Unit Four

Electronics

Electric Charge

• Electrification is the process that produces electric charge on an object

• If that charge is confined, it is called an electrostatic charge

• Static electricity is stationary electricity in the form of an electric charge at rest– Commonly caused by friction

Two Kinds of Charge

• Positive

• Negative

• Basic Law of Electrostatics – objects with similar charges repel each other; objects with opposite charges attract

Conductors

• A conductor is a material through which an electric charge is readily transferred

• Have large numbers of free electrons

• Examples– ?– ?– ?

Insulators

• An insulator is a material through which an electric current is not readily transferred

• Insulators lack free electrons

• Examples– ?– ?– ?

Force Between Charges• Quantity of charge is measured in coulombs (C)• 1 coulomb = the charge on 6.25 x 1018 electrons• Coulombs law of electrostatics – the force

between two point charges is directly proportional to the product of their magnitudes and inversely proportional to the square of the distance between them

• F = k Q1Q2

d2

• Practice problem on page 390

Electric Fields

• An electric field is said to exist in a region of space if an electric charge placed in that region is subject to an electric force

• An electric line of force is a line so drawn that a tangent to it at any point indicates the orientation of the electric field at that point

Electric Fields

Electric Fields

Electric Fields

Electric Fields

• The electric field intensity, E, at any point in an electric field is the force per unit positive charge at that point (newtons per coulomb)

• E = F

q

• Example on page 392

Assignment

• Questions 1-10 page 392, and problems 1, 3, 11 page 393

• Due tomorrow

• 10 points

Electric Potential

• The potential difference, V, between two points in an electric field is the work done per unit charge as a charge is moved between these points

• V = Work (W)/charge (q)

Electric Potential

• The potential gradient of an electric field is the change in potential per unit of distance

• When the potential difference between a conductive object and the earth is zero, it is said to be grounded

Distribution of charges• Faradays experiment1. All the static charge on a conductor lies on its

surface2. There can be no potential difference between

two points on the surface of a charged conductor3. The surface of a conductor is an equipotential

surface4. Electric lines of force are normal to

equipotential surfaces5. Lines of force originate or terminate normal to

the conductive surface of a charged object

Capacitors

Capacitors

• A capacitor is a combination of conducting plates separated by an insulator that is used to store an electric charge

• Capacitance (C) is the ratio of the charge on either plate of a capacitor to the potential difference between the plates– C = Q/V

• C = capacitance• Q = quantity of charge• V = the potential difference between the conducting

plates

Dielectric Materials

• Dielectrics are materials used to separate the plates of capacitors

• The ratio of the capacitance with a particular material separating the plates to the capacitance with a vacuum between the plates is called the dielectric constant (K) of the material

• K= C2/C1

Combinations of Capacitors

• For capacitors connected in parallel– CT = C1 + C2 + C3…

• For capacitors connected in series– 1/CT = 1/C1 + 1/C2 + 1/C3…

• Try practice problems 1&2 on page 407

Assignment

• Q’s 1-10, Problems 1-6, 7, 10

• Due tomorrow

• 10 points

Direct-Current Circuits

• Current– Current (I) = Charge (Q) / time (t)– Unit of current is the ampere (A)

• One ampere is a current of 1 coulomb per second

Resistance

• Resistance (R) is the opposition to electric current– The unit of resistance is the ohm (Ω)– Resistance is caused by collisions between the

free electrons and the fixed particles of a conductor

Continuous Current

• In order to utilize electricity, a closed-loop conducting path is needed– This is called an Electric Circuit

• An electric circuit is a conducting loop in which a current can transfer electric energy from a suitable source to a useful load

• Look at figure 17-4 (pg 414) in you text for symbols commonly used in schematic diagrams of electric circuits

Sources of continuous current

1. Electromagnetic

2. Photoelectric

3. Thermoelectric

4. Piezoelectric

5. Chemical

Combinations of cells

• If the cells are connected in series– The emf* of the battery is equal to the sum of the emfs

of the individual cells– The current in each cell and in the external circuit has

the same magnitude throughout– The internal resistances of the battery is equal to the

sum of the internal resistances of the individual cells

*The emf (electromotive force) of a source is the energy per unit charge supplied by the source, measured in volts

Combinations of cells

• If the cells are connected in parallel – The emf is equal to the emf of each separate

cell– The total current in the circuit is divided

equally among the cells– The reciprocal of the internal resistance of the

battery is equal to the sum of the reciprocals of the internal resistances of the cells

Assignment

• Answer questions 1-8 and 13 on pages 423-424

• Worth a possible 10 points

• Due tomorrow

Ohm’s Law for dc circuits

• Ohm’s law of resistance – in a closed circuit the ratio of the emf of the source to the current in the circuit is a constant

• The constant is the resistance of the circuit

• V=IR

• P=IV

• P=I2R

The Laws of Resistance

1. The resistance of all substances changes with temperature

2. The resistance of a uniform conductor is directly proportional to the length of the conductor

3. The resistance of a uniform conductor is inversely proportional to its cross-sectional area

4. The resistance of a given conductor depends on the material of which it is made

Review