# 4.3.4 Use of Circuit Components

### Potentiometers

• When two resistors are connected in series, the potential difference across the power source is shared between them

A potential divider splits the potential difference of a power source between two components

• The potential difference across each resistor depends upon its resistance:
• The resistor with the largest resistance will have a greater potential difference than the other one
• If the resistance of one of the resistors is increased, it will get a greater share of the potential difference, whilst the other resistor will get a smaller share

• A potentiometer is a single component that (in its simplest form) consists of a coil of wire with a sliding contact, midway along it

A potentiometer is a kind of variable resistor

• The sliding contact has the effect of separating the potentiometer into two parts – an upper part and a lower part – both of which have different resistances

Moving the slider (the arrow in the diagram) changes the resistances (and hence potential differences) of the upper and lower parts of the potentiometer

• If the slider in the above diagram is moved upwards, the resistance of the lower part will increase and so the potential difference across it will also increase

### Relay

• A relay consists of two parts:
• A coil (electromagnet)
• A magnetic switch

When there is a current in the coil, it attracts the switch which completed the right-hand part of the circuit

• When there is a current in the coil, it creates a magnetic field which attracts the switch, closing it
• This can be used to control a switch in a separate circuit, as shown in the above diagram

### Thermistors & LDRs

The symbol for a thermistor

• A thermistor is a type of resistor (a thermal resistor) which changes its resistance when it is heated up:
• As a thermistor gets hotter, its resistance decreases
• As it gets colder, its resistance increases

The resistance through a thermistor is dependent on the temperature of it

LDRs

The resistance through an LDR is dependent on the intensity of light falling on it

• An LDR (Light Dependent Resistor) is a type of resistor which changes its resistance when light is shone on it:
• When more light is shone on an LDR its resistance decreases
• If an LDR is covered up (reducing the light hitting it) its resistance increases.
• Note: The symbol for an LDR is often drawn with a circle going around it

Diagram showing an LDR circuit

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### Sensing Circuits

• Thermistors (or LDRs) may be used to create heat (or light) sensitive circuits, which perform certain actions when the temperature (or light) reaches a certain level

LDRs can be used in circuits where detecting changes in light intensity are necessary (such as in street lamps)

• In the above circuit, an LDR is used, along with a relay, to control a bulb
• When it gets darker, the resistance of the LDR increases
• This causes the potential difference across the LDR to increase
• When this potential difference increases the current in the coil (part of the relay) will increase
• When the current gets large enough the coil will attract the switch, closing it and allowing a current to pass through the right-hand side of the circuit

• Temperature controlled circuits may be created by replacing the LDR with a thermistor, and replacing the LDR with a suitable component such as a buzzer or heater

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### Diode Action

A diode is a component which only allows a current when the potential difference is in the direction of the arrow

• A diode is a component which only allows charge to flow in one direction through it
• If a power source is connected back to front then there will be no current

In order to have a current, the diode must point around the circuit from positive to negative

• If a diode is connected to an a.c. (alternating current) power supply, it will only allow a current half of the time
(This is called rectification)

A diode can be used to rectify an alternating current

### Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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