B.5- Current and Circuits

• What is current?

  Amount of positive charge flowing past a point per second
• What does EMF stand for?

  Electromotive Force
• What does EMF represent?

  Work done by a cell or source per unit charge
• What is the definition of PD?

  Work done per unit charge in moving a positive charge between two points
• What is resistance?

  Ratio of PD across a component to the current through it
• What is the formula for voltage?

  V = I R
• What is the formula for charge?

  Q = I t
• What is the formula for power in terms of current and resistance?

  P = I² R
• What is the formula for power in terms of current and voltage?

  P = I V
• What is the relationship between EMF and work done?


  Work done = EMF × charge
• What happens to current in a series circuit?

  Current is the same throughout the circuit
• What happens to potential difference (PD) in a series circuit?

  PD is shared among components
• What is the fundamental charge (e)?

  e = 1.60 x 10⁻¹⁹ C
• What does Kirchhoff's First Law state?

  At a junction, the sum of currents in equals the sum of currents out

  It is a conservation of charge law
• What does Kirchhoff's Second Law state?

  The sum of EMF in a loop equals the sum of PD in that loop

  It is a conservation of energy law
• What are the formulas for total resistance in series and parallel circuits?

  • Series: RT = R1 + R2 + ...
  • Parallel: 1/R total = 1/R1 + 1/R2 + ...
• What is the characteristic of a filament lamp?

  It is non-Ohmic; as voltage increases, current increases, temperature increases but resistance also increases
• What is the characteristic of a diode?

  It allows current to flow in one direction and is non-Ohmic, exponential past 0.5V
• What is the relationship between voltage and current in a resistor?

  Voltage is directly proportional to current (Ohmic behavior)
• What are the factors affecting resistance?

  1. Temperature: R proportional to T
  2. Length of conductor: R proportional to L
  3. Cross-sectional area: R proportional to 1/A
  4. Resistivity: R = (resistivity x L) / A
• What is internal resistance in a cell or battery?

  It is the resistance within the cell that causes a drop in potential difference when current flows
• What is the equation relating EMF, terminal voltage, and lost voltage?

  ε = V + v
• What does the equation ε = I(R + r) represent?

  It represents the relationship between EMF, current, total resistance, and internal resistance
• What are the steps to solve two-loop problems in circuits?

  1. Predict direction of current (identify loops)
  2. Set up Kirchhoff's equations for each loop
  3. Solve for unknowns (number of equations = number of unknowns)
• What is the relationship between voltage and resistance in a potential divider?

  The ratio of the resistances determines the ratio of the voltages
• What is a potential divider?

  A series circuit in which voltage is divided based on the resistance of the components (ratio of the resistance = ratio of the voltage)
• What are the characteristics of a primary cell?

  • Cannot be recharged
  • Chemical to electrical energy
  • Direct current
  • Cheap but expensive to keep replacing
• What are the characteristics of a secondary cell?

  • Can be recharged
  • Electrical to chemical
  • Direct current
  • Less wasteful but more expensive
• What are the characteristics of a solar cell?

  • Electrons in semiconductors absorb energy from EM radiation
  • Solar to electrical
  • Direct current
  • Renewable but weather dependent