Electricity

• The flow of charge generates electrical current.
• Current is measured in Amperes (A), which is one of the base SI units.
• One amp is equivalent to one coulomb of charge passing a point in one second.
• I = $\frac Q t$
• Charge can be either positive or negative.
• Conventional current flows from the positive to the negative terminal, while electrons actually flow the from the negative to the positive terminal.
• Like charges repel and opposite charges attract.
• Electrical charge is measured in Coulombs, with one Coulomb equivalent to one Ampere second.
• Elementary charge (the charge of a proton or electron) is $1.6*$$10^{-19}$ Coulombs.
• The total charge is given by Q = ne, where n is the number of charge carriers and e is elemental charge.
• Delocalised electrons or ions carry charge in a compound/mixture.
• An ammeter is used to measure current. It should have the lowest possible resistance and should be placed in series.
• Charge is always conserved: the current and charge before an interaction should always be the same after the interaction - this is Kirchhoff’s first law.
• Number density is the number of free electrons per unit volume of a material.
• Conductors have a high number density, followed by semiconductor, and insulators have the lowest number density.
• The current can also be calculated using the formula I = nAve, where A is the cross-sectional area of the wire and v is the mean drift velocity of the charge carriers.
• A narrower wire increases the mean drift velocity of the charge carriers, while a wider wire decreases the drift velocity of the carriers (as long as current is constant).
• Potential difference is a measure of the energy transferred by charge carriers per unit charge.
• Potential difference is measured in volts, with one volt equivalent to one joule of energy transferred by one coulomb of charge.
• The equation to calculate potential difference is $V =$$\frac W Q$
• Potential difference is measured with a voltmeter placed in parallel. An ideal voltmeter would have infinite resistance.
• Electromotive force (e.m.f) describes when work is done on the charge carriers, while Potential difference describes when the charge carriers are doing work.
• Electromotive force is the energy that is transferred into electrical energy per unit charge, and is measured in volts.
• Energy transferred can be calculated using the formula E = QV, where E is the energy transferred in Joules.
• An electron gun produces a beam of electrons:

  • A cathode causes electrons to accelerate down the tube towards the anode.
  • The anode has a small hole in it for electrons to pass through.
  • A beam of electrons is produced.
• Resistance is the opposition to the flow of charge in a circuit.
• Resistance is measured in ohms (Ω), with one ohm meaning that it takes 1 volt to allow one amp of current to flow.
• Resistance can be calculated using the equation: $R =$$\frac V I$
• Ohm’s law states that for an ohmic device at a constant temperature, the current is directly proportional to the potential difference.
• The resistance of a component increases as temperature increases - the atoms vibrate and collide with charge carriers more frequently.
• The I-V characteristic for a fixed resistor is a straight line.
• The I-V graph for a filament lamp is a curve, with the increase of current slowing down as voltage increases.
• A diode allows current to pass through in only one direction.
• The I-V graph for a diode shows current increasing exponentially with potential difference, but no current for a negative potential difference.
• Resistivity describes the ability of a material to oppose the flow of charge.
• Resistivity can be calculated using the equation:

  $\rho =$$\frac {RA} L$
• If the temperature of a thermistor increases or the light intensity around an LDR increases, the charge carriers gain energy and become free, so the number density increases. This means that current increases, so resistance decrease. These components have a negative temperature coefficient.
• Power is the rate of energy transfer by an electrical component, and it is measured in Watts (W).
• The equations for power are:

  • $P =$$IV$
  • $P =$$\frac {V^2} R$
  • $P =$$I^2R$
• Electricity bills are generally given in kilowatt hours, which is the energy transferred by a device of power 1 kW in 1 hour.