Electric fields Dieric Baola. Electric Field Property of space around a charged object that causes forces on other charged objects Measuring the Field Intensity of a Charged Object + To determine the intensity of a field, use a test charge q’ at any given point from the object being tested. The test charge is a According to Coulombs Law the closer the test small positive particle. charge the stronger the force The field can be determined by the effects on the test charge in different locations Field Intensity = E = is proportional to the + force on the test charge and inversely proportional to the charge itself. This ratio is independent of the test charge. The direction is the direction of a positive (+) charge toward a negative (-) charge The electric field in the electrical system is analogous to gravity (gravitational field) in the mechanical system. The Analogous Relationship of Electric Field to a Mechanical System The electric field caused by point charge Q at location r. The gravitational field at location r caused by mass M. The electric field experienced by q in a region of space based on its charge amount and the electrical force exerted upon it. The gravitation field experienced at the location of mass, m. + + + Test Charge ++ - Field Line s Field Lines Represent the Intensity and Direction of the Electric Field Unlike Charges Like Charges Direction of Electric Field The electric field direction is always directed away from positive source charges and towards negative source charges. Positive charges always move in the direction of the electric field lines, negative moves opposite. motion + motion Positive charges always move in the direction of the electric field line, negative the opposite. - The Electric Field Between Parallel Plates Parallel plates – two parallel sheets of equal quantity and opposite charges on each plate. + + + + + + + - - - - - - - •The electric field is uniform between parallel plates. Parallel Plates Diagram Two parallel sheets of equal quantities of opposite charges. The electric field has the same strength at every location between the plates. Electric Field Intensity The greater the number of electric field lines. The greater the field intensity. The density of electric field lines around these three objects reveals that the quantity of charge on C is greater than that on B which is greater than that on A. Electric Field Facts Part 1 1. The closer together the lines of force, the stronger the electric field. 2. The direction of the electric field is tangent to the lines of force. 3. The electric field lines start at positive charges and end on negative charges. 4. The number of lines leaving or entering a charge is proportional to the magnit de of the charge Electric Field Facts Part 2 • The electric field is a vector quantity. • The electric field can be measured in Newton/Coulomb (N/C) or Volts/meter (V/m). • The electric field represents the force per unit charge on a charge in the field. In other words, the electric field can be considered as the relative capability of a region to exert an electrical force on a charge in the field. • The electric field is defined by the direction a small positive charge (called a test charge), would move if placed in the field. • A small charge is a charge too small in quantity to influence electric fields in its vicinity. • Positive charges always move in the direction of the electric field and negative charges in the opposite direction to the electric field. Capacitance • Capacitance is the ability of a component or circuit to collect and store energy in the form of an electrical charge. It can also be defined as the ratio of the change in an electric charge in a system to the corresponding change in its electric potential. • Capacitors are energy-storing devices available in many sizes and shapes. They consist of two plates of conducting material (usually a thin metal) sandwiched between an insulator made of ceramic, film, glass or other materials, even air. • The insulator boosts a capacitor's charging capacity. • The internal plates are wired to two external terminals, which sometimes are long and thin and can resemble tiny metallic antennae or legs. These terminals can be plugged into a circuit. • A capacitor collects energy (voltage) as current flows through an electrical circuit. Both plates hold equal charges, and as the positive plate collects a charge, an equal charge flows off the negative plate. • When the circuit is switched off, a capacitor retains the energy it has gathered, though slight leakage usually occurs. Capacitance If charge +Q is placed on one plate, and -Q on the other, the potential difference between them is V, and then the capacitance is defined as The SI unit is C/V, which is called the Farad (F), named after the famous and creative scientist Michael Faraday from the early 1800’s. Applications: Radio tuner circuit uses variable capacitor Blocks DC voltages in ac circuits Act as switches in computer circuits Triggers the flash bulb in a camera Converts AC to DC in a filter circuit Determination of Charge Value (The Milikan Oil Drop Experiment) •Performed by Robert Milikan in 1909 •Received Nobel Prize for experiment Milikan’s Method of Calculation F e F e= electrical force on the charged oil drop W= weight charged of oil drop F e=qE W=mg + Fe=W when the charged oil drop was suspended by the electric field. qE=mg q=mg/E