Unit 2: Electricity & Magnetism Lesson 2 – Electric Fields Two types of forces - Contact and Non-contact Contact – friction, normal force, pushing, etc. Objects touch. Non-contact – gravity, electric force etc. Objects do not have to touch. Forces act over a distance. Electric Field – Area around a charged object where another charge experiences an electric force ie. electric field exerts a force on any object in its vicinity. The closer a charged object is brought to the charged object creating the field, the greater the force exerted on it. How can we tell if an electric field exists? By placing a small test charge (+) at the location to be tested. Test charge: A positive charge of very small magnitude, used to determine the direction of the electric field. (+): because of convention small: so that it does not interfere with the electric field we are trying to measure Electric field lines: Lines of electric force that represent electric field. Density measures strength of electric field (closer field lines = greater field strength or intensity) Direction is same as the direction a small positive test charge would move if in the field. Electric field lines never cross (if they crossed, it would mean that a (+) test charge could be moving in multiple directions simultaneously - not possible) Field lines are directed from (+) charge to (-) charge. Unlike charges Equipotential Lines (curves) Lines of constant voltage Perpendicular to the electric field lines Have no direction Represent the amount of electric potential at a given region http://www.cco.caltech.edu/~phys1/java/phys1/EField/EField.html Electric field strength (or intensity) E: ratio of force to the amount of test charge; force per unit charge at that point SI unit: Newtons/Coulomb (N/C) E = F / qo E - is electric field strength (N/C) F - Force produced on charge (N) qo - Charge on object in the field (test charge) - (C) Show that electric field exists regardless of the presence of the test charge qo ie., E - is electric field strength (N/C) q - source charge (charge producing the E field) (C) d - distance of test charge from source charge (m) By definition, Example: How strong would an electric field have to be to produce a force of 1.00 N if the charge in the field was 1.00 x 103μC? Electric Potential Two points are said to differ in electric potential if work is done to move a charge from one point to another point in an electric field. The Potential Difference (PD - measured in Volts, V) between two points in an electric field is the work done per unit charge as a charge is moved between these two points. W ΔV = /q ΔV = Voltage, Potential Difference W = Work done to move charge between two points PE = Potential energy due to position (separation) of charge q = Amount of charge How is potential difference (voltage) related to E? ∴The electric field strength between 2 points can be found by dividing the pot. diff. between them by the distance separating them. Example Problems: 1. 30 J of work is done to move a +5 C charge from one point to another in an electric field. What is the difference in potential between the two points? 2. A test charge of +5 x 10-4 C is in an electric field which exerts a force of 2.5 x 10-4N upon it. What is the strength of the electric field? 3. A test charge of +80 μC is placed in a 50 N/C field. What force does it experience? 4. A +19 μC charged sphere produces a 1.7 x 10 3 N/C upon a 0.5 μC test charge.What force is the charge subjected to? How far away is the test charge from the sphere? 5. What is the charge on an object that experiences a force of 0.6 N in an electric field, between two plates separated by 12 cm, and having a potential difference of 225 V? 6. A +4.9 μC charge produces an electric field of 3.6 x 104 N/C upon a (+) test charge. How far away is the charge? (1.11 m) 7. How much energy is needed to move a charge of +5μC from A to B if the potential of A relative to ground is +100 V and the potential of B relative to ground is +112 V? .