Physics Section 16.3 Apply the Properties of Electric Fields Recall: An electrically charged object will exerts a force on another charged object without any contact. An electric field is a region in space surrounding a charged object in which another charged object experiences an electrical force. +q experiences an electrical force due to the charge +Q. The closer the charges are to each, the larger the force. A measure of this force is called the electric field strength.

The strength of the electric field is the ratio of the magnitude of the electric force acting on a test charge and the magnitude of the test charge. E = F e q o E = electric field strength N/C F e = electric force (N) q o = test charge (C) Note: The electric field is created by +Q, not by +q.

Example: A test charge of 3.0 x C experiences an electric force of 6.0 x N. Find the electric field strength.

example A charged object creates an electric field with a strength of 4.0 x 10 5 N/C at a given point. Find the force exerted on an electron at this point.

Note: the electric field strength varies inversely as the distance from the object creating it. E = K c Q_ r 2 E = electric field strength N/C K c = 8.99 x 10 9 N m 2 /C 2 r = distance from the object creating the electric field (m) Q = charge on object creating electric field (C)

example An object with a charge of 8.2μC creates an electric field. Find the electric field strength at a distance of 65 cm.

Note: A convenient way to represent an electric field is to use electric lines of force. Electric field lines are lines that indicate the path a test point would follow in an electric field. Properties of electric field lines 1. Electric field lines originate from a positively charged object and terminate at a negatively charge object. 2. Electric field lines originate and terminate perpendicular to the surface of a charged object. 3. The density of the electric field lines indicate the strength of the electric field. 4. No two field lines from the same source can cross.

Electric fields from compound systems

A charge can be given to a conductor. Properties of an isolated charged conductor. 1. The electric field is zero everywhere inside the conductor. 2. All excess electric charge lies on the outer surface. 3. The electric field is perpendicular to the conductor’s surface. 4. On an irregular shaped conductor, charge accumulates at the point(s) of greatest curvature.

A Faraday Cage is an enclosure made of a conducting material. No electric field can exist in a Faraday Cage. The repairman wears a protective suit made of shielding material. The helicopter must ground off before he steps onto the line since the chopper and the line have different electrical charges. Bonding, or equalizing the charges is a very risky operation. Too close and the chopper gets fried, too far away and the bonding process fails. The power line repairman's suit acts as a "faraday cage" or electrical shield. 150,000 volts flows around his body and down the powerline, leaving him unharmed.

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