LOS ANGELES — Microsoft has unveiled Surface, a tablet computer to compete with Apple’s iPad.CEO Steve Ballmer announced the new tablet, calling it part of a “whole new family of devices” the company is developing.One version of the device, which won’t go on sale until sometime in the fall, is 9.3 millimeters thick and works on the Windows RT operating system. It comes with a kickstand to hold it upright and a touch keyboard cover that snaps on using magnets. The device weighs less than 1.5 pounds and will cost about as much as other tablet computers.The size is similar to the latest iPad, which is 9.4 millimeters thick and weighs 1.3 pounds. Microsoft also promised that the Surface’s price tag will be similar to the iPad, which sells for $499 to $829, depending on the model.Microsoft’s broadside against the iPad is a dramatic step to ensure that its Windows software plays a major role in the increasingly important mobile computing market.“They are saying it’s a different world now and are trying to put the sexy back into the Microsoft brand,” said Gartner Inc. analyst Carolina Milanesi.Microsoft is linking the Surface’s debut with the release of its much-anticipated Windows 8 operating system, which has been designed with tablets in mind. The company hasn’t specified when Windows 8 will hit the market, but most analysts expect the software to come out in September or October.