Electric charge is the
physical property of
matter that causes it to experience a
force when placed in an
electromagnetic field. There are two types of electric charges:
positive and
negative. Positively charged substances are repelled from other positively charged substances, but attracted to negatively charged substances; negatively charged substances are repelled from negative and attracted to positive. An object is negatively charged if it has an excess of
electrons, and is otherwise positively charged or uncharged. The
SI derived unit of electric charge is the
coulomb (C), although in electrical engineering it is also common to use the
ampere-hour (Ah), and in
chemistry it is common to use the
elementary charge (
e) as a unit. The symbol
Q is often used to denote charge. The early knowledge of how charged substances interact is now called
classical electrodynamics, and is still very accurate if
quantum effects do not need to be considered.