Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
It depends if those apps are actively processing in the background. If they are not, there will be no impact on battery life. The majority of apps fit that criteria, even many apps that use data for their content, such as news apps. Usually they only update their content when you launch the app or manually refresh. If left running in the background they wouldn't be processing anything or accessing data. In fact, multi-tasking would probably improve battery life in this case as you wouldn't need to close/re-launch when switching tasks which would force it to fetch data uneccessarily.

is there a way to check what is running on my iphone 3GS? or do they automatically close when you press the home button? I read something about holding the top button then holding the bottom button to actually close the app, but hardly ever do it.
 
is there a way to check what is running on my iphone 3GS? or do they automatically close when you press the home button? I read something about holding the top button then holding the bottom button to actually close the app, but hardly ever do it.

Any third-party app will be completely closed. Some default Apple apps are allowed to continue to process in the background such as Safari and the iPod. For example, if you start loading a web page in Safari and immediately return to the homescreen, when you launch Safari again it the webpage will have been fully loaded. The force-quit method will work no different for third-party apps but will terminate the process for any default Apple app that would normally continue to run as a processes in the background.

In general, there is no way to tell which processes are running and which are not, although there are apps in the app store that will show you this. All of the processes that run are associated with the OS (and the few default Apple apps, if running) and do things like monitor the sensors: accelerometer, ambient light, proximity, capacitance, etc, and fetch for mail and things like that.
 
Any third-party app will be completely closed. Some default Apple apps are allowed to continue to process in the background such as Safari and the iPod. For example, if you start loading a web page in Safari and immediately return to the homescreen, when you launch Safari again it the webpage will have been fully loaded. The force-quit method will work no different for third-party apps but will terminate the process for any default Apple app that would normally continue to run as a processes in the background.

In general, there is no way to tell which processes are running and which are not, although there are apps in the app store that will show you this. All of the processes that run are associated with the OS (and the few default Apple apps, if running) and do things like monitor the sensors: accelerometer, ambient light, proximity, capacitance, etc, and fetch for mail and things like that.

thanks.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.