magnetic tape embedded in a plastic credit card.
    – uhoh
    Commented Oct 6, 2022 at 19:22

3 Answers 3

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You say "power off", but then you mention battery drain. If the battery drains, then the MBA is on, maybe in sleep mode, but that is still on.

Every electro-magnetic-wave (as the name says) that is used for WiFi, Bluetooth, cellular and so on also "creates" a magnetic field. The magnitude is much lower though (the constant connecting the magnitude of the electric field and the magnetic field is interestingly the speed of light).

So if your MBA is indeed still on and maybe also connected to a network or bluetooth device it does have a magnetic field. Further any current creates a magnetic field, however (although I haven't done any calculation) I suspect that the currents flowing inside the MacBook (i.e. not from the power brick to the MBA) are so small that the magnetic field is tiny.

I haven't done any calculations whatsoever so I can't say anything about the magnitude, but your phone can simply measure it!

The app is called "phyphox" and available for both Android and iPhone:

I don't know about the precision, however, it should give you an indication of where there the magnetic field is the strongest.

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I have a MacBook Pro (16-inch, 2019), so not exactly the same. But for many Mac laptops there are magnets along the top edge of the screen. They hold the laptop shut when it's shut. If they weren't there, the laptop screen might bounce open and shut as you walk, and send a lot of sleep/wake events.

Take something made of basic metal (I had a small flat-head screwdriver on my desk) and run it along the black plastic between the visible screen image and the edge of your laptop's lid. You'll feel the magnets tug on your piece of metal. It's obvious. I have about 4 magnets on the left side of my laptop screen and 4 on the right.

So everyone else is correct: there are magnets in the speakers. But there are also magnets in the lid.

"Power Nap" is a concept, too. Your Mac will wake up once in a while, join the wifi, mess around on the Internet (download email, update calendars, etc). So even though you put it to "sleep", it won't stay asleep. Go to System Preferences --> Battery, and then select 'Battery' and you'll see the option to Enable/Disable "Power Nap" if you want your computer to stay asleep when you put it to sleep.

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The strongest magnetic field from a Mac is the speakers. Passing a magnetic card past them a few times hasn’t been an issue for me but I suppose you could have cards get degaussed but that.

Here’s what Apple has to say about magnets and their products.

I don’t have any physics references on field strength to provide, but Apple sells a MagSafe wallet to hold your (and possibly their) credit card right next to the phone magnets. In my unscientific testing, the magnet on my phone and wallet are stronger than anything I can get from the top or bottom of my Mac. With strong magnets in purses and wallets, I’m guessing you have another issue with your cards besides a Mac.

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