5.1 current electricity

• What is current?

  The rate of flow of charge
• What is potential difference?

  The work done/energy per unit charge
• What does resistance do?

  impedes/obstructs the flow of current
• What is the equation for current, involving charge and time?

  $I =$$\frac{\Delta Q}{\Delta t}$where:

  • I = current
  • ΔQ = the change in charge
  • Δt = time
• What is the resistance of an ideal voltmeter?

  infinite
• What is the resistance of an ideal ammeter?

  zero
• What is the resistance of wires?

  zero
• What is Ohm’s Law?

  $V\propto I$ as long as external, physical conditions - e.g. temperature, pressure, humidity - remain constant
• What happens to the resistance of a material when its resistivity is high?

  Resistance is also high
• What does the resistance of a material depend on?

  • length of the wire
  • cross-sectional area of the wire
  • resistivity of the material
• If the length of the wire increases what happens to the resistance of the wire?

  The resistance would also increase as there would be more collisions between the ions and electrons.
• If the cross sectional area of the wire increases what happens to the resistance of the wire?

  The resistance decreases as the electrons now have more paths to take and are less likely to collide with the metal ions.
• If the diameter of a wire doubles, what happens to the resistance?

  • it decreases by a quarter
  • as the cross sectional area $\propto$ to the diameter squared
  • so if the diameter doubles, the area quadruples
  • causing the resistance to drop by a quarter
• What does the I-V graph of an ohmic resistor look like?

  A straight line that crosses the origin i.e. V is directly proportional to I as according to Ohm’s law (V $\propto$ I)
• On an I-V where I is on the y-axis and V is on the x-axis, what does a steep line represent?

  a small resistance
• On an I-V where I is on the y-axis and V is on the x-axis, what does a shallow line represent?

  a high resistance
• What does the gradient of an I-V graph where I is on the y-axis and V on the x-axis represent?

  $\frac {1}{R}$
• Explain the I-V graph of a filament bulb?

  • filament in the bulb is a metal wire
  • as the electrons (the current) try to flow through the wire the positive ions in the metal obstruct the electron flow
  • ions vibrate due to collisions w/ electrons —> wire heats up
  • even more collisions decreases current and resistance increases making it harder for the electrons to pass through wire
  • filament bulb is not an ohmic conductor as external physical factor, temp, has changed and Ohm’s law isn’t obeyed
  • at low currents wire won’t heat significantly so Ohm’s law obeyed so straight line around origin
• What is the total resistance in a series circuit?

  • the sum of the individual resistances
  • $R_T =$$R_1 +$$R_2 +$$R_3 +$$…$
• What is the total resistance in a parallel circuit?

  $\frac {1}{R_T} =$$\frac{1}{R_1} +$$\frac{1}{R_2} +$$\frac{1}{R_3} +$$…$
• What is the current in a series circuit?

  the same for all components
• How is the current in a parallel circuit shared?

  • the current is split across the different branches/junctions
  • the total current into a junction must equal the total current out of a junction
• What is the electromotive force (emf)?

  The maximum energy supplied to charge carriers per unit charge. It is also the terminal potential difference when no current is drawn.
• Why is the terminal potential difference always lower than the emf?

  • the cell has internal resistance
  • since resistance opposes current the current decreases and so the voltage available to the rest of the external circuit also reduces
• How do you calculate the lost volts?

  The lost volts is the voltage lost due to internal resistance so V=IR using the internal resistance of the cell and the current in the circuit.
• What is a superconductor?

  A material which when cooled below a certain temperature (the critical temperature) had zero resistivity. This means the material has no resistance below a critical temperature.
• What is the critical temperature?

  The temperature below which a material becomes a superconductor.
• What are some applications of superconductors?

  Superconductors are useful for applications that require large electric currents so they are useful for the production of strong magnetic fields and in the reduction of energy loss in the transmission of electric power:

  • MRI scanners
  • maglev trains
  • motors
  • fusion reactors
  • electromagnets
  • particle accelerators
• Explain what is meant by a potential divider?

  A combination of resistors in series connected across a voltage source to produce a required potential difference.
• What is a semi-conductor?

  A material where the number of free charge carrier varies (e.g. with temperature).
• How do you calculate the net charge?

  magnitude of positive charge - magnitude of negative charge
• How do you calculate the number of coulombs?

  number of coulombs = charge of particle x number of particles
• What is current?

  number of coulombs of charge moving past a given point per second in a circuit
• What is an equation for current?

  $I=$$\frac {\Delta Q}{\Delta t}$ where:

  • I = current
  • $\Delta Q$ = change in charge
  • $\Delta t$ = change in time
• What is the direction conventional current?

  the direction of the drift of positive charges so the opposite to the direction of the drift of negative charges
• What direction is the current?

  same direction as conventional current
• What does the number of volts on a battery refer to?

  number of volts between the two terminals of a battery
• What does the voltage of a battery refer to?

  the difference between the (electric potential) energy of one coulomb of charge at one terminal and at the other
• What happens to the electric potential energy of electrons as the drift along the circuit from one terminal to the next?

  • decreases by some amount (as the electrons are moving towards the terminal with lower electric potential energy)
  • the energy dissipated by the components of the circuit increases by the same amount
  • and the kinetic energy of the electrons stays the same
• What does the potential difference between two points in a circuit show?

  the change in potential energy per coulomb between those two points