Unit Four Electronics. Electric Charge Electrification is the process that produces electric charge...
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Transcript of Unit Four Electronics. Electric Charge Electrification is the process that produces electric charge...
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