Ammeters and current    Circuit Symbols     Switches

Whenever an electric current flows when we must have a complete circuit.

complete_circuit
incomplete_circuit

The components in a circuit are represented by symbols

 

The battery (or cells) push the electricity around the circuit.

cell

Symbol for a cell

Battery

Battery

The wires are made of conductors , usually copper, covered with plastic insulation.

 

lamp lamp2

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.

 


Lamps in Series
 (end to end)

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.

image016


Lamps in Parallel

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.

image014


A Short Circuit

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.

short_circuit

Ammeters and current
:
How to Measure current:   Use an instrument called an AMMETERUnit for Current:    Current is measured in AMPS Symbol for an ammeter:    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)

 ammeter 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
ammeter_series
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
ammeter_parallel Typical values:

A1=0.4 amp
A2=0.2 amp
A3= 0.2amp

Note: Current through A1= Current through A2+ current through A3

 

The three effects of an electric current

An electric current has three main effects:

i.    Heating effect

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:
 i.  by adding more turns to the coil
ii. by increasing the voltage. This caused the current to go up.

Warning: After about 4 amps the wire will get very hot.

coil

 

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.
junction1
Solution
Current X = current Y + current Z
Current at Y = 2 – 1.7   Current at Y = 0.3 amps  (ans)

 Cells in series

The voltage of each cell are added together cells


Example

circuit1

Total voltage = 1.5v

 

circuit2

Total voltage = 3v

 

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. opposite_cells

Cells in parallel

cells_papallel 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:
diode

  A diode only allows a current to flow in one direction.

diode_on
diode_off

Problem

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

ammeters
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?
a) what will happen to bulb Q?  
b) what will happen to ammeter A1?  

0.3A
A2 = 0.2A, A3=0.1A
Q is normal, R is bright
Bulb Q will GO OUT
A1 would GET LESS (0.2A)