If you have an iPhone 4 with a 10-watt charger cable, you can use it to charge your iPad. Any other iPhone cable will deliver only half the necessary wattage to your iPad if you use it with the included wall socket adapter. The differences between iPhone and iPad cables lie in the power transformer portion of the cable.
Your iPad and iPhone adapters, if you use the models included with your devices at purchase, have a 30-pin-to-USB cable, a transformer and a detachable wall prong unit. The transformer and wall prong unit form the square part of the adapter. The transformer is the larger portion, converting your standard household electricity to the lower-capacity charge needed by the iPad. Standard household electrical current in the U.S. is 120 volts at 15 to 20 amps. The iPad requires only 5.1 volts and 2.1 amps. iPhone models prior to the fourth generation require even fewer. Look on the side of this square to find the transformer's wattage. If it says "10w USB Power Adapter," it will charge your iPad, whether you bought it with the iPad or iPhone 4.
Other than your transformer, the portions of the charging cables are interchangeable. You can swap your iPhone's cable or wall prong assembly with your iPad charger's and still charge your iPad properly as long as the transformer is the correct one. If you have a high-powered USB port on your computer, which many newer Macs and some newer Windows PCs do, you can use either the iPhone or iPad cable to charge the device using your computer.
Your iPad will receive some electrical charge if you use your iPhone charger cable, including its transformer, even on older-model iPhones. However, these cables deliver only half the necessary wattage to your iPad, so charging wil l proceed very slowly or not at all if the screen is on; only sleep-mode charging is supported. You'll face the same issue using either the iPad or iPhone cable to charge your iPad on a computer that lacks a high-powered USB port. Standard and high-speed USB ports can't deliver the 10 watts the iPad needs.
Replacement charger cables from Apple or other manufacturers, including car chargers, don't all have the same power output. If you use a replacement iPhone charger, check the fine print on the transformer portion of the charger to find the power output. It may be listed in volts and amps, such as "Output 5.1V---2.1A." Multiply the amps and volts to get the approximate wattage. In this case, 5.1 x 2.1 = 10.71, so the charger delivers approximately 10 watts, the appropriate amount for the iPad.
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