# Topic 3: Electric Circuits

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• Created by: Laura,
• Created on: 04-11-20 11:27
• Topic 3: Electric Circuits
• Current (the rate of flow of charged particles // flow of charge per unit time)
• I = ?Q ÷ ?t
• Principle of charge conservation
• Total electric charge in a closed system doesn't change
• K1 Law: The total current flowing into a junction is equal to the current flowing out of that junction
• Series Circuit: Current ? everywhere in the circuit
• Parallel circuit: sum of currents in each parallel set of branches ? total current
• The sum of the inputs ? the sum of the outputs
• Equations
• P = VI
• W = VIt
• P = I²R
• P = V²/R
• Resistance
• Measure of how difficult it is for charge to pass through a component
• R = P.d. ÷ Current
• Untitled
• Ohm's Law: For an ohmic conductor, I ? V
• (as long as physical conditions (temp) remain constant)
• Ohmic conductor: Line straight through the origin
• I-V Graphs
• Filament bulb: metal wire heats as current increases, so resistance increases
• Obeyed for very low currents
• (Negtaive temperature coefficient) Thermistor: as it heats up, resistance decreases
• increasing the temperature of a thermistor causes electrons to be emitted from atoms, therefore the number of charge carriers increases and so current increases causing resistance to decrease
• Obeyed for constant temperatures
• Ohm's Law: For an ohmic conductor, I ? V
• (as long as physical conditions (temp) remain constant)
• Ohmic conductor: Line straight through the origin
• I-V Graphs
• Filament bulb: metal wire heats as current increases, so resistance increases
• Obeyed for very low currents
• (Negtaive temperature coefficient) Thermistor: as it heats up, resistance decreases
• increasing the temperature of a thermistor causes electrons to be emitted from atoms, therefore the number of charge carriers increases and so current increases causing resistance to decrease
• Obeyed for constant temperatures
• Combining resistances
• Series: Rt = R1+R2+..+Rn
• Parallel: 1/Rt =1/R1 + 1/R2 ...
• (can sub in V=IR for voltage too)
• Resistivity:  measure of how easily a material conducts electricity
• R= ?l / A
• Resistance is directly proportional to the length
• Resistivity is useful for comparing materials of differing sizes
• Resistivity is also dependent on temperature
• I = nqvA
• Current (the rate of flow of charged particles // flow of charge per unit time)
• I = ?Q ÷ ?t
• Principle of charge conservation
• Total electric charge in a closed system doesn't change
• K1 Law: The total current flowing into a junction is equal to the current flowing out of that junction
• Series Circuit: Current ? everywhere in the circuit
• Parallel circuit: sum of currents in each parallel set of branches ? total current
• The sum of the inputs ? the sum of the outputs
• Equations
• P = VI
• W = VIt
• P = I²R
• P = V²/R
• n of a material describes the number of charged carriers it contains per unit volume
• v: speed at which the charged particles are travelling
• Constantly colliding in conductor. So the average speed at which they move is considered - drift velocity
• q: charge that a single charged particle carries, 1.6 x10^-19C for electrons
• Large range of  resistivities for different materials and for the same material at different temp
• (some release more charge carriers when temp increased)
• The potential along a uniform current-carrying wire increases uniformly with the distance along it.
• Potential Difference
• Potential divider: circuit with several resistors in series connected across a voltage source (used to provide required fraction of source p.d. which remains constant)
• Terminal Potential Difference: the p.d. across the resistance R
• Lost Volts: The p.d. across the resistance r - equal to the energy wasted by the cell per coloumb of charge
• Lattice vibrations: due to atom arrangement, atoms vibrate about their equilibrium. As temp increases, intensity of atoms does too.                More intense vibrations, more difficult for free electrons to pass through. >likely to collide if >oscillating; causing them to slow down and increasing resistance of material
• (intensity refers to speed & amplitude of oscillations)
• Untitled
• s the temperature of a metal or semiconductor increases, its atoms gain energy, and once they gain enough energy they begin to release electrons (thermionic emission). This increases the number of charge carriers available in the conductor, hence decreasing the resistance.
• Potential Difference: energy transferred // unit charge between 2 points in a circuit
• V = W÷Q
• Principle of conservation of energy
• K2 Law: Sum of voltages in a series circuit ? battery voltage, Or sum of the voltages is zero
• Series: Battery p.d. shared across all elements, totoal sum of voltages across all elements ? supply p.d.
• Parallel: P.d. across each branch ?
• Graphs
• I: Electric Current (A)  Q: Charge,     t: Time (s)    P: Power (W)  V: Voltage (V)  W: Energy  Transferred     (J,kWh)        R: Resistance (?)
• ?=resistivity, l=length  A=cross sectional area, I=current, n= charge carrier density, v = drift velocity