| Electric circuits |
Lamps in series
Lamps in parallel
Short circuit
Ammeters and
current Cells in parallel
Circuit symbols
The three effects of an
electric current Fuses
Switches (including a relay)
LED and LDR
Problems to solve
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. |  the symbol for a cell |
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The wires are made of conductors , usually copper, covered with plastic insulation.
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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. |
Lamps in Series (end to
end)
| When two lamps are wired in SERIES
they will be DIMMER than a single lamp.
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 root. 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 AMMETER Unit for Current: Current is measured in AMPS |
Symbol for an ammeter: |
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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 the 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: A1= A2+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:
| 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
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.
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Problem: 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 |
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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.
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. |
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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? Q2 What are the readings on A2 and A3? Q3 If bulb P lights with 'normal' brightness , what will be the brightness of Q and R (Dim, normal, bright or out) Q4 Suppose bulb P is now unscrewed taken out of its holder? |
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