When you rub a balloon against your jersey, you create a static charge on the surface of the balloon. This happens due to a phenomenon called "triboelectric charging," where two different materials, in this case, the balloon and the jersey, come into contact and exchange electrons. Some materials have a stronger affinity for electrons and tend to gain them, becoming negatively charged, while others lose electrons and become positively charged.
In this case, when you rub the balloon against your jersey, electrons from the jersey are transferred to the balloon. As a result, the balloon becomes negatively charged and the jersey becomes positively charged.
Now, let's see what happens when you hold the charged balloon near your hair. Your hair is made up of individual strands that are neutral, with an equal number of positive and negative charges, so they naturally attract and repel each other, staying in a relatively straight and neutral position.
However, when you bring the negatively charged balloon close to your hair, something interesting occurs. The balloon's negative charge repels the negative charges in your hair, causing those charges to move away from the surface of your hair strands and distribute themselves more evenly within the hair. This redistribution of charges leaves the surface of the hair strands positively charged.
Since like charges repel each other, the positively charged hair strands now repel one another. As a result, the hair strands try to move away from each other, causing them to stand on end. This phenomenon is commonly known as "static electricity" or "static cling."
The effect is more noticeable if the air is relatively dry, as moisture in the air helps dissipate the static charge, making the hair less likely to stand on end. The static charge also gradually dissipates over time, returning the hair to its usual state.
So, in summary, the rubbing of the balloon on your jersey causes it to become negatively charged. When you bring the charged balloon near your hair, the hair strands become positively charged, causing them to repel each other and stand on end due to the principle of like charges repelling.