Current, Potential Difference & Resistance (AQA GCSE Physics): Revision Notes
Required practical - Investigating resistance
What this practical investigates
This required practical helps you understand how electrical resistance works in different types of circuits. You will explore how combining resistors in series and parallel affects the total resistance. You'll also investigate how the length of a wire changes its resistance.
This practical combines two important electrical concepts - circuit arrangements (series vs parallel) and the physical properties of conductors (wire length). Understanding both will help you predict and calculate resistance in real electrical systems.
Aim
The aim is to investigate electrical resistance in series and parallel circuits. You will also find out how the resistance of a wire varies with its length.
Apparatus you need
- Battery or suitable power supply
- Ammeter (to measure current)
- Voltmeter (to measure potential difference)
- Switch
- Crocodile clips
- Two resistors
- Resistance wire
- Metre ruler
- Variable resistor
- Connecting leads
Make sure your ammeter and voltmeter have appropriate ranges for your power supply. Using metres with ranges that are too high will make small readings difficult to measure accurately.
Method 1: Series and parallel circuits
Setting up the circuits
You need to set up two different circuits - one with resistors in series and one with resistors in parallel.
Circuit Setup Example: Series vs Parallel
Series circuit:
- Connect the resistors one after the other in a single loop
- The current flows through both resistors
- Think of it like a single path with obstacles in line
Parallel circuit:
- Connect the resistors so they each have their own separate path
- The current splits between the two paths
- Think of it like a road that splits into two lanes
Taking measurements
For both circuits:
- Measure the potential difference across the resistors using a voltmeter
- Measure the current flowing through the circuit using an ammeter
- Calculate the total effective resistance using R = V ÷ I
The resistance calculation uses Ohm's Law:
Where:
- = resistance (in ohms, Ω)
- = potential difference (in volts, V)
- = current (in amperes, A)
Key differences between series and parallel
In series circuits:
- The potential difference splits across the resistors
- Each resistor gets part of the total voltage
- Total resistance =
In parallel circuits:
- Each resistor gets the same potential difference
- This is the same as the power supply voltage
- Total resistance =
Method 2: Resistance of a wire
Setting up
- Set up a circuit with a piece of resistance wire
- Use crocodile clips to change the length of wire in the circuit
- Include a switch so you can turn the circuit on and off
Keep the circuit switched off between measurements to prevent the wire heating up. Hot wires have different resistance properties, which could affect your results.
Taking measurements
- Measure the potential difference across different lengths of wire
- Measure the current flowing through the wire
- Work out the resistance for each length by dividing voltage by current:
- Take at least two sets of readings for accuracy
- Plot a graph with resistance on the y-axis and length on the x-axis
Worked Example: Calculating Wire Resistance
Step 1: Measure the values
- Length of wire = 50 cm
- Potential difference = 2.4 V
- Current = 0.3 A
Step 2: Apply Ohm's Law
Step 3: Record in your table The 50 cm length of wire has a resistance of 8.0 Ω
Recording your results
Create a table to record your measurements. Include columns for:
- Length of wire (in cm) OR arrangement of resistors
- Potential difference (in V)
- Current (in A)
- Calculated resistance (in Ω)
Table Organisation Tip: Keep separate tables for the series/parallel investigation and the wire length investigation. This makes it easier to analyse your data and spot patterns in your results.
Key findings
Key Points About Series vs Parallel:
- Series circuits have higher total resistance than parallel circuits
- When resistors are in series, you add up their individual resistances:
- When resistors are in parallel, the total resistance is lower than the smallest individual resistor
Key Points About Wire Length:
- Longer wires have higher resistance
- The relationship is directly proportional - double the length means double the resistance
- A graph of resistance against length should give a straight line through the origin
The mathematical relationship for wire resistance and length is: or where is a constant that depends on the wire material and thickness.
Common Mistakes to Avoid:
- Don't confuse voltage splitting (series) with current splitting (parallel)
- Remember that parallel circuits provide multiple paths, so total resistance decreases
- Always check your calculated values - if resistance seems too high or low, double-check your measurements
Remember!
Essential Points to Remember:
- Series circuits split voltage - each component gets part of the total voltage
- Parallel circuits share voltage - each component gets the full voltage
- Longer wires = higher resistance - resistance increases with length
- Always use R = V ÷ I to calculate resistance from your measurements
- Take repeat readings to make your results more reliable