Although they have similarities, they have differences that make them appropriate for certain uses. The biggest difference between them is their size (pun intended). The iPad is almost 7 times bigger than the iPhone but is slightly thinner. The bigger device allows Apple to put better specs into the iPad.
For one, the iPad has a 9.7 inch display while the iPhone has a 3.5 inch display but with only a slightly higher resolution. Despite that, the bigger screen of the iPad makes it much better when watching video or browsing the Internet. You can still do the same with the iPhone but you need to make do with the smaller screen and very frequent zooms when online. The iPad is also equipped with a bigger battery, to provide the same amount of video viewing or browsing time. The iPad has bigger power requirements and installing a battery with identical specs to that of the iPhone would mean lesser available power.
Although the iPad has some advantages over the iPhone, there are also roles that only the iPhone can play. The iPad is not able to make calls or send text messages like the iPhone; after all who would want to hold the iPad to their face to make or receive a call. The iPad was also not equipped with a video camera, so you cannot record videos or even take pictures with it. The iPhone 4 actually has 2 cameras, one on the rear for pictures and video and one in front for video calling.
The iPad only has a few of the sensors you would find on the latest iPhone. The most important sensor right now is the gyroscope which allows for better motion sensing than the accelerometer. It should open up new possibilities for game controls and other stuff. Sadly, the current iPad cannot run those apps since it lacks the gyroscope of the iPhone.
Summary:
The iPad is much bigger than the iPhone
The iPad has a bigger battery than the iPhone
You can make calls on the iPhone but not on the iPad
The iPhone has a camera but not the iPad
The iPhone has more sensors than the iPad
Read more: Difference Between iPad and iPhone | Difference Between | iPad vs iPhone
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