Resistance (CIE IGCSE Physics)

• Resistance is the opposition to current
  • For a given potential difference, the higher the resistance, the lower the current
  • Therefore resistors are used in circuits to control the current 
  • The unit of resistance is the ohm, represented by the Greek symbol omega Ω 

     

       

Ohm's Law

• The definition of resistance can be given using the equation

• Where
  • R = resistance (ohms, Ω)
  • V = potential difference (volts, V)
  • I = current (amperes, A)

    

• Ohm's Law can be stated in words:

Current is directly proportional to potential difference as long as the temperature remains constant

Use the formula triangle to help you rearrange the equation

 

Consequences of Ohm's Law

• Resistors are used in circuits to control either
  • The current in branches of the circuit (through certain components)
  • The potential difference across certain components
• This is due to 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 graphs for a resistor and a filament lamp

• The IV graph for a resistor is very simple:

The current is proportional to the potential difference

• This is because the resistor has a constant resistance

 

• For a lamp the relationship is more complicated:

The current increases at a proportionally slower rate than the potential difference

• This is because:
  • 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

I-V graph for a diode

 

• A diode is a non-ohmic conductor that allows current to flow in one direction only
  • The direction is shown by the triangular arrow of the diode symbol 
  • This is called forward bias
• In the reverse direction, the diode has very high resistance, and therefore no current flows
  • This is called reverse bias

• The I–V 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)