5.1. Current Electricity

• What is electric current? State its units.

  The rate of flow of charge. It is measured in amperes (amps).
• What is potential difference?

  The work done moving a unit charge between 2 points in a circuit.V = W / Q
• What is resistance?

  How difficult it is for current to flow through an appliance. A component has a resistance of 1Ω if 1A flows through it when a p.d of 1V is applied across it.R = V / I
• What is meant by an ohmic conductor?

  A conductor that obeys Ohm’s law, meaning that current is directly proportional to potential difference providing physical conditions (such as temperature) remain constant.
• How can you measure the current in a circuit?

  You can measure the current in a circuit with an ammeter connected in series with the component.
• How do you measure potential difference across a component?

  Using a voltmeter, connected in parallel across the component being measured.
• What does the gradient of a current - potential difference graph represent?

  Rate of change of current with respect to voltage. This is not the same as 1/R
• Why does the current increasing on a filament lamp cause an increase in the resistance?

  As current flows through the lamp, electrical energy is converted to heat energy so the metal ions vibrate with increased amplitude. This impedes the movement of electrons through the lamp as they collide with the ions (resistance has increased).
• What is a diode?

  A diode is a appliance that only allows current to flow in one direction.
• Unless stated in the question, should you assume that voltmeters to have zero resistance or infinite resistance?

  You should assume they have infinite resistance. Current takes the path of least resistance so, if the voltmeter has infinite resistance, when applied in parallel to the appliance, no current will flow through it and all the current will flow through the appliance.
• Why should you assume that an ammeter has zero resistance unless stated otherwise?

  This assumption means that there would be 0 potential difference across the ammeter and no energy is lost across it; it does not affect the circuit.
• What is an Light Dependent Resistor (LDR)?

  A semiconductor that is sensitive to light. As the light intensity increases, its resistance decreases.
• How does a thermistor work?

  Similar to an LDR but, as the temperature increases, the resistance decreases (This means thermistors have a negative temperature coefficient).
• What is resistivity?

  The resistance of a 1m cylinder with a cross sectional area of 1 m^2. Resistivity is an inherent property of a material.

  ρ = RA/Lρ = resistivity (Ωm)R = resistance (Ω)A = cross-sectional area (m^2)L = length (m)
• What is a superconductor?

  A material that has a resistivity of zero at or below a critical temperature. The critical temperature is an inherent property of the material.
• How do you find the total resistance in a series circuit?

  R_Total = R_1 + R_2 + R_3 + …

  Add the individual resistances of each component.
• If 6 cells, each of voltage 5V, are arranged in parallel what is the voltage in the circuit?

  V = 5V
• If 6 cells, each of voltage 5V, are arranged in series what voltage is provided to the circuit?

  30V(5V x 6)
• How does the current vary between each component of a series circuit?

  The current through all of the components is the same so the current does not vary.
• Is the current in parallel components the same?

  No, each branch of a parallel circuit can have different currents through them according to Kirchhoff’s first law.
• What is Kirchhoff’s first law?


  All of the current going into a junction is equal to the current leaving the junction.
• What is Kirchhoff's second law?


  For any path (loop) of a circuit, the sum of all of the potential differences must equal to total emf of the circuit.
• In a series circuit, if two cells are connected negative to negative, would their emf add up or cancel out?

  They will cancel out. The total emf would be equal to:

  ε_total = ε_1 - ε_2
• What is power?

  The rate of energy transfer.
• Give an equation for power in terms of current and voltage.

  Power = Current x VoltageP = I⋅V
• What is the purpose of a potential divider?

  ● To provide variable potential difference, or

  ● To provide a constant specific potential difference
• A circuit is set up with a cell providing a voltage of 12V to 2 resistors of 6Ω and 7Ω respectively, in series. What is the voltage across the 7Ω resistor?

  Total resistance = 6 + 7 = 13 Ω(7/13) x 12 =6.5 V
• What is emf?

  Electromotive force: the electrical energy transferred by a power supply per unit charge.
• Rearrange the equation Ɛ = I(R + r) into the form y = mx + c


  y = mx + c

  
  

  Ɛ = IR + Ir

  Ɛ = V + Ir

  
  

  So for a graph of V against I

  V = -rI + Ɛ

  Gradient = -r and y-intercept = Ɛ
• What are two applications of superconductors?

  ● Power cables, which would reduce energy loss through heating to zero during transmission.

  ● Strong magnetic fields, which would not require a constant power source. These could be used in maglev trains, where there would be no friction between the train and rail, and in certain medical applications.
• What is the total resistance of a parallel arrangement of 3Ω, 7Ω and 9Ω resistors?

  1 / R_T = ⅓ + 1/7 + 1/91 / R_T = 37 / 63R_T = 63 / 37 =1.7 Ω