[理学]大学物理学专业英语2.ppt

Electricity Magnetism Electric charges and fields Electric force: Charges attract or repel each other with an electric force. If point charges Q1 and Q2 are distance r apart, and F is the force on each, then according to Coulomb’s law: This is an example of an inverse square law. If r doubles, then force F drops to one quarter, and so on. With a suitable constant, the above proportion can be turned into an equation: The unit of charge for Q1 and Q2 is the coulomb (C). The value of k is found by experiment. It depends on the medium (material) between the charges. For a vacuum, k is 8.99×10-9 N m2 C-2, and is effectively the same for air. In practice, it is more convenient to use another constant, ε0, and rewrite the equation on the left in the following form: In vacuum ε0 is called the permittivity of free space. Its value is 8.85×10-12 C2 N-1 m-2. Note: Although ‘4π’ complicates the above equation, it simplifies others derived from it. In the above equation, if, say Q1 and Q2 are like charges (e.g. – and –), then F is positive. So a positive F is a force of repulsion. Similarly, it follows that a negative F is a force of attraction. Electric field If a charge feels an electric force, then it is in an electric field. If a charge q feels a force F, then the electric field strength E is defined like this: In symbols For example, if a charge of 2 C feels an electric force of 10 N, then E is 5 N C-1. Note: Electric field strength is a vector. Its direction is that of the force on a positive (+) charge. The force acting on a charge in an electric field can be found by rearranging the equation above: Charge Q produces an electric field which acts on a small charge q. As F= qE So: The electric filed round a charged, spherical conductor is shown on the right. It is a radial field. r Note: The charge is on the surface of the conductor. Outside the conductor, the electric field is the same as if all the charge were concentrated at the centre, and the above equation applies.