Uniform Electric Fields and Potential Difference Forces and Fields 8
POS Checklist compare, qualitatively, gravitational potential energy and electric potential energy. define electric potential difference as a change in electric potential energy per unit of charge. calculate the electric potential difference between two points in a uniform electric field. explain, quantitatively, electric fields in terms of intensity strength and direction, relative to the source of the field and to the effect on an electric charge.
Hollow Conducting Object Field lines are perpendicular to outer surface. The electric field inside a hollow conductor is zero. Excess charge is repelled outward
Like a solid sphere, the charge ( or ) will spread out uniformly. However, because of the balanced vector nature of the fields, there is no electric field inside the sphere.
This phenomenon led to the production of Faraday Cages: hollow objects which have a electric field of zero inside of them. No electric field means no charge acting inside the sphere/object! http://www.youtube.com/watch?v=muwxyesr5wo http://www.youtube.com/watch?v=nidjwyv7qcg&feature=related http://www.youtube.com/watch?v=mcbp8q2fdoa&feature=related
Electric Fields on Parallel Plates If two conducting plates are charged, a uniform electric field is produced between them. The field will be of uniform strength and spacing. Also, the field only exists between the plates (and to a small extent around the edges).
Let's describe what is happening between these parallel plates. To do this, we need to take an aside and go back in time to Physics 20. Ahh, P20! Feel's Nice! We will look at the idea of Potential Energy Differences.
P20 Review:
Back to Physics 30: Consider a charge placed within an electric field Electric Potential Energy Work E p d F up F e E p E p E greater lesser E p F down d F e E p E p E lesser greater E p charge charge moving the charge away from the positive charge requires work which increases electric potential energy and moving the charge toward the negative charge also does the same
Electrical E p changes when work is done in moving a charge from one point to another within an electric field q 1 E F applied E p lesser E p greater F e Work Fd E increases E p To increase E p between two unlike charges, move them farther apart. d p Low E p d d Higher E p
To increase electric E p between two like charges, move them closer together d d Low E p Higher E p So...
Electrical Potential Difference the ratio of the change in energy (work, potential or kinetic) per unit charge (1.00 C) within an electrical field
Electric potential difference or voltage is the change in potential energy per unit charge. Equation: In Joules V E q In Coulombs The ratio of E/q is called a volt and is named after Alex Volta who invented the battery
Another name for potential difference is Voltage
Example: A charge of 2.60 X 10 3 uc is moved from a positive plate to a negative plate by doing 1.17 X 105 J of work. What is the potential difference between the plates? F e F applied W q V V 1.17 10 2.60 10 E q 1.17 10 2.60 10 4.50 10 3 5 5 9 9 V J C J C
Moving a small charge of 1.6 x 10 19 C against an electric field produced by parallel plates increases its electric potential energy by 3.2 x 10 16 J. What is the potential difference between the plates?
In moving a charge of 5.0 C from one charged parallel plate to another, a battery raises the electric potential energy of the charge by 60 J. Determine the potential difference between the battery terminals.
2 nd Equation for Voltage W E So V qed q V Ed and E V d Only for parallel plates (ie: uniform electric fields)
Example: Determine the electric field strength between 2 oppositely charged plates 25 cm apart to which 120 V is supplied. 0.25 m E V E E Ed V d 120V 0.250m 480 V / m V = 120 v
ex) A voltmeter measures the potential difference between two parallel plates to be 60.0 V. The plates are 3.00 cm apart. What is the magnitude of the electric field between them?
The Electron Volt