Electrical Circuits in A Level Physics AS

Electrical Circuits

This section focuses on the analysis of electrical circuits, including the study of series and parallel combinations of resistors, voltage dividers, potential dividers, internal resistance, electromotive force (EMF), terminal voltage, and electrical measurements.

Series and Parallel Circuits

In a series circuit, components are connected end-to-end, so the same current flows through each component. The total resistance Rtotal in a series circuit is the sum of the individual resistances:

Worked Example: Series Circuit

Problem: Calculate the total resistance of three resistors in series: R1 = 2 Ω, R2 = 3 Ω, R3 = 5 Ω.

Solution:

• Rtotal = R1 + R2 + R3
• Rtotal = 2 + 3 + 5 = 10 Ω

In a parallel circuit, components are connected across the same two points, and the voltage across each component is the same. The total resistance Rtotal in a parallel circuit can be calculated using:

1/Rtotal = 1/R1 + 1/R2 + 1/R3

Worked Example: Parallel Circuit

Problem: Calculate the total resistance of three resistors in parallel: R1 = 2 Ω, R2 = 3 Ω, R3 = 6 Ω.

Solution:

• 1/Rtotal = 1/2 + 1/3 + 1/6
• 1/Rtotal = 3/6 + 2/6 + 1/6 = 6/6
• Rtotal = 1 Ω

Voltage and Potential Dividers

A voltage divider is a simple circuit that turns a large voltage into a smaller one. It consists of two resistors in series. The output voltage Vout across one of the resistors can be calculated as:

Vout = Vin * (R2 / (R1 + R2))

Internal Resistance and EMF

The electromotive force (EMF) of a source is the voltage when no current flows. However, when a load is connected, the terminal voltage is affected by the internal resistance r of the source:

Vterminal = EMF - I * r

Electrical Measurements

To measure current and voltage in circuits, ammeters and voltmeters are used. Ammeters are connected in series to measure current, while voltmeters are connected in parallel to measure voltage across components.

Worked Example: Measuring Current

Problem: If an ammeter shows a current of 5 A in a circuit, what does this indicate?

Solution:

• This indicates that 5 coulombs of charge pass through a point in the circuit every second.