(5.1) Current Electricity

• What is electric current (I)?

  The flow of charge per unit time
• How is electric current defined mathematically?

  I = $\frac{\Delta Q}{\Delta t}$
• What is potential difference (V)?

  The energy transferred per unit charge
• How is potential difference defined mathematically?

  V = $\frac{W}{Q}$
• What is resistance (R)?

  Measure of difficulty for charge carriers
• How is resistance defined mathematically?

  R = $\frac{V}{I}$
• What does Ohm's law state about ohmic conductors?

  Current is directly proportional to potential difference
• What does the current-voltage graph of an ohmic conductor look like?

  A straight line through the origin

  
• What is the forward bias of a semiconductor diode?

  Direction allowing current to flow easily
• What happens to the resistance of a diode in reverse bias?

  Resistance is extremely high, small current flows
• How does a filament lamp behave as current increases?

  Resistance increases as current increases
• Under what condition does a filament lamp obey Ohm's law?

  At very low currents
• What is assumed about ammeters in circuits?

  They have zero resistance
• What is assumed about voltmeters in circuits?

  They have infinite resistance
• What is resistivity (ρ)?

  Measure of how easily a material conducts electricity
• How is resistivity defined mathematically?

  ρ = $\frac{RA}{L}$
• How does temperature affect the resistance of a metal conductor?

  Resistance increases as temperature increases

  
• What happens to a thermistor's resistance as temperature increases?

  Resistance decreases as temperature increases

  
• What is one application of a thermistor?

  Used as a temperature sensor in circuits
• What is a superconductor?

  A material with zero resistivity below critical temperature

  
• What is the critical temperature of a superconductor?

  The temperature below which it has zero resistivity

  
• What are two applications of superconductors?

  Power cables and strong magnetic fields
• What are the rules for adding resistances in series?

  RT = R1 + R2 + R3 + ...
• What are the rules for adding resistances in parallel?

  $\frac{1}{RT} =$$\frac{1}{R1} +$$\frac{1}{R2} +$$\frac{1}{R3} +$$...$
• How do you calculate total resistance in a circuit with both series and parallel resistors?

  Use both series and parallel rules
• What is the formula for power (P)?

  P = $\frac{E}{t}$
• What is another formula for power involving voltage and current?

  P = V I
• How can power be expressed in terms of resistance?

  P = $\frac{V^2}{R}$
• What is the relationship between energy transferred and power?

  E = P t
• How do you find the energy transferred by a lamp with 60 W power in 2 minutes?

  E = 7200 J
• How do you find the current in a lamp connected to 240 V with 60 W power?

  I = 0.25 A
• What is true about current in a series circuit?

  The current is the same everywhere
• What is true about voltage in a series circuit?

  Total voltage equals the sum across all elements
• What is true about current in a parallel circuit?

  The sum of currents equals the total current
• What is true about voltage in a parallel circuit?

  The potential difference across each branch is the same
• What happens to voltage when identical cells are joined in series?

  Total voltage equals the sum of individual voltages
• What happens to voltage when identical cells are joined in parallel?

  Total voltage equals the voltage of one cell
• What do Kirchoff's first law state?

  Total current into a junction equals current out
• What do Kirchoff's second law state?

  Sum of voltages in series equals battery voltage
• What is a potential divider?

  A circuit producing a fraction of source voltage