Ch 21 sec 2 w 6 and 7 tr 2 Applications_of_Electric_Fields

Electric Potential

• Electric potential is the electric potential energy per unit charge, commonly referred to as voltage.

Applications of Electric Fields

K-W-L Chart

• K: What I Know

• W: What I Want to Find Out

• L: What I Learned

Essential Questions

• What is an electric potential difference?

• How is potential difference related to the work required to move a charge?

• What are properties of capacitors?

Key Vocabulary

• Electric potential difference

• Volt (V)

• Equipotential

• Capacitor

• Capacitance

Energy and Electric Potential

• Electric potential difference (ΔV):

  • Defined as the work (W) needed to move a positive test charge from one point to another, divided by the magnitude of the test charge.

  • Also understood as the change in electric potential energy (ΔPE) per unit charge.

  • Measured in joules per coulomb (J/C), where 1 J/C = 1 V.

  • When the electric potential difference between positions is zero, those positions are at equipotential.

Electric Potential in a Uniform Field

• A uniform electric field can be created using two parallel conducting plates (+ and -).

• The electric field is consistent between plates except at the edges, directed from positive to negative.

• The electric potential is higher near the positively charged plate and lower near the negatively charged plate.

• The electric potential difference (ΔV) can be calculated using the formula linking electric field strength, separation distance, and potential difference.

Example Problem 1

• Situation: Two charged parallel plates, 4.0 cm apart, with an electric field of 2400 N/C.

• Known: E = 2400 N/C, d = 4.0 cm

• Unknown: ΔV and work (W) to move a proton from one plate to another.

Millikan’s Oil-Drop Experiment

• Experiment: An oil drop weighing 1.5×10−14 N is suspended between charged plates with potential difference of 450 V (2.4 cm apart).

• Known: ΔV = 450 V, Fg = 1.5×10−14 N

• Unknown: Charge (q) and number of excess electrons (n) on the oil drop.

• The electric force equals gravitational force in a balanced state.

Electric Fields Near Conductors

• Electrons in conductors repel each other and spread out to minimize potential energy.

• Charges rest on the surface of conductors; the electric field is zero inside a closed charged metal container.

• The electric field is stronger at sharp points of conductors.

Capacitors

• A capacitor stores electrical energy in an electric field.

• With a 1-V connection, the potential difference results in a net positive charge (+q) on one plate and an equal negative charge (-q) on the other.

• The capacitance (C) is defined in a straight line relation between charge and potential difference in a charge vs. potential graph.

• Capacitance is measured in farads (F), where 1 F = 1 C/V.

Example Problem 2

• Situation: Charging a sphere with potential difference of 76.0 V and charge of 3.8×10−4 C.

• Known: q = 3.8×10−4 C, ΔV = 76.0 V.

• Unknown: Capacitance (C).

Shapes and Sizes of Capacitors

• Capacitors come in various forms for use in electronics such as computers and televisions.

• Capacitance can be adjusted by changing the surface area, distance between plates, or the insulating material used.

• Typical capacitance values range from 10 picofarads to 500 microfarads, with specialized memory capacitors having capacitance ranging from 0.5 F to 1.0 F.

Conclusion

Review of Essential Questions

• Electric potential difference is crucial for understanding the work needed to move charges and capacitor properties.

• Vocabulary concerning electric potential is essential for a comprehensive understanding of electric fields.