P12 electric current

• electric current is the rate of flow of charge in the wire or component, it is due to the movement of charged particles called charge carriers, electrons or ions
• to make an electric current pass round a circuit, the circuit must be complete and there must be a source of potential difference
• the unit of current is the ampere, represented by A
• the unit of charge is the coulomb, represented by C
• equation for current: I = Q/t

  I = current in A

  Q = charge in C

  t = time in s
• potential difference is the work done per unit charge
• the unit of potential difference is the volt, represented by V
• equation for potential difference: V = W/Q

  V = potential difference in V

  W = work done in J

  Q = charge in C
• the emf of a source of electricity is defined as the amount of chemical energy transferred to electrical energy per unit charge passing through the source
• emf is represented by $\epsilon$
• the unit of emf is the volt
• an equation for power: P = IV

  P = power in W

  I = current in A

  V = potential difference in V
• resistance is a measure of how difficult it is for charge carriers to pass through a component
• resistance is defined as the potential difference across a component divided by the current through the component
• the unit of resistance is the ohm, represented by $\Omega$
• equation for resistance: R = V/I

  R = resistance in ohms

  V = potential difference in V

  I = current in A
• Ohm’s law states that:

  the potential difference across a metallic conductor is proportional to the current through it, as long as the physical conditions are constant
• resistivity is a measure of how easily a material conducts electricity
• resistivity is represented by $\rho$
• resistivity is defined as the product of resistance and cross-sectional area, divided by the length of the material
• equation for resistivity: rho = RA/L

  rho = resistivity in $\Omega m$

  R = resistance in $\Omega$

  A = cross-sectional area in $m^2$

  L = length in $m$
• a superconductor is a wire or component made of material which has zero resistivity at and below a critical temperature
• the critical temperature is different for each superconductor material
• a superconductor has zero resistance below the critical temperature of the material
• applications of superconductors:

  • power cables as this 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 for frictionless trains and in medicine
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  A) ammeter

  
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  A) voltmeter

  
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  A) cell

  
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  A) lamp

  
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  A) diode

  
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  A) LED

  
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  A) resistor

  
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  A) variable resistor

  
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  A) thermistor

  
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  A) LDR

  
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  A) heater

  
• an ohmic conductor follows ohm’s law, so potential difference is proportional to current
• a semiconductor diode has a forward and reverse bias

  the forward bias is the direction in which it will easily allow current to flow, past a threshold voltage

  the reverse bias is the direction in which it allows very little current to flow as resistance is very high
• a filament lamp contains a length of metal wire which heats up as current increases, so it obeys ohm’s law at low current but not at high current
• unless otherwise stated, an ammeter has zero resistance so do not affect the measurement of current in a circuit at all, and a voltmeter has infinite resistance so measures potential difference exactly as no current can flow through them