Your biceps are stronger than your triceps, so why use the same five-pound dumbbell to work both muscles? You might think it's annoying or a waste of time to change weights for different exercises, but you'll actually save time because you won't have to do as many reps.
Keep a set of three different-sized dumbbells on the floor in front of you so you can easily grab the weight you need. Not sure what size dumbbell to use? You'll know it's the right amount when your muscles fatigue after 10 to 12 reps. As your muscles become stronger, increase the weight to respond to each muscle's ability.