# A-level Physics/Electrons, Waves and Photons/D.C. circuits

A direct current (DC) circuit usually has a steady and constant voltage supplied to it. A direct current does not have a continually changing polarity, unlike an alternating current (AC), but instead a constant direction and rate of flow. DC is generally provided by batteries or via a transformer, rather than generators.

## Circuit diagrams

Below are the symbols and names for all of the components that you are required to know:

## Series circuits

When resistors are set up in series, the formula to work out the total resistance is:

${\displaystyle R_{T}=r_{1}+r_{2}+r_{n}...etc}$

Where ${\displaystyle r_{1},r_{2}}$ etc., are the resistance of each resistor in series.

## Parallel circuits

When resistors are set up in parallel, the formula to work out the total resistance is:

${\displaystyle {\frac {1}{r_{t}}}={\frac {1}{r_{1}}}+{\frac {1}{r_{2}}}+{\frac {1}{r_{n}}}...etc}$

Where ${\displaystyle r_{1},r_{2}}$ etc., are the resistance of each resistor in parallel.

## Internal resistance

A electrical source has its own resistance, known as Internal Resistance. This is caused by the electrons in the source having to flow through wires within it, or in the case of a chemical battery, the charge may have to flow through the electrolytes and electrodes that make up the cell.

By considering a battery of EMF E, in series with a resistor of resistance R we can calculate the internal resistance r:

${\displaystyle E=I(R+r)}$ (See Series Circuits above)

Combining with V=IR:

${\displaystyle V=E-Ir}$

The quantity Ir is called the lost volts. The lost volts shows us the energy transferred to the internal resistance of the source, so if you short circuit a battery, I is very high and the battery gets warm.

## Potential dividers

A potential (or voltage) divider is made up of two resistors. The output voltage from a potential divider will be a proportion of the input voltage and is determined by the resistor values.

The values of a battery with voltage V1 passing through two resistors in series of resistance R1 and R2, with an output circuit in parallel with Resistor R1 with output voltage V2 are related by the equation: ${\displaystyle V1=(R1/(R1+R2))}$ × ${\displaystyle V2}$

## Kirchhoff's laws

First Law states "The sum of the current (A) entering a junction is equal to the sum of the current (A) leaving the junction". This is a consequence of conservation of charge.

Second law states that the EMF is equal to the voltage of the circuit. This is a consequence of conservation of energy.

## Use of other components

Thermistors can be placed in circuits when temperature plays a role. As the temperature increases, the resistance of the device decreases. This does not obey the Ohms law. Light dependent resistors are resistors that decrease their resistance when exposed to light.