Circuits
Ammeters and current Circuit Symbols Switches
Whenever an electric current flows when we must have a complete circuit.
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The components in a circuit are represented by symbols
| The battery (or cells) push the electricity around the circuit. |
 Symbol for a cell |
 Battery |
The wires are made of conductors , usually copper, covered with plastic insulation.
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symbols for a light bulb |
A light bulb has a thin metal filament, often made out of tungsten, which glows very hot when an electric current flows through it. The higher the current, the hotter the filament gets until it MELTS. |
| When two lamps are wired in SERIES they will be DIMMER than a single lamp. This is because two lamps in series have more resistance than a single lamp so the current flowing through them is less. When one lamp is unscrewed the other goes out. |
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| When two lamps are wired in PARALLEL they both stay BRIGHT.
More current will flow through the cell so it will run down more quickly. The electricity can divide and when one lamp is unscrewed the other lamp stays bright. |
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| The electricity always travels more down the easier route.
In a short circuit ALL the electricity goes down the wire and the lamps go OUT. A short circuit can be quite dangerous as a high current can flow and this often makes the battery run out quickly. |
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Ammeters and current : |
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| How to Measure current: Use an instrument called an AMMETERUnit for Current: Current is measured in AMPS | Symbol for an ammeter: |  |
An ammeter must be placed in SERIES with the component being tested . Current always flows away from the positive terminal.
Note:
Brighter bulb = more current flowing
Dimmer bulb = less current flowing
(this assumes that the bulbs are all the same as each other)
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Typical value: A=0.2amp Note: the current at every part of a series circuit will be the same. The bulb does NOT use up any current it just makes it less. The arrows show the direction of the current |
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Typical value: A=0.1 amp This value will always be LESS than circuit I because two lamps in series have more resistance than a single lamp and so less current will flow |
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Typical values:
A1=0.4 amp Note: Current through A1= Current through A2+ current through A3 |
The three effects of an electric current
An electric current has three main effects:
ii. Magnetic effect
iii. Chemical effect
Heating effect
An electric current can make a wire get hot. A higher current makes the wire hotter.
Sometimes this is useful e.g. in the element of an electric fire or the filament of a light bulb.
Sometimes it is a disadvantage e.g. when an electric motor gets too hot and breaks or a wire over-heats in a house and starts a fire.
A fuse
A fuse is a short length of copper wire designed to melt when more than a certain amount of current flows through it.
The thin piece of wire is usually inside a china case to make it safer and easier to handle.
What a fuse is for:
When the fuse melts it breaks the circuit and stops the current flowing.
This helps stops a wire from melting or burning someone.
Magnetic effect
An electric current flowing through a wire creates a magnetic field (see magnetism)
An electromagnet can be made by wrapping a coil of wire around a soft iron rod.
The strength of the electromagnet will depend on three things:
- The number of turns of wire on the coil
- The strength of the current
- The material that the rod (core) is made from
| Experiment:An electromagnet can be made by wrapping a length of insulated wire around a rod made of soft iron.
We can measure the strength of the electromagnet by seeing how many paper-clips it can pick up. We found that we could increase the strength of our magnet two ways: |
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Uses for the magnetic effect of a current:
i. an electro-magnet to separate iron from copper
ii. An electric motor
LINK: Electricity This link explains how electricity is made and also how dynamos and motors work
Chemical effect
An electric current can cause a compound in solution to decompose
Rule for current at a junction: :
The total current entering the junction must be the same value as the total current leaving the junction.
| Problem: The diagram shows a junction where 3 wires join together. You know the current at X = 2A and the current at Z=1.7A What is the current at Y. |
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| Solution Current X = current Y + current Z Current at Y = 2 – 1.7 Current at Y = 0.3 amps (ans) |
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Cells in series
| The voltage of each cell are added together | |
Example
 Total voltage = 1.5v |
 Total voltage = 3v
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The Life of a battery depends on the current flowing through it. depends on the current flowing through it.
More current = shorter life.
Less current = longer life.
The cell in circuit 1 (above) will last longer than the cells in circuit 2 because more current is flowing in circuit 2
In circuit 1 (above) the cell will last longer than the ones in circuit 2. This because more current is flowing. (note the brighter bulb)
| If two cells are facing in opposite directions then no current will flow and the bulb will be out. |  |
Cells in parallel
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The voltage is the same as it would be for one cell, but the cells:
i. would take longer to run out ii.would be able to supply more current if they need to (powering several bulbs in parallel for example) |
Other components
| The diode | Symbol: |
A diode only allows a current to flow in one direction.
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| Look at the circuit shown:Three bulbs (P,Q and R) are connected to a battery of cells.
There are also 4 ammeters to measure the current in various parts of the circuit You are told that A1 reads 0.3 amps |
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| Q1 what is the reading on ammeter A4? Answer
Q2 What are the readings on A2 and A3? Answer Q3 If bulb P lights with ‘normal’ brightness , what will be the Q4 Suppose bulb P is now unscrewed taken out of its holder? |
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