Resistance

• Resistance is the opposition to current, how difficult it is for current to flow

  • Symbol is R
  • The unit of resistance is the ohm, represented by the Greek symbol omega Ω
  • Uses an ohmmeter to measure
• For a given potential difference, the higher the resistance, the lower the current so resistors are used in circuits to control the current
• Ohm’s law - current is directly proportional to potential difference as long as the temperature remains constant

  • graph has a linear line
  • graph goes through origin
  • when potential differences doubles, current doubles
• Formula for resistance: R = V/I

  • R = resistance (ohms, Ω)
  • V = potential difference (volts, V)
  • I = current (amperes, A)
• Resistance formula triangle:

  A) voltage

  B) current

  C) resistance

  
• Resistors are used in circuits to control either:

  • The current in branches of the circuit (through certain components)
  • The potential difference across certain components
• The consequences of Ohm's Law:

  • The current in an electrical conductor decreases as its resistance increases (for a constant p.d.)
  • The p.d. across an electrical conductor increases as its resistance increases (for a constant current)
• As the potential difference (voltage) across a component is increased, the current in the component also increases
• The precise relationship between voltage and current can be different for different types of components and is shown by an IV graph:

  • IV graph for a resistor is very simple - the current is proportional to the potential difference because the resistor has a constant resistance
  • IV graph for a lamp is more complicated - the current increases at a proportionally slower rate than the potential difference
• IV graph for resistor and lamp

  A) resistor

  B) filament lamp

  
• Why the relationship between current and potential difference in a lamp is more complicated:

  • The current causes the filament in the lamp to heat up
  • As the filament gets hot, its resistance increases
  • This opposes the current, causing it to increase at a slower rate
• A diode is a non-ohmic conductor
• Forward bias - current flows in one direction only

  • Happens in the diode
  • The direction is shown by the triangular arrow of the diode symbol
• Reverse bias - in the reverse direction, the diode has very high resistance, and therefore no current flows
• The IV graph for a diode has a unique shape

  • When the diode is in forward bias, the graph shows a sharp increase in voltage and current (on the right side of the graph)
  • When the diode is switched around, in reverse bias, the graph shows a flat line where current is zero at all voltages (on the left side of the graph)
• IV graph for diode

  A) semiconductor diode

  
• Wires resist the flow of electrons so those electrons require voltage to flow aka push which is why the formula is its P.D/current as its the voltage used to push the electrons over the amount of charge passing through the wire
• Short circuit - a wire that has little resistance
• Resistance is measured using currents and potential difference, using the 2 values of currents and potential differences to calculate resistance
• Variable resistor - can change its resistance 

  
• Fixed resistor - resistance doesn’t change

  
• Ohmic conductor - follows Ohm’s law
• Lightbulbs are non-ohmic conductors because their graph isn’t a straight line
• 4 factors affecting resistance

  • length - when the length increases, the length increases
  • cross sectional area - it is easier for electrons to move through a wider space than a smaller one which is why thin wires have more resistance while thicker wires have less resistance
  • matieral - the type of material affects its conductivitiy which influences resistance
  • temperature - resistance increases with temperatures because more particles are moving around inside the material so it’s more difficult to move acorss
  • semiconductors decrease in resistance when temperature increases
• Resistance occurs because of the collisions between electrons
• 2 wires with the same material and temperature will have the same resistance

  • resistance A x area A/ length A = resistance B x area B/length B
• All batteries have internal resistance
• • As electrons pass through a wire, they collide with the metal ions in the wire
  • The ions get in the way of the electrons, resisting their flow
• Inside a wire

  A) metal ion

  B) electron

  
• The longer a wire, the greater its resistance

  • If the wire is longer, each electron will collide with more ions and so there will be more resistance
• The thicker a wire, the smaller its resistance

  • If the wire is thicker (greater diameter) there is more space for the electrons and so more electrons can flow
• The relationship between resistance, length and cross-sectional area can be represented mathematically

  • Resistance is directly proportional to length
  • Resistance is inversely proportional to cross-sectional area (width, or thickness)-
• Resistance in relation to length and cross sectional area

  A) doubling

  B) double

  C) doubling

  D) half

  
• Series circuit - on a singular wire
• Formula for resistance for resistors on a series circuit:

  • Rtotal = R1 + R2 …
• Formula for resistance for only 2 resistors on a parallel circuit

  • resistance multiplied/resistance added
• What happens to a thermistor if temperature increases?

  Resistance decreases
• What happens to potential differences if resistance decrease?

  Increases
• Resistors in parallel are like water draining out of a bucket with a hole. If a second hole is drilled into the bucket the water (like adding another resistor in parallel) will drain faster. It does not matter how narrow the second hole is (how high its resistance is), it is always faster for water to flow through two holes instead of one
• What is the relationship between length and resistance?

  directly proportional