Ubuntu come configured to use the CPU in its limit when it is needed. You can configure your computer to use less processing power than it is available and things will cool down.
Open a terminal and type:
sudo nano /etc/rc.local
add this lines before the exit(0)
for i in `ls /sys/bus/cpu/devices/ | egrep "^cpu[0-9]+"`; do echo "2000000" > /sys/bus/cpu/devices/$i/cpufreq/scaling_max_freq; done
sysctl -w vm.swappiness=0
and then reboot your computer or execute this command:
This will make your computer a little slower, a lot colder and your battery time will increase.
Tweak a bit with the frequency. You can see the valid frequencies with this command:
On my i5-3230M valid values are: 2601000 2600000 2500000 2400000 2300000 2200000 2100000 2000000 1900000 1800000 1700000 1600000 1500000 1400000 1300000 1200000. But you should have more high frequencies available.
Most of the time I limit to the minumum frequency (12000000) and still have a very good desktop experience on ubuntu gnome. I use ubuntu gnome 3 desktop, lots of openoffice, google sketchup and eclipse. I draw a lot.