PHYS 115 1st Edition Exam # 2 Study Guide Lectures: 6-12- Coulomb’s Lawoo K = 9 *109N-m2/C2o r12 = distance between chargeso This is the force exerted by q1 on q1- Conductors and Insulatorso Inside a conductor, the electric field is zero, otherwise the molecules would movearound; charges can’t move moleculeso Electric field is the same along plateso- Electric potential and potential energyo A region has a uniform electric field E pointing to the right (produced by two oppositely-charged plates, not shown) o A positive charge +qis placed in the field and is initially at rest. o The magnitude of the force Feq is proportional to the magnitude of the electric field E and to the magnitude of the charge q:FEq = qEo The force causes the charge to undergo a displacement Δx.o It is useful to describe the change in electric potential energy in a way that does not depend on the size or sign of the charge q.o We therefore define the electric potentialo As for potential energy, only changes in the electric potential ΔV are meaningful and the configuration to which we assign V = 0 is arbitrary. o The units of electric potential are energy/charge = J/Co If we move a charge q through a potential difference ΔV the electric potential energy of the system changes by ΔU = q ΔV.o The most common way of producing a potential difference is to use a battery.o A battery produces an electric potential difference ΔV between its two terminals.o An electric potential difference is also called a voltage and is measured in volts (1volt = 1 V = 1 J/C)o The positive terminal is at a value of the electric potential that is higher than the value at the negative terminalo The potential difference is produced by separation of charge inside the battery.o If the electric field is uniform (same everywhere), the change in electric potential from the initial to the final position of the charge iso The difference in the electric potential between two locations is the work per unit charge needed to move a charge between the two locations.o The value of the electric potential decreases in the direction in which the electric field points. - Direction of the electric fieldoThe value of the electric potential energy decreases as the positive charge movesin the direction in which the electric field pointsoIf change in electric potential energy is negative, the value is negative. The electric field points in the decreasing.- Electrical currentso You can’t see these- Proportionalityo Brightness is an indicator of flow- Resistanceo Adding more bulbs increases resistanceo Resistance increases, current decreaseso Third rule of our modelo When a bulb is added in series to a circuit, the total resistance of the circuit increases, and the current through the battery increases. o If a bulb which is connected in series is removed from a circuit, the total resistance of the circuit decreases, and the current through the battery increases.- Kirchhoff’s Junction Lawo The splitting and combining of current in a parallel circuit is a consequence of conservation of charge.o Kirchhoff’s Junction Law states that the sum of the currents into a junction equals the sum of the currents leaving.o Fourth rule of our model- Bulbs in parallelo Brightness is equal IF same circuito Brightness equal in parallelo Current iD = iE = iAo Fifth rule of our modelo When a bulb is added in parallel to a circuit, the total resistance of the circuit decreases, and the total current through the battery increases.o When a bulb connected in parallel is removed from a circuit, the total resistance of the circuit increases, and the total current through the battery decreases.- Electric Circuit Modelo 7. The voltage across the terminals of an ideal battery is constant.o 8. Kirchhoff’s Loop Law: The sum of the voltages around any single current loop must be zero.o 9. Ohm’s Law: For a conductor of constant resistance, R, the potential difference, V, necessary to create a current, I, through the conductoro 10. The current through a battery is equal to the ratio of the voltage across the battery and the resistance of the circuit. o 11. For resistors that are connected in series, the equivalent resistance of the network of resistors is the sum of the individual resistors.o 12. For a network of resistors connected in parallel, the equivalent resistance is the inverse of the sum of the inverse of the individual resistors.- Real Batterieso All real batteries can be model as an ideal battery in series with a resistor, often referred to as the internal resistance.o If there is current through the battery, the terminal voltage is always less than the voltage.- Capacitanceo The ratio Q/ΔV is equal to the capacitance of the plates.o We have shown that it is only dependent on the physical parameters of the plates. - Dielectricso An insulator consists of vast numbers of atoms. When an insulator is placed in anelectric field, each atom polarizes.o Polarization occurs when an atom’s negative electron cloud and positive nucleus shift very slightly in opposite directions in response to an applied electric field.o An induced positive charge builds up on one surface of the insulator, and an induced negative charge builds up on the other surface. o When a dielectric is placed between two charged plates, the electric field between the plates decreases.o When a dielectric is inserted, the electric field between the plates decreases, which implies that the potential difference decreases as well.o The presence of a dielectric results in an increased capacitance.- RC Circuitso The current and the capacitor voltage decay to zero after the switch closes, but not linearly.o The decays of the voltage and the current are exponential decays.o The time constant τis a characteristic time for a circuit. A long time constant implies a slow decay; a short time constant, a rapid decay: RCo The voltage across a charging of a capacitor is also described by an exponential