1、Assessment for this course,Final exam: 60% Mid-semester exam: 20% Quiz x 3: 10% Homework: 10%,If you do the homework, the rest should be easy!,In this course, we learn about four fundamental principles.,(1) Momentum principle动量原理(2) Energy principle 能原理(3) Angular momentum principle 角动量原理(4) Fields
2、场,Electric forces,Two kinds of charge (电荷),One type from rubbing amber 琥珀(Greek word for amber: elektron),One type from rubbing glass玻璃,Particles with the same charge repel each other (相斥),Particles with the same charge repel each other (相斥),Particles with opposite charges attract each other (相吸),Be
3、njamin Franklin (1706-1790)Believed that electricity was a fluid (流体) that can flow between objects.Too much fluid = “positive” chargeToo little fluid = “negative” charge,+,-,Physics would be less confusing if he named them the other way around!,Unit of charge: Coulomb (C),Charge is quantized (量子化).
4、Everything is charged in multiples of e, the elementary charge.Except,Quarks (夸克) have a charge less than e, but are never seen alone.,Proton,Neutron,Pion,Nuclei and atoms,原子,核,质子,中子,电子,Electrostatic force静电力,Coulombs Law:,; 0 is called the “permittivity of free space”,Example,How strong is the repu
5、lsive electrostatic force between two protons in a nucleus?,Example,How strong is the attractive gravitational force between two protons in a nucleus?,The strong nuclear force is what keeps nuclei from blowing themselves apart.,Electrostatic forces are strong.We usually dont notice because most thin
6、gs including us have the same amounts of positive and negative charge.We are electrically neutral (中性).,+,+,+,+,We can divide the world into two parts: The source charges, which are causing the force The test charge, which feels the force,This is a bit like the system/surroundings idea in mechanics.
7、,+,+,+,What if we remove the test charge?,The “influence” of the source charges is still there we call it the electric field.,Electric field,is a vector field. It has a value and a direction at every position in space and every moment in time:,Electric field,Two steps when thinking about electric forces:Calculate the field caused by the source charges.Calculate the effect the field has on other charges.,Electric field of one point charge,q,
