chapter 17 electric potential. question 1 answer

Chapter 17

Electric Potential

Question 1 answer

Question 2 Answer

Question 3 Answer

Objectives: The students will be able to:

• Determine the magnitude of the potential at a point a known distance from a point charge or an arrangement of point charges.

• State the relationship between electric potential and electric field and determine the potential difference between two points a fixed distance apart in a region where the electric field is uniform.

• Explain what is meant by an electric dipole and determine the magnitude of the electric dipole moment between two point charges.

17.5 Electric Potential Due to Point Charges

The electric potential due to a point charge can be derived using calculus.

(17-5)


The potential in this case is usually taken to be zero at infinity; this is also where the electric field (E=kQ/r2) is zero. The result is

(17-5)

Where k = 8.99 x 109 Nm2/C2. V is the absolute potential at a distance r from the charge Q, where V = 0 at r = ∞, or think of V as the potential difference between r and infinity.


These plots show the potential due to (a) positive and (b) negative charge.

17.5 Electric Potential Due to Point ChargesNotice that the potential V decreases with the first power of the distance, whereas the electric field decreases as the square of the distance.


Using potentials instead of fields can make solving problems much easier – potential is a scalar quantity, whereas the field is a vector.

Example 17-4 page 477:Determine the potential at a point 0.50 m (a) from a +20 μC point charge,(b) From a -20 μC point charge.

Example 17-5 page 477: Work done to bring two Positive charges close together.

What minimum work must be done by an external force to bringa charge q = 3.00 μC from a great distance away (take r = ∞) to a point 0.500m from a charge Q = 20.0 μC?

Example 17-6: Potential above two charges.

Calculate the electric potential (a) at point A in Fig. 17-10 Due to the two charges shown, and (b) at point B. This is The same situation as Example 16-9, Fig. 16-28, where weCalculated the electric field at these points.

Example 17-6: Potential above two charges.Calculate the electric potential (a) at point A in Fig. 17-10 Due to the two charges shown, and (b) at point B. This is the same situation as Example 16-9, Fig. 16-28, where we calculated the electric field at these points.

Figure 17-11Example 17-7

Potential energies.

Consider the three pairsof charges. Call them Q1

and Q2. (a)Which set has a Positive potential energy?

Potential energies.



Potential energies.



Set (iii) has a positive potential energy becausethe charges have the same sign.


Potential energies.


and Q2. (b) Which set has the most negative potential energy?


Potential energies.


and Q2. (b) Which set has the most negative potential energy?

Both (i) and (ii) haveopposite signs of charge and negativePE. Because r is smaller in (i), the PEis most negative for(i).


Potential energies.


and Q2. (c) Which set requires themost work to separate thecharges to infinity? Assume the charges allhave the same magnitude.


Potential energies.


and Q2. (c) Which set requires themost work to separate thecharges to infinity? Assume the charges allhave the same magnitude.

Set (i) will require the most work for separationto infinity. The more negative the potential energy, the more work required to separate the charges and bring PE up to zero (r = ∞).

17.6 Potential Due to Electric Dipole; Dipole Moment

The potential due to an electric dipole is just the sum of the potentials due to each charge, and can be calculated exactly.

Two equal point charges Q,of opposite sign, separated by a distance l, are calledan electric dipole.

See figure to the left.


Approximation for potential far from dipole:

(17-6a)

Or, defining the dipole moment p = Ql,

(17-6b)


Table 17-2Dipole Moments of Selected Molecules

Homework

• Chapter 17• #18, 19, 24, 26

Closure

• Kahoot 17-5 and 17-6

chapter 17 electric potential. question 1 answer

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