21.2 Applications of Electric Field

1. 21.2 Applications of Electric Field • If you do work to lift a ball against gravity the PE of the ball will increase. • The same works with charges. • How do you get unlike charges apart? • The farther apart the charges are moved the more work that is done and more energy is gained. • Electric Potential Difference, V, is defined as the work done in moving a positive test charge between two points in the electric field.

2. The difference in electrical potential is the ratio of the work needed to move a charge to the strength of that charge. • Units of volt (J/C = V)

3. Equipotential positions • Equipotential lines are similar to altitude lines on a map

4. The Electric Potential in a Uniform Field • Use 2 plates, one charged positive and one negative • The electric field is constant between the plates and the direction is from positive to negative • Place a positive test charge, q’ in the field and move it a distance d towards the positive plate

5. Won q’ = Fd • Won q’ = Vq’ • So, V=Fd/q’=(F/q’)d • V=Ed • E=V/d • So the units for E can be either N/C or V/m • The electric potential increases in the direction opposite of the electric field direction • The electric potential is higher near the positively charged plate

6. By dimensional analysis the product of the units of E and d is (N/C)(m) • N.m is a J (Joule) • So 1 V = 1J/C

7. Millikan’s Oil-Drop Experiment • Used to find the charge on an electron • A fine mist was sprayed into and Electric Field • Gravity causes the drops to fall • Field increased until drops rise • Field adjusted until drops are suspended • Downward force of gravity equals upward electric force

8. Magnitude of E determined form the electric potential difference • Weight of electron had to be determined • Drop was suspended and then the electric field was turned off so the drop could fall • Because friction of tiny drop so large terminal velocity was reached quickly • Using a complex equation the mass was found • Then using mg the weight was calculated.

9. Sharing of Charge • All systems come to equilibrium to make the energy of the system at minimum. • Ball on a hill will end up resting in a valley • If one object is charge and comes in contact with an uncharged object the charges with spread out evenly across both objects. • If objects are not of same size then the charges equal out until voltage is the same • Charges are closer together at sharper points

10. Storing Charges: The Capacitor • Lift a book to a shelf and you increase GPE. • In a sense you are storing GPE • A device that stores electric potential energy is called a capacitor. • As a charge is added to an object the electric potential difference between that object and the Earth increases. • For a given situation the ratio of the charge stored to the electric potential difference is a constant

11. That ratio is called the capacitance, C • The unit of capacitance is a farad, F

12. Farad • Farad, F, named after Michael Faraday • One Coulomb/volt • 1 C is a very large charge so 1 F is also very large • Often F or pF

13. Uses • Storage of charge • Memory capacitors to prevent losses of memory in computers • Televisions have very large capacitors • Power giant lasers • Small ones power a flash of a camera