knowledge-9 CE

• electric current is the rate of flow of charge:

  • current = change in charge / time
• electric current of 1 amp is a rate of flow of charge of 1 coulomb per second.
• Current is measured using an ammeter, which must be connected in series in a circuit.
• Potential difference is the work done per unit charge:

  • voltage = work done / charge
• A p.d of 1 volt across a component or between two points in a circuit means 1 joule of energy is transferred per coulomb of charge through the component or between points.
• A voltmeter measures p.d and must be parallel to the circuit.
• Resistance is a measure of how much a component opposes the flow of current through it. Resistance is measured in ohms (Ω).

  • R=V/I
• Resistivity:

  The resistivity of a material is a measure of how difficult it is for a current to flow through it , and is defined as the resistance of a 1 meter length of material with a cross-sectional area of 1 square metre.
• Resistivity:

  R = resistance x cross-sectional area / length
• resistivity is not constant and can depend on temperature and light intensity.
• Ohms law states that the current through a metallic conductor is directly proportional to the p.d across it, provided the physical conditions are constant.
• Conductors that follow ohms law have constant resistance and are known as ohmic conductors.
• I-V characteristics on Ohmic conductors:

  • current is proportional to p.d, so the I-V graph is a straight line through the origin.
  • resistance is constant and can be found by 1/ gradient
• I-V characteristics on Filament lamps:

  • the gradient decreases , showing that the resistance of the filament lamp increases.
  • As more current flows through the filament , its temp increases. The atoms and ions in wire vibrate more , and collide more often with electrons flowing through it , so there is more resistance.
  • resistance is found by r=v/i
• I-V characteristics on a diode:

  • A diode only lets current flow in one direction , known as the forward direction.
  • The p.d must be above a certain threshold voltage before any current can flow.
• A superconductor is a material which has zero resistivity at and below critical temperature , which depends on the particular material.
• when current flows through a material in superconducting state , there will be no p.d across it because V=IR and R is zero. This means the current has no heating effect and no energy is lost.
• Superconductors are used to produce strong magnetic fields in devices such as MRI scanners and particle accelerators.
• Superconducting transmission cables would allow much more efficient transfer of electrical energy because none of it would be lost to heating. Superconducting wires would also increase the efficiency of electric motors and generators.
• A metal has positive temperature coefficient because the resistance of a metal increases with temperature.
• At higher temperatures, the atoms and ions in a metal vibrate more , increasing the chance of collisions with charge carriers. This makes it more difficult for them to pass through the metal so there is less current per volt across the metal and therefore , greater resistance.
• The resistance of a negative temperature coefficient thermistor decreases as temperature increases.
• resistivity = resistance x cross sectional area / length