Electric Current

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Charge, Current and Potential Difference

An electric current is defined as the rate at which charged particles pass through a point in a circuit.

  • Current is measured in coulombs per second or amperes.
  • In metals, the charged particles are electrons which move from the negative to the positive terminals in d.c supply.
  • In circuit diagrams, the charged particles move from +ve to -ve, which is known as conventional current.
  • Current = change in charge / change in time
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Charge, Current and Potential Difference (cont)

  • A potential difference is what makes a current flow.
  •  A potential difference is the electrical energy transfered/converted per unit of charge passing between 2 points. 
  • P.D is measured in joules per coulomb, or volts. Potential difference = Energy / Charge.
  • A charged particle gains energy when it passes through a cell, and it releases this gained energy when it passes through a component in the circuit.
  • Thus both the cell and a component have a p.d across them when charge flows in a circuit. 
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Charge, Current and Potential Difference (cont)

  • Resistance is the opposition charged particles face when they flow around a circuit.
  • The potential difference needed to make a current flow in a circuit depends on the resistance of circuit.
  • The bigger the resistance, the more potential difference (energy per coulomb) is required to make a certain current flow.
  • Resistance = potential difference / current
  • It is measured in ohms
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Current/Voltage Characteristics

  • The effect of varied potential difference on the current through a component can be investigated using a variable p.d cell connected to an ammeter, switch and component in series, with a voltmeter in parallel to the component.
  • By varying and recording the supply p.d, a range of current values can be recored for the component.
  • The battery is reversed and the supply p.d is varied over the same range.
  • An I/V characteristics curve for that component can be plotted from the results. 
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Current/Voltage Characteristics (cont)

  • In a resistor or wire, a proportional straight line is plotted. 
  • The proportionality between current and p.d means that the conductor follows Ohm's law.

                                   (http://www.bbc.co.uk/schools/gcsebitesize/science/images/ph_elect08.gif)

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Current/Voltage Characteristics (cont)

  • For a semiconductor diode, the shape of the graph depends on the direction the current is flowing.
  • When the diode is forward biased (facing direction of convectional current), between 0-0.7V the diode offers a large resistance to the current.
  • From 0.7V onwards the resistance of the diode falls rapidly and a large current flows, shown by a steep rise in the graph.
  • When the diode is reversed biased, the diode offers high resistance until the breakdown voltage, where the diode is destroyed and a large current flows. 
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Current/Voltage Characteristics (cont)

          (http://www.learnabout-electronics.org/images/diode-IV.gif)

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Current/Voltage Characteristics (cont)

  • When the p.d across a filament lamp is steadily increased, the graph becomes less and less steep.
  • The p.d and the current are not proportional because the increasing current heats the filament lamp.
  • An increase in temperature increases the resistance of the filament and so decreases the rate of increase of current with p.d.
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Current/Voltage Characteristics (cont)

                   (http://www.antonine-education.co.uk/Image_library/Physics_1/Electricity/graph_1A.gif)

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Current/Voltage Characteristics (cont)

  • Ohm's law states that the current in a conductor is directly proportional to the p.d across it, provided that the temperature and other physical conditions remain the same.
  • A proportional I/V characteristics graph shows that a component obeys Ohm's law (wires and resistors).
  • These are called Ohmic conductors, while components that do not obey Ohm's law are called non-Ohmic conductors. 
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Resistivity

  • The 2 factors that affect the resistance of a conductor are its length and its cross-sectional area.
  • Resistance is proportional to length.
  • Resistance is inversely proportional to cross-sectional area.
  • The resistivity of a conductor=
    (cross sectional area x resistance of conductor) / length of conductor.
  • The resistivity is a constant of the material from which the conductor is made.
  • Its units are ohm metres
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Resistivity (cont)

  • Resistivity of a wire can be measured using a battery, a switch, an ammeter, and a 100cm piece of wire under test taped to a ruler, all in series. Add a voltmeter in parallel to the wire on the ruler.
  • Record the p.d and current for the full 100cm of wire.
  • Vary the length of the wire that it across the voltmeter from 100-30cm, recording the current and votlage throughout. 
  • Calculate the resistance for each recorded length.
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Resistivity (cont)

  • Measure the diameter of the wire using a micrometer, and use this value to calculate the cross-sectional area of the wire.
  • Plot resistance against length (y=mx+c).
  • The gradient is the resistivity/cross-sectional area, so the resistivity can be calculated.
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Resistivity (cont)

  • Temperature always affects conduction, no matter wha the material (conductor, semiconductor ect)
  • In conductors, as the temperature increases the resistance increases.
  • Metal conductors (wires and resistors) contain +ve ions as well as free electrons. The electrons collide with the ions as they try to carry charge through, causing a resistance
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Resistivity (cont)

  • As temperature increases, the +ve ioons and electrons have more kinetic energy, so the +ve ions vibrate more (greater amplitude), and electrons move faster.
  • Both of these increase the number of collisions of the charge carriers with the +ve ions (frequency), so resistance increases.
  • The resistance does not change greatly, so in small circuits we consider wires and restitors ohmic conductors. 
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Resistivity (cont)

  • In a thermistor, the resistance decreases significantly as temperature increases.
  • A thermistor is made from semiconductor material and so there are few free electrons to produce a current.
  • As temperature increases, extra electrons are released from the semiconductor ions due to increased thermal energy.
  • This makes the thermistor far more conducting and far less resistive
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Resistivity (cont)

             (http://www.arroyoinstruments.com/images/cms/sized/Thermistor%20Response%20Curve1.gif)

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Resistivity (cont)

  • When the temperature of a conductor approaches absolute zero, the electrical resistance disappears completely.
  • This occurs at a specific temperature for a material known as the critical temperature
  • At and below this temperature, no energy is transferred to the conductor as a current passes through it. 
  • This is called a superconductor. 
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