Why are P and T waves curved and not straight (ECG)
P wave represents atrial depolarization (hence atrial contraction).

Atrial depolarization takes only a split second, therefore its corresponding wave will be smooth and rounded.

Variations from this shape could be consequences of various heart diseases: for example,a spiked P wave is called P pulmonale and suggests right atrial enlargement.

T wave represents ventricular repolarization: a normal T wave looks like an 


  upright low amplitude broad hump following the QRS complex.


Abnormalities in T wave could be the signs for multiple heart diseases, varying from myocardial ischemia to hyperkalemia.

In particular, peaked T waves suggest an hyperkalemia diagnosis.

T wave abnormalities are well documented here

The QRS complex corresponds to ventricular depolarization, which is faster and more intense than atrial or ventricular repolarization: hence a normal QRS complex is peaked.

In conclusion, P wave and T wave are rounded because these correspond to low-intensity electrical events while the QRS complex is provoked by an intense electrical event.