 Assess. Statements due Monday, 10/20/14

 Like the gravitational field around masses, an electric field occupies the space surrounding charged objects.  To test for the presence of an electric field: o Bring a small positive charge (q) into the space o Release the small positive charge If the charge experiences a force, then we know there is an electric field present No force, no electrical field  Known as a Test Charge

 Attractive forces  The test charge will accelerate towards the negatively charged object  The test charge will follow the path of the electric field of the negative object:

 Electric charges exert forces on other electric charges through the electric fields  Quantified through Coulomb’s Law: o The electric force between two point charges, Q 1 and Q 2, is inversely proportional to the square of their separation distance and directly proportional to the product of the two charges:   0  electric permittivity of a vacuum o (= 8.85 x C 2 ·N -1 ·m -2 )

 Two charges, 4.00  C and 6.00  C, are placed along a straight line separated by a distance of 2.00 cm. Find the force exerted on each charge.

 Two equally charged lightweight balls, q, are suspended from strings that are each 10.0 cm long. They repel each other and have a separation distance between the charged particles is cm. Assume the mass of each of the lightweight balls is g. What is the charge on each of the balls?

 Electric field lines are drawn in the same direction as the force the small postive test charge would experience at that point.  The electric field strength is defined as the force per unit charge experienced by a small positive test charge, q.  The electric field strength is FELT by the charge in the field itself.  The electric field strength is CAUSED by the charge creating the field

 Electric field strength depends on the charge that is creating the field, and it depends on how far away from the charge the field is being measured:

 What is the electric field strength 7.50 cm from a particle with a charge of 5.00  C?  What force would a charge of nC experience at that point?

 The electric field between two parallel plates is N·C -1. What acceleration would a charge of 2.00  C and mass 1.00 x kg experience if placed in this field? (ignore the weight of the charged mass)

 Imagine an electric field generated by a charge Q, and consider a positive test charge, q, in that field.  What must be done to move q closer to Q?  Electric Potential is the work done per unit charge to bring a positive test charge from far away to the some point P in an electric field created by Q  The work done in moving the charge to point P from infinity increases the electrical potential energy of the test charge.

 The work done in moving a test charge of 2.0  C from very far away to a point P is 1.50 x J. What is the electric potential at point P?

 The potential at a point P is 12.0 V and a charge of 3.00 C is placed there. What is the electric potential energy of the charge?  What is the electric potential energy if the charge placed at P is – 2.00 C?

 Electric potential difference: the work done to move a charge from point A in an electric field to point B in an electric field (neither is very far away from the source of the field).  Mathematically, it’s the difference of potential energy for the charge at each of the positions (W =  U)  Potential difference is the total work done to move the charge (change in energy)  Potential is the work per unit charge.

 What work must be performed in order to move a charge of 5.00  C from the negative plate to the positive plate if a potential difference of 250. V is established between the plates?

 A charge of 5.00  C and mass 2.00 x kg is shot with an initial velocity of 3.00 x 10 2 m·s -1 between two parallel plates kept at a potential of V and V, respectively. (the charge starts at the low potential end) o What will the speed of the charged mass be when it gets to the other (higher potential) plate?

 The amount of work needed to move a charge equal to one electron’s charge through a potential difference of exactly 1 volt.  What is the work needed to move a charge of +2e across a potential difference of 2.0 V?  What is the work needed to move a charge of +3e across a potential difference of 5.0 V?

 What is the equivalent of 1 eV, measured in Joules?  1 eV = work to move 1e through 1V  What’s the charge of 1e?  W = qV=(1.61x C)·(1 V) = 1.61 x J