Current, Resistance & Potential Difference (OCR GCSE Combined Science A (Gateway)) : Revision Note

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

Resistance & Current

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:

• This equation can be rearranged with the help of the following formula triangle:

Voltage, current, resistance formula triangle