Components and circuit devices

In a nutshell

A standard test circuit can be used to investigate a range of components, such as diodes, thermistors and LDRs. An $I-V$ graph shows how the current and voltage varies through a given component.

The standard test circuit

A standard test circuit can be used to investigate the relationship between the current, potential difference and resistance of components. The standard test circuit contains:

Physics; Electricity and circuits; KS4 Year 10; Components and circuit devices

	component	Description
1.	Power source	A cell or battery to power the circuit.
2.	Variable resistor	To change the current through the circuit. If the resistance value is increased, this will reduce the current that is able to flow through the circuit, and vice versa.
3.	Test component	The component to be tested (a fixed resistor in this case).
4.	Ammeter	Connected in series anywhere in the circuit, to measure the current.
5.	Voltmeter	Connected in parallel across the component to be tested, to measure the voltage.

The ammeter and voltmeter

The ammeter measures the current flowing through a component. The ammeter must be placed in series with the component. It can be placed anywhere in series within the circuit, because the current will be the same value at any point in the circuit.

The voltmeter measures the potential difference across the component. The voltmeter must only be placed in parallel with the component being tested.

Investigating components

Using the standard test circuit, it is possible to create current-potential difference graphs ( $I-V$ graphs) of different components. $I-V$ graphs show how current changes as the potential difference across a component is varied.

Three important $I-V$ graphs are the following.

Physics; Electricity and circuits; KS4 Year 10; Components and circuit devices

A	Wire or resistor
B	Filament bulb
C	Diode

Physics; Electricity and circuits; KS4 Year 10; Components and circuit devices

Physics; Electricity and circuits; KS4 Year 10; Components and circuit devices

Linear components have an $I-V$ graph that is a straight line at constant temperatures. This is the case for a fixed resistor, for example.
Non-linear components have an $I-V$ graph that is curved. This is the case for a filament lamp or diode, for example.
All $I-V$ graphs pass through the origin ( $0,0$ ), For a linear component, the resistance equals the inverse of the gradient of the line ( $\frac{1}{gradient}$ ).
It is possible to find the resistance of any point on any $I-V$ graph (linear or non-linear) by reading the potential difference and current values, and then using the formula $R = \frac{V}{I}$ .

Diodes

A diode is a device made of a semiconductor material such as silicon. A diode allows current to flow through it in one direction only as the reverse direction has an extremely high resistance. Below is the circuit diagram of a diode:

Physics; Electricity and circuits; KS4 Year 10; Components and circuit devices

Diodes have many uses in various electronic circuits, such as radio receivers and logic gates.

Light-dependent resistors

A light-dependent resistor (LDR) is a resistor whose resistance value depends on the intensity of the surrounding light.

In darkness, the resistance is highest. As the light level increases, the resistance drops, which means the current through the LDR increases. Below on the left is the circuit symbol of an LDR. Below on the right is the graphical relationship between light intensity and resistance for an LDR.

Physics; Electricity and circuits; KS4 Year 10; Components and circuit devices

Physics; Electricity and circuits; KS4 Year 10; Components and circuit devices

LDRs have many applications, including automatic night lights, outdoor lighting and burglar detectors.

Themistors

A thermistor is a temperature-sensitive resistor. For an NTC (Negative Temperature Coefficient) thermistor, the following properties apply:

In higher temperatures, the resistance drops.
In lower temperatures, the resistance increases.

Below on the left is the circuit symbol of a thermistor. Below on the right is the graphical relationship between temperature and resistance for an NTC thermistor.

Physics; Electricity and circuits; KS4 Year 10; Components and circuit devices

Physics; Electricity and circuits; KS4 Year 10; Components and circuit devices

In constant conditions, a thermistor's IV-graph is curved. This is because as the current increases, the thermistor warms up, so the resistance decreases.

Thermistors have applications in car engine temperature sensors, as well as electronic thermostats.

Note: All thermistors you will be asked about will be NTC thermistors. If an exam question refers to a thermistor, you can assume it to be an NTC thermistor.