In so far as using iPhone charger for iPad or vice versa is concerned, the basic question which arises is the design of the charger. As already stated, the Apple 4 iPad uses charger with 30 pin and this cannot be used on iPhone or with other iPad(iPad Pro) because of the mismatching of the pins and the slot provided for inserting the charger. Barring this, technically it is possible to charge an iPad with a charger meant for iPhone but it may take a long time and also it may damage the circuit because of the use of wrong type of charger. At the same time, the batter may not perform at its optimum level because of the use of wrong chargers. It may take hours of charging because of low current in the case of iPhone. On the other hand if you use a charger with higher amps then the iPad or iPhone may charge much faster, but it may result in the damage to the circuitry system and performance of the touch screen, sometimes the touch screen response carry static charges. Extra heating may cause damage to the battery because when you are using a much higher charger, 2 Amp on a phone which can take only 1 Amp you will overheat the batteries and damage the circuitry system. On the other hand when you are using 1 Amp charger to charge a tablet battery which usually works on 2 Amp charger you may be causing less damage but you are making the charging slow and most probably this may affect the performance of the battery in the future.
If we get into technical details we can see why the iPhone charger does not work to charge iPad batteries because all Apple chargers are 5 volts and amps is the amount of current flowing in either iPhone or iPad to charge their batteries. The formula for calculating Watts is Volts x Amps = Watts. For example the charger with 5 volts with 1 Amp requires 5 watts; this is needed to charge iPhone battery. The charger with 5 volts and 2.1 Amps has 10 watts; this is needed to recharge iPad batteries. If we look at this formula we can understand how using an iPad charger to recharge batteries of iPhone is technically not very satisfactory. From the technical aspect, iPad charger is better than iPhone charger considering its higher capacity of power absorption. However, it is advisable to use the specific type of charger for respective device for optimum results.
Thank you for the detailed article. I use my iPad charger to charge my iPhone and never had any problems. Now getting ready to go on a trip and do not want to carry my Sony HD Video camera charger which already has a small USB cable built in it.