# Lec 02 08

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### Slide 1:

Chapter 17 Electric Potential

### Slide 2:

Units of Chapter 17 Electric Potential Energy and Potential Difference Relation between Electric Potential and Electric Field Equipotential Lines The Electron Volt, a Unit of Energy Electric Potential Due to Point Charges Potential Due to Electric Dipole; Dipole Moment

### Slide 3:

Units of Chapter 17 Capacitance Dielectrics Storage of Electric Energy Cathode Ray Tube: TV and Computer Monitors, Oscilloscope The Electrocardiogram (ECG or EKG)

### Slide 4:

Electrostatic Potential Energy and Potential Difference When a force is “conservative” ie gravitational and the electrostatic force a potential energy can be defined Change in electric potential energy is negative of work done by electric force:

### Slide 5:

Electrostatic Potential Energy and Potential Difference Electric potential is defined as potential energy per unit charge: Unit of electric potential: the volt (V). 1 V = I J/C

### Slide 6:

Electrostatic Potential Energy and Potential Difference Only changes in potential can be measured, allowing free assignment of V = 0.

### Slide 7:

Electrostatic Potential Energy and Potential Difference Analogy between gravitational and electrical potential energy:

### Slide 8:

Relation between Electric Potential and Electric Field Work is charge multiplied by potential Work is also force multiplied by distance

### Slide 9:

Relation between Electric Potential and Electric Field Therefore If the field is not uniform, it can be calculated at multiple points:

### Slide 10:

Equipotential Lines An equipotential is a line or surface over which the potential is constant. Electric field lines are perpendicular to equipotentials. The surface of a conductor is an equipotential.

### Slide 11:

Equipotential Lines of Dipole

### Slide 12:

If you cross contour lines especially perpendicular you will be changing your gravitational potential and very rapidly if the lines are close together

### Slide 13:

The Electron Volt, a Unit of Energy One electron volt (eV) is the energy gained by an electron moving through a potential difference of one volt.

### Example 17-2 :

Example 17-2 PE Speed ve ?

### Electric Field from Potential :

Electric Field from Potential a) Find E field

### Slide 16:

17.5 Electric Potential Due to Point Charges The electric potential due to a point charge can be derived using calculus.

### Slide 17:

17.5 Electric Potential Due to Point Charges These plots show the potential due to (a) positive and (b) negative charge.

### Slide 18:

17.5 Electric Potential Due to Point Charges Using potentials instead of fields can make solving problems much easier – potential is a scalar quantity, whereas the field is a vector.

### Slide 19:

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.

### Slide 20:

17.6 Potential Due to Electric Dipole; Dipole Moment Approximation for potential far from dipole: (17-6a)

### Slide 21:

Or, defining the dipole moment p = Ql, (17-6b) 17.6 Potential Due to Electric Dipole; Dipole Moment

### Slide 22:

17.7 Capacitance A capacitor consists of two conductors that are close but not touching. A capacitor has the ability to store electric charge.

### Slide 23:

17.7 Capacitance Parallel-plate capacitor connected to battery. (b) is a circuit diagram.

### Slide 24:

17.7 Capacitance When a capacitor is connected to a battery, the charge on its plates is proportional to the voltage: (17-7) The quantity C is called the capacitance. Unit of capacitance: the farad (F) 1 F = 1 C/V

### Slide 25:

17.7 Capacitance The capacitance does not depend on the voltage; it is a function of the geometry and materials of the capacitor. For a parallel-plate capacitor: (17-8)

### Slide 26:

17.8 Dielectrics A dielectric is an insulator, and is characterized by a dielectric constant K. Capacitance of a parallel-plate capacitor filled with dielectric: (17-9)

### Slide 27:

17.8 Dielectrics Dielectric strength is the maximum field a dielectric can experience without breaking down.

### Slide 28:

17.8 Dielectrics The molecules in a dielectric tend to become oriented in a way that reduces the external field.

### Slide 29:

17.8 Dielectrics This means that the electric field within the dielectric is less than it would be in air, allowing more charge to be stored for the same potential.

### Slide 30:

17.9 Storage of Electric Energy A charged capacitor stores electric energy; the energy stored is equal to the work done to charge the capacitor. (17-10)

### Slide 31:

17.9 Storage of Electric Energy The energy density, defined as the energy per unit volume, is the same no matter the origin of the electric field: (17-11) The sudden discharge of electric energy can be harmful or fatal. Capacitors can retain their charge indefinitely even when disconnected from a voltage source – be careful!

### Slide 32:

17.9 Storage of Electric Energy Heart defibrillators use electric discharge to “jump-start” the heart, and can save lives.

### Slide 33:

17.10 Cathode Ray Tube: TV and Computer Monitors, Oscilloscope A cathode ray tube contains a wire cathode that, when heated, emits electrons. A voltage source causes the electrons to travel to the anode.

### Slide 34:

17.10 Cathode Ray Tube: TV and Computer Monitors, Oscilloscope The electrons can be steered using electric or magnetic fields.

### Slide 35:

17.10 Cathode Ray Tube: TV and Computer Monitors, Oscilloscope Televisions and computer monitors (except for LCD and plasma models) have a large cathode ray tube as their display. Variations in the field steer the electrons on their way to the screen.

### Slide 36:

17.10 Cathode Ray Tube: TV and Computer Monitors, Oscilloscope An oscilloscope displays en electrical signal on a screen, using it to deflect the beam vertically while it sweeps horizontally.

### Slide 37:

17.11 The Electrocardiogram (ECG or EKG) The electrocardiogram detects heart defects by measuring changes in potential on the surface of the heart.

### Slide 38:

Summary of Chapter 17 Electric potential energy: Electric potential difference: work done to move charge from one point to another Relationship between potential difference and field:

### Slide 39:

Summary of Chapter 17 Equipotential: line or surface along which potential is the same Electric potential of a point charge: Electric dipole potential:

### Slide 40:

Summary of Chapter 17 Capacitor: nontouching conductors carrying equal and opposite charge Capacitance: Capacitance of a parallel-plate capacitor:

### Slide 41:

Summary of Chapter 17 A dielectric is an insulator Dielectric constant gives ratio of total field to external field Energy density in electric field: 