PRS04 Masses in Potentials Consider 3 equal masses sitting in different gravitational potentials: A) Constant, zero potential B) Constant, non-zero potential C) Linear potential (V ∝ x) but sitting at V = 0 Which statement is true? 1. 2. 3. 4. None of the masses will accelerate Only B will accelerate Only C will accelerate All masses will accelerate, but B will have the largest acceleration 5. All masses will accelerate, but C will have the largest acceleration PRS04 Masses in Potentials (3) Only C (in linear potential) will accelerate When you think about potential, think “height.” For example, near the Earth: U = mgh so V = gh Constant potential (think constant height) thus will not cause acceleration. The value of the potential (height) is irrelevant. PRS04 Positive Charge Place a positive charge in an electric field. It will move from 1. higher to lower electric potential; lower to higher potential energy 2. higher to lower electric potential; higher to lower potential energy 3. lower to higher electric potential; lower to higher potential energy 4. lower to higher electric potential; higher to lower potential energy PRS04 Positive Charge (2) From higher to lower potential and higher to lower potential energy Objects always move to reduce their potential energy. Positive charges do this by moving towards a lower potential: U = qV PRS04 Negative Charge Place a negative charge in an electric field. It will move from 1. higher to lower electric potential; lower to higher potential energy 2. higher to lower electric potential; higher to lower potential energy 3. lower to higher electric potential; lower to higher potential energy 4. lower to higher electric potential; higher to lower potential energy PRS04 Negative Charge (4) From lower to higher potential and higher to lower potential energy Objects always move to reduce their potential energy. Negative charges do this by moving towards a higher potential: U = qV PRS04 Potential and Energy Which is true? I. It takes positive work to bring like charges together. II. Electric field lines always point in the direction of decreasing electric potential. III. If a negative charge moves in the direction of the electric field, its potential energy decreases. 1. II only. 2. II and III only. 3. I, II and III. 4. I and II only. 5. I only. PRS04 Potential and Energy (4) I and II Only I. It takes positive work to bring like charges together. TRUE II. Electric field lines always point in the direction of decreasing electric potential. TRUE III. If a negative charge moves in the direction of the electric field, its electric potential energy decreases. FALSE – potential decreases so potential energy increases (U = qV) PRS04 Two Point Charges The work done in moving a positive test charge from infinity to the point P midway between two charges of magnitude +q and –q: +q P -q 1. is positive. 2. is negative. 3. is zero. 4. can not be determined since not enough information is given. 5. I don’t know PRS04 Two Point Charges (3) Work from ∞ to P is zero +q P -q The potential at ∞ is zero. The potential at P is zero because equal and opposite potentials are superimposed from the two point charges (remember: V is a scalar, not a vector) PRS04 Potential Landscape If I think of the electric potential as a mountain range, then the electric field points: 1) Up the mountain sides 2) Down the mountain sides 3) Around the mountain sides 4) I don’t know PRS04 Potential Landscape (2) The electric field points the fastest way down the potential mountain – in the same way that the gravitational field tells you to come down