Current, Resistance & Potential Difference (OCR Gateway GCSE Physics: Combined Science)

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

• 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

Step 1: List the known quantities

• • Resistance, R = 10 Ω
  • Current, I = 0.3 A

Step 2: Write the equation relating resistance, potential difference and current

V = IR

Step 3: Substitute in the values

V = 0.3 × 10 = 3 V