Current, Resistance & Potential Difference (AQA GCSE Combined Science: Trilogy: Physics): Revision Note

Current, Resistance & Potential Difference

• Potential difference is the energy transferred per unit charge passing through a component (the work done per coulomb), measured in units of volts (V)

• The potential difference between two points in a circuit is a measure of the amount of energy transferred between those points

• Electric current is the flow of electrical charge measured in units of amperes (A) or amps

• The size of the electric current is the rate of flow of electrical charge

  • In other words, how much charge passes through a fixed point each second

• Resistance is defined as the opposition to current:

  • The vibration of the atoms in the wire slows down the flow of electrons

  • 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

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

• Every electrical component has a resistance, even wires

  • In exam questions, the resistance of the wires and batteries are assumed to be negligible

• 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:

potential difference (V) = current (A) × resistance (Ω)

$V=I\times R$

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

• For example, rearranging for current gives

$I=\frac{V}{R}$

• This shows that, for a given potential difference, current is inversely proportional to resistance, so

  • doubling the resistance halves the current

  • the product of current and resistance stays constant ($IR=V$)