Current, Resistance & Potential Difference

• Potential Difference:

  • The terminals of a cell make one end of the circuit positive and the other negative
  • This sets up a potential difference across the circuit
  • This is sometimes known as the voltage
• • Potential difference is defined as:The amount of energy transferred per unit of charge passing through the terminals
  • This means that one volt (the unit of potential difference) is equivalent to one joule (the unit of energy) per coulomb (the unit of charge):

  1 V = 1 J / C
• Measuring Potential Difference:

  • Potential difference (or voltage) is measured using a voltmeter
  • A voltmeter is always set up in parallel to the component being measured
  • This is described as measuring voltage 'across' the component
• Potential difference (voltage) can be measured by connecting a voltmeter in parallel between two points in a circuit. Here, the voltmeter is measuring the voltage across the resistor:

  
• Current & Potential Difference:

  • Current and potential difference are directly proportional to each other
  • When there is more current then that current has more potential difference
  • There are more electrons with potential difference moving around the circuit
• Resistance:

  • Resistance is defined as the opposition to current:
  • The higher the resistance of a circuit, the lower the current
  • This means that good conductors have a low resistance and insulators have a high resistance
• The symbol for resistance is R

  • It is measured in Ohms (Ω)
  • Ω is the Greek capital letter ‘Omega’
  • An Ohm is defined as one volt per ampere (1 V / A)
• • The resistance of a circuit can be increased by adding resistors (or variable resistors) to it
  • In exam questions, the resistance of the wires and batteries are assumed to be negligible. Every electrical component has a resistance, even wires
• High resistance means there is lower current and vice versa:

  
• The current I through a component depends on both the resistance R of the component and the potential difference V across the component

  • The greater the resistance R of the component, the lower the current I for a given potential difference V across the component
  • The lower the resistance R of the component, the greater the current I for a given potential difference V across the component
• Calculating Current, Resistance & Potential Difference:

  • The current, resistance and potential difference of a component in a circuit are calculated using the equation:

  
• Voltage, current, resistance formula triangle: