Real circuits can be very complex and it can be difficult to understand their layout, this is why a circuit diagram is used, rather than a drawing of the actual circuit.
Circuit diagrams use standard symbols to represent components and use simple rules so that everyone draws them in a similar and easy to understand design.
- Wires are draw in straight lines with a ruler that meet, join and turn at right angles only.
- Place the power supply, cell, battery or LVS at the top middle of the diagram.
- Use the standard symbols only.
__Symbols __(these are all required for the exam)
Cell: A single ‘battery’ as they are often called is correctly called a cell.
A Battery: Where two or more cells are used together this is called a battery.
Switches: Used to make or break circuits. The top switch is the sort used for turning on or off a light, the second is the sort that might be used for a doorbell.
Ammeter: Placed in series in a circuit to measure the current in amps.
Voltmeter: Place in parallel in a circuit to measure the voltage in Volts.
Resistor: Used to reduce the current in parts of a circuit. The size of the resistance is normally written in the middle of the symbol in Ohms.
Variable Resistor: Placed in a circuit to allow the current to be changed. These are used as dimmer switches for lights and volume controls on audio equipment.
Thermistor: Similar to a variable resistor, it can increase or decrease the current in a circuit, but it does so in response to a change in temperature. More current flows as the temperature increases.
Light Dependant Resistor (LDR): Acts like a thermistor but in response to light. More current flows as the light intensity increases.
Lamp or Bulb
Diode: A device that only allows current to flow in one direction, indicated by the direction of the arrow.
Light Emitting Diode (LED): A diode that glows and emits light as current passes through it. They are very efficient at transferring electrical energy to light.
Simple Circuit designs
This circuit contains a battery of two cells connected to a ammeter and 2 bulbs in series. A voltmeter is connected in parallel to measure the voltage across the second bulb.
A series circuit is one in which the components follow one another in a line, the current must flow through each component in turn. The current in a series circuit is the same in all part of the circuit.
In this simple series circuit the ammeter will give the same reading no matter where it placed in the circuit.
Series circuits are easy to construct, but because the current has to flow through all part of the circuit if one component fails the circuit will be broken and stop working.
There are no branches in a series circuit.
Parallel circuits have branch that run alongside each other. The current in each branch of the circuit is independent of the current in the other branches. Each parts acts as it own series circuit.
Parallel circuits can be more complicated to assemble, but have the advantage that if a component fails on one branch it will not affect the other branches.
Series Circuits for Measuring
Simple series circuits, such as the one below, are useful in making simple measurements of the current and the voltages in different parts of a circuit, from these other quantities can be calculated, such as Energy, Charge and Resistance.
The position of the ammeter is unimportant as the current will be the same in all parts of the circuit. The voltmeter can be placed in parallel with any one component or with a number of components to measure the voltage or potential difference across those components.
In this example the current is being measured, and so is the voltage of one of the bulbs. To measure the voltage of the other bulb the contacts would have to be placed either side of that bulb. Remember voltmeters compare potential in two places and report the difference, hence they must be in parallel with the component(s).
- State the differences and similarities of a thermistor and a LDR.
- Your answer should include: variable resistors / temperature changes
Explanation: They are both types of variable resistors that respond to environmental conditions. The thermistors responds to temperature changes and the LDR to light.