Physics Edexcel A Level 3: Electricity

Ohm's Law

Ohm's Law: At a constant temperature, the current through a conductor is proportional to the p.d. applied across it

Non-Ohmic Components

  • Filament Lamp  Image result for filament lamp iv graph 
    • As the temperature increases, the resistance increases, as metals have a high resistance when hot. Higher resistance causes a lower gradient
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Ohm's Law 2/

Image result for diode iv graph

Diode

  • Has a low resistance when 'forward biased', i.e. when v > 0.6. But when v < 0.6, the resistance becomes so high it is virtually non-conducting. 
  • 0.6V = threshold voltage
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Ohm's Law 3

Image result for thermistor iv graphThermistor

  • Shown is a NTC thermistor, with the resistance decreasing at it gets hotter.
  • This is due to a semi-conducting material, whose atoms release more conduction electrons as the temperature rises
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Conservation Laws

Quantity Series Rules Parallel Rules Current (I) Current is the same throughout (I1 = I2 = I3) Itotal = I1 +I2 + I3 Voltage (V) Vtotal = V1 +V2 + V3 Voltage is the same throughout (V1 = V2 = V3) Resistance (R) Rtotal = R1 +R2 + R3 1/Rtotal = 1/R1 +1/R2 + 1/R3

  • Sum of the currents entering a junction = 0
  • In a closed loop the emf sum = sum of p.ds (IR)
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Resistivity

  • It is a property of materials that doesn't vary with the size, unlike resistance
  • R = pl/A
  • Experiment: Measuring wires of different lengths to plot a graph to find resistivity
    • Using a screw-gauge to measure cross-sectional area
    • Graph with length on x-axis, and resistance on y-axis, gradient p/A
    • You could also vary the area instead, with the current on the x-axis, and gradient pl

Image result for resistivity physics

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Curr

I = nqva

  • We use the drift velocity (v) as charged particles do not travel in a straight line through a conductor, because they collide with other particles in the material
  • n = carrier density, how many charge carriers in the material

Carrier densities

  • In metals, it is very large, which makes them good conductors as they have little resistivity
  • Insulators have almost none, meaning they have very high resistivity
  • Semiconductors have low amounts, meaning they have higher resistivity than metals
    • LDRs are semiconductors, therefore is there is more light energy, more charge carriers are freed.
    • As the temperature in an NTC thermistors, the charge carriers also increases. The lattice vibrations here have a very minimal effect.
  • If the temperature is increased in a metal, there is an increase in lattice vibration, which reduces the drift velocity. Therefore the resistivity increases with temperature.
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E.M.F and Internal Resistance

  • E = V+ Ir
    • E = emf (total voltage in a cell)
    • V = voltage of the circuit
    • I = current
    • r = internal resistance
  • When a cell is connected and current is drawn, some of the energy is dissipated within the cell itself.
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Potential Divider Circuits

  • Potential divider circuit:
    • I = Vin / (R1 + R2)
    • I = Vout / R1
    • Vout = Vin x R/ (R1 + R2)
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Semiconductors

  • Semiconductors start with a low amount of charge carriers, which are free electrons
  • As the energy increases, the amount of charge carriers increases, due to the electrons passing amount of energy needed to break from the other atoms. 
  • This means that the current increases, therefore the resistivity decreases
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