Ch 6 - Circuits Flashcards
What is a current?
the movement of charge that occurs between 2 points that have different electrical potentials ( A = C/s)
I = Q/t
What is the difference between the conventional definition of a current and reality?
- by convention, current is defined as the movement of positive charge from the high potential end of a voltage source to the low potential end
- in reality, it is negatively charged particles (electrons) that move in a circuit, from low potential to high potential
Where does currents flow?
only in conductive materials
What is the difference between metallic conduction, electrolytic conduction, and insulators?
- metallic relies on uniform movement of free electrons in metallic bonds
- electrolytic relies on the ion concentration of a solution
- insulators are materials that do not conduct a current
What is Kirchhoff’s law?
express conservation of charge and energy
What is Kirchhoff’s junction rule?
states that the sum of currents directed into a point within a circuit equals the sum of the currents directed away from that point
I into junction = I leaving junction
What is Kirchhoff’s loop rule?
states that in a closed loop, the sum voltage sources is always equal to the sum of voltage drops
- if all the voltage wasn’t used up in each loop of the circuit, then the voltage would build after each trip around the circuit, which is impossible
V source = V drop
- the total potential difference around any closed loop is 0
What is resistance?
opposition to movement of electrons through a material
R = (resistivity x length)/cross sectional area
R = pL/A
What are resistors?
conductive materials with a moderate amount of resistance that slow down electrons without stopping them
What is Ohm’s law?
states that for a given resistance, the magnitude of the current through a resistor is proportional to the voltage drop across the resistor
V = IR
What is the purpose of combining resistors in a circuit and how can they be combined?
- can be combined to calculate the equivalent resistance of a full or partial circuit
- resistors in series are additive and sum together to create the total resistance of a circuit
- resistors in parallel cause a decrease in equivalent resistance of a circuit
What does the amount of dissipated power across each resistor in a circuit depend on?
dependent on the current through the resistor and the voltage drop across the resistor
What are capacitors?
have the ability to store and discharge electrical potential energy
How is capacitance in parallel plate capacitors determined?
by the area of the plates and the distance between the plates
What do capacitors in series cause?
a decrease in the equivalent capacitance of a circuit
What do capacitors in parallel create?
they sum together to create a larger equivalent capacitance
What are dielectric materials?
insulators placed between the plates of a capacitor that increase capacitance by a factor equal to the material’s dielectric constant, k
C’ = kC
- can never decrease capacitance, k never less than 1
What are ammeters?
inserted in series in a circuit to measure current; they have negligible resistance
- minimum impact on circuit
What are voltmeters?
inserted in parallel in a circuit to measure a voltage drop; they have very large resistance
- minimum impact on circuit
What are ohmmeters?
inserted around a resistive element to measure resistance; they are self powered and have negligible resistance
What is voltage?
potential difference between 2 points (Volts = J/C)
What is electromotive force (emf)?
the potential difference of the voltage source for a circuit, usually a battery, and is given in volts
What is conductivity?
the reciprocal of resistance and is a measure of permissiveness to current flow, measured in siemens (S)
Which likely has a higher conductivity and why: 1 M glucose and 0.25 M NaCl?
NaCl higher because it is a salt and will increase the ion content of water
- glucose does not dissociate, and therefore it has a near zero impact on conductivity
How is the power of a resistor or collection of resistors calculated?
P = IV = I^2R = V^2/R
How are resistors calculated in series v in parallel?
- in series: Rs = R1 + R2 + R3…
- in parallel: 1/Rs = 1/R1 + 1/R2 …
How does adding or removing a resistor change the total resistance of a circuit with resistors in series? in parallel?
- adding a resistor in series increases the total resistance of a circuit; removing one in a series decreases the total resistance in the circuit
- these relationships are reversed in parallel: adding a resistor decreases resistance while removing one increases it
How does internal resistance affect the voltage of the circuit?
- it will lower the available voltage which will also lower current for any given resistance
What is capacitance?
the ratio of the magnitude of the charge stored on one plate to the potential difference (voltage) across the capacitor
C = Q/V
How are capacitors calculated in series v in parallel?
- series: 1/Cs = 1/C1 + 1/C2 …
- parallel: Cp = C1 + C2 …
Assuming the plates are attached by conducting material, how does a capacitor behave after the voltage source has been removed from a circuit?
the capacitor discharges, providing a current in the opposite direction of the initial current
How does a dielectric material impact capacitance?
always increases capacitance
How does a dielectric material impact voltage?
- if capacitor is isolated, voltage will decrease
- if capacitor is in circuit, voltage is constant because it is dictated by the voltage source
How does a dielectric material impact charge?
- if capacitor is isolated, charge will remain constant because there is no additional source of charge
- if capacitor in circuit, charge will increase
How does adding or removing a capacitor change the total capacitance of a circuit with capacitors in series? in parallel?
- adding a capacitor in series decreases the total capacitance of a circuit, removing one in a series increases total capacitance
- reversed in parallel: adding capacitor increases capacitance, removing decreases it
What physical qualities contribute to the capacitance of a capacitor?
surface area, distance, and dielectric constant
C = Q/V
