Physics - Electrons, Waves and Photons

• Electric current, I


  Rate of flow of charge
• SI base unit for current

  Amperes (A)
• Ammeter


  Device used to measure current in an electrical circuit, placed in series
• Charge, Q


  Physical quantity, can be positive or negative, measured in coulombs (C)
• 1 coulomb is defined as the flow of charge in a time of 1 second when the current is 1 ampere</b>
• Like charges repel each other, whereas opposite charges attract each other
• Proton


  Charge of +1
• Electron


  Charge of -1
• Elementary charge, e


  1.6x10-19 C
• Net charge of a particle

  Due to the gain or loss of electrons
• In an atom, the number of protons equals the number of electrons, so the charges cancel each other out and the overall charge is neutral
• Negative ion


  Produced by increasing the number of electrons
• Positive ion


  Produced by removing electrons
• Net charge on a particle, Q


  Q = ± ne, where n is the number of electrons added or removed
• Electrolytes


  Conducting liquids that contain positive and negative ions
• Anode


  Positive electrode
• Cathode


  Negative electrode
• Conventional current


  Rate of flow of charge from the positive to the negative terminal
• In metals, the electrons flow from negative to positive, so the electron flow is in the opposite direction to the conventional current
• Circuit symbols


  • Drawn carefully, with arrows pointing in the correct directions
  • Wires in circuits should be shown as straight lines, with wires in junctions drawn at 90° angles to each other
• Kirchhoff's first law


  The sum of the currents in to a point in an electrical circuit is equal to the sum of the currents coming out of that point
• Charge is a fundamental physical property, which cannot be created or destroyed, so it must be conserved
• Potential difference (p.d.)


  Used to measure the work done by charge carriers, which lose energy as they pass through the components in a circuit. Defined as the energy transferred from electrical energy to other forms, per unit charge.
• Mean drift velocity, v

  The average velocity of the electrons as they travel down the wire, colliding with positive metal ions
• Number density, n


  The number of free electrons per unit volume
• Electromotive force (e.m.f.)


  Used to measure the work done to charge carriers, when they gain energy as they pass through a cell or power supply. Defined as the energy transferred from chemical energy to electrical energy per unit charge.
• Conductors have very high number densities, around 1028 per m3, insulators have much smaller number densities, and semi-conductors have in-between values
• The volume of the wire is equal to its cross sectional area, A, multiplied by its length. The length of the wire divided by the time taken for the electrons to cross this distance is equal to the mean drift velocity, v
• Kirchhoff's second law


  The sum of the electromotive force is equal to the sum of the potential difference in a closed loop
• Electron gun


  1. Heating a small metal filament (cathode) to cause thermionic emission

  2. Accelerating the freed electrons towards the anode using a high p.d.

  3. Passing the electron beam through a small hole in the anode
• Series circuit


  • Current at every point in the circuit is the same
  • E.m.f. is split so that the total p.d. across each component is equal to the e.m.f.
  • Total resistance is the sum of the resistance of each component
• The velocity of the electrons in an electron gun can be determined using the conservation of energy
• Parallel circuit


  • Current in each loop adds up to the total current
  • P.d. across each loop is the same
  • Total resistance is given by the formula 1/Rt = 1/R1 + 1/R2 + ...
• Internal resistance


  Not all of the energy transferred to the charge carriers is available to the circuit, as some is transferred to the internal resistance of the cell
• Resistance


  The potential difference across a component divided by the current in the component. Measured in ohms (Ω).
• Lost volts


  Difference between the measured p.d. across the terminals of the power supply, and the actual e.m.f. of the cell
• Modelling internal resistance


  1. Source of e.m.f. ε being in series with its internal resistance r

  2. ε = I(R + r)
• Techniques to determine internal resistance

  1. Cell with internal resistance r connected in series to an ammeter and a variable resistor

  2. Voltmeter connected in parallel around the cell

  3. Resistance of variable resistor varied, V and I readings recorded

  4. Plot graph of terminal p.d. (V) against current (I), y-intercept is e.m.f. and negative gradient is internal resistance
• Determining resistance of a component

  1. Set up circuit with variable power supply, ammeter in series, voltmeter in parallel

  2. Vary power supply and record p.d. and current

  3. Calculate resistance from I-V graph
• Potential divider


  • P.d. splits in a ratio proportional to the resistance of each component
  • Vout = (R2/(R1 + R2)) * Vin