Feel the force, Luke • Electricity behaves much like gravity, the same equations actually just different variables. • As stated yesterday, the electric force varies inversely with the square of the distance. • If a small charged object has an electric force on it then there is an electric field, basically an electric field is a force on a charged object. • Faraday theorized that the electric force changes the properties of space and that the object acted upon experiences this change in space • That object (charge) has to be considered a point source.
The Force • The electric field strength or force can be summarized by the following: • E = F on q/q E is in N/C • E is the electric field, divided by the force on q divided by the strength of q • Lets try one: • An electric field is measured using a positive test charge of 3.0 x 10-6 C. The test charge experiences a force of 0.12 N. What is the magnitude of the electric field strength on the test charge
Force 2 • Just like vectors, arrows can be used to represent the magnitude and direction of the electric field. • The electric field is represented by field lines. These are drawn perpendicular to the point source or charge. Positive charges have field lines pointing away from the source. Negative charges have field lines pointing towards the source
Make Sense ? Janky pictures, I know Limited budget
The Dark Side • Field lines extend out straight unless there are two or more charges then the field becomes the vector sum of the fields , the field lines become curved and more complex • Remember, field lines are simply a way of representing the electric field around an object or charge.
Why?!@ Why?@! Why@#$? • What happens to the potential energy of an object as you lift it into the air? • In the same way, as you perform work on a charge by pulling it away from another one of opposite charge you increase its potential energy. The larger the test charge the greater the potential energy because more work is required. V = W on q/q • This is the difference in electrical potential, the ration of the work done on a charge/by the strength of that charge
Ok, back to business • The electric potential difference is measured in J/C also known as a volt = 1 joule/1 coulomb • Can you move a charge to two different positions and have the same potential difference? • If the charge is moved in a circle. The distance remains the same. • Equipotential is when the electrical potential difference is the same in two or more positions. • Only differences in electric potential can be measured
The force is strong in this one • Putting different equations together we get the following : V = Ed , remember V = W on q/q as well, basically W/q • Where V is the potential difference, E is electric field intensity, and d is the distance the charge moves. E is in N/C and d is in m putting the two together we get J/C which is 1 volt.
Let’s try one • Two charged parallel plates are 1.5 cm apart with a single proton between them. The magnitude of the electric field between the plates is 1800 N/C • What is the potential difference between the plates? • What work is required to move from the negative to the positive plate?
More • Uniform electric fields allowed for the charge of an electron to be established which means that an object can have only a charge with a magnitude that is some integral multiple of the charge of an electron (1.6 x 10-19 C). • Robert Millikan did this in 1909 with the oil drop experiment.
Pictures of Millikan’s Stuff REAL DEAL Easier version
Shared Charges • All systems come to equilibrium when the energy of the system is at a minimum. If there excess charges on one charged object they will migrate to another object in contact with it until the balance of charges is equal on both sides. After equilibrium is reached, work would have to be performed to move any more charges. • At equilibrium there is no potential difference.
More Sharing • Two metal objects (spheres) of different sizes would result in an imbalance of charges at equilibrium due the proximity of charges and their repulsive forces • Any potential difference results in the movement of electrons (charges) aka electricity. • This is the purpose of grounding. Providing a path for a potential difference to flow. The book references gas trucks and electrical devices.
Electric Fields and Conductors • Sort of common sense, but charges on a conductive surface will spread apart as far as possible. A solid sphere or a hollow sphere for example will have all the accumulated charges migrate to the outside. This explains why a car (hollow metal sphere) shields a driver from a lightning strike or downed power line. Also because the outside provides the easiest path to ground.
More conductor info. • Charges accumulate more in smaller areas on an irregularly shaped object, a point or protrusion of some kind. For this reason smooth, regularly shape objects are used to reduce the electric fields and thus reduce the chance of a spark. Lighting rods are pointed in order to maximize the electric field and the subsequent potential difference to form the stepped leader (conducting path) for lightning.
Storing Charge • A capacitor is a device used to store electrical charge. First created in 1746 by a Dutch dude, the device was called the Leydon Jar. • Capacitance is the ratio of the stored charge in an object to the electrical potential difference • C = q/ V • Capacitors vary widely in size and capacitance are independent of the charge. If charge goes up so does the potential difference.
Capacitors • Capacitors are nothing more than two conducting plates of equal and opposite charge separated by an insulator, usually some type of foil separated by thin plastic. The capacitance is varied by manipulating the amount of surface area of the conductors, the distance between them, and the insulating material used. Ceramic, polyester, and air are common dielectrics or insulating materials.
Lastly • Capacitance is measured in farads (F) which is one coulomb per volt. • A farad is a relatively large unit and is most often measured in very small fractional amounts.