Devil physics The baddest class on campus IB Physics

Devil physicsThe baddest class on campusIB Physics

Tsokos Lesson 5-2Electric Field and Electric potential

IB Assessment Statements • Electric Potential Difference • 5.1.1. Define electric potential difference. • 5.1.2. Determine the change in potential energy when a charge moves between two points at different potentials. • 5.1.3. Define the electronvolt. • 5.1.4. Solve problems involving electric potential difference.

IB Assessment Statements • Electric Current and Resistance • 5.1.5. Define electric current. • 5.1.6. Define resistance. • 5.1.7. Apply the equation for resistance in the form where ρ is the resistivity of the material of the resistor.

IB Assessment Statements • Electric Current and Resistance • 5.1.8. State Ohm’s Law. • 5.1.9. Compare ohmic and non-ohmic behavior. • 5.1.10. Derive and apply expressions for electrical power dissipation in resistors. • 5.1.11. Solve problems involving potential difference, current and resistance.

Introductory Video: Electric Fields and Potential

Review • In 5.1 we learned: • There is a force between electric charges and that vector methods must be used to find the net force on a given charge. • How to charge by friction and by induction and the difference between the two.

Review • In 5.1 we learned: • What an electroscope is and how to use it. • That the charge in a conductor resides on the outside surface of the conductor and that the net charge on the interior of the conductor is zero.

Review • In 5.1 we learned: • The formula for the electric force between two point charges is • Or, • where

Objectives • Appreciate that a charge q in an electric field of magnitude E will experience a force of magnitude

Objectives • Understand that the electric field of a point or spherical charge Q a distance r away has a magnitude of given by and is radial in direction. The field is zero inside the charged conductor

Objectives • Understand that the electric field inside parallel plates is uniform and its magnitude is given by

Objectives • Understand that the work done in moving a charge q across a potential difference V is

Objectives • Understand that a charge q that is at a point where there is potential V will have an electric potential energy of

Objectives • Understand that a charge moving in an electric potential satisfies the law of conservation of energy,

Electric Field • An electric field exists around any charged object and extends/radiates either into or out of the object • By convention, charge flows from positive to negative so, • For a positively charged object, the field lines extend outward

Electric Field • For a positively charged object, the field lines extend outward • For a negatively charged object, the field lines extend inward + -

Electric Field • The field does not “exist” unless shown to exist by a charge • We use a small positive test charge, q, to determine if a field exists – bring the test charge close and if it experiences a force, then a field exists

Electric Field • Electric field is defined as the force per unit charge experienced by a small positive test charge, q, The electric field is a vector with direction being the same as the force a positive charge would experience at the given point +

Electric Field • Units for electric field is N/C

Electric Field • The electric field from a single point charge, Q, at a point a distance r away is

Electric Field • Likewise, the charge on the surface of a spherical conductor is given by where R is the radius of the sphere. Inside the conductor the field is zero

Electric Field Lines • For a positively charged object, the field lines extend outward • For a negatively charged object, the field lines extend inward +

Electric Field Lines • A convention for drawing field lines is that the number of field lines should be proportional to the strength of the charge • The stronger the electric field, the closer the field lines are to each other

Electric Field Lines • Field lines will flow toward opposite charges and away from like charges. For example, two equal and opposite charges:

Electric Field Lines • Two unequal but opposite charges:

Electric Field Lines • Two equal, but like (+) charges

Electric Field Lines • Uniform Electric Field exists when the field has a constant magnitude and direction such as that generated by two oppositely charged parallel plates.

Electric Field Lines • The field lines at the edges begin to curve • The field is uniform if the length of the field is large compared to the distance between the plates

Electric Potential • Consider an electric field and a positive test charge q • In order to move the charge from its equilibrium position, work must be done

Electric Potential • If held in that new position, the test charge now has potential energy like a compressed spring because it wants to go back to its equilibrium position

Electric Potential • “V” is the electric potential and is defined in terms of the work, W, needed to bring a positive test charge, q, from very far away to a position close to the charged body Remember that work is based on displacement and not distance travelled!

Electric Potential • The unit of potential is: 1V = 1J/1C • The potential energy, U, is: U = qV • The unit of potential energy is: (1C) x (1J/1C) = 1J

Potential Difference • The amount of work needed to move a test charge from one point to another is equal to the change in potential energy of the charge • Just like gravity

Charge Moving In A Region of Electric Potential • A point charge moving in a region of an electric potential will have kinetic energy due to its mass and velocity • It also has potential energy due to the electric potential • As it moves through the region the potential impacts the velocity as potential changes

Charge Moving In A Region of Electric Potential • Law of Conservation of Energy states that the sum of the potential energy and kinetic energy at one point must equal the sum of the potential energy and kinetic energy at any other point

Electric Field between parallel plates • The electric field, E, between two parallel plates is equal to the potential difference between the plates, V, divided by the distance between the plates, d • Note that E is the electric field – E does not stand for energy!

How Does It Fall? • Consider a positively charged metal sphere with some mass m falling vertically through an electric potential between two plates • What is the direction of motion if it starts at point P? • What is the direction of motion if it starts at point Q?

Electronvolt • Atomic physics deals with extremely small amounts of energy where the Joule is not really appropriate • The electronvolt, eV, is equal to the work done when the charge on one electron is moved across a potential difference of 1 volt