Bài giảng Electrical circuits

S.MORRIS 2006 
ELECTRICAL CIRCUITS 
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The CELL 
The cell stores chemical energy and transfers it to electrical energy when a circuit is connected. 
When two or more cells are connected together we call this a Battery . 
The cells chemical energy is used up pushing a current round a circuit. 
What is an electric current? 
An electric current is a flow of microscopic particles called electrons flowing through wires and components. 
+ 
- 
In which direction does the current flow? 
from the Negative terminal to the Positive terminal of a cell. 
simple circuits 
Here is a simple electric circuit. It has a cell , a lamp and a switch . 
To make the circuit, these components are connected together with metal connecting wires. 
cell 
lamp 
switch 
wires 
simple circuits 
When the switch is closed, the lamp lights up . This is because there is a continuous path of metal for the electric current to flow around. 
If there were any breaks in the circuit, the current could not flow. 
circuit diagram 
cell 
switch 
lamp 
wires 
Scientists usually draw electric circuits using symbols; 
circuit diagrams 
In circuit diagrams components are represented by the following symbols; 
cell 
battery 
switch 
lamp 
motor 
ammeter 
voltmeter 
buzzer 
resistor 
variable resistor 
types of circuit 
There are two types of electrical circuits; 
SERIES CIRCUITS 
PARALLEL CIRCUITS 
The components are connected end-to-end, one after the other. 
They make a simple loop for the current to flow round. 
SERIES CIRCUITS 
If one bulb ‘blows’ it breaks the whole circuit and all the bulbs go out. 
PARALLEL CIRCUITS 
The current has a choice of routes. 
The components are connected side by side. 
If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it stays alight. 
measuring current 
Electric current is measured in amps (A) using an ammeter connected in series in the circuit. 
A 
measuring current 
A 
A 
This is how we draw an ammeter in a circuit. 
SERIES CIRCUIT 
PARALLEL CIRCUIT 
measuring current 
SERIES CIRCUIT 
PARALLEL CIRCUIT 
 current is the same 
 at all points in the 
 circuit. 
2A 
2A 
2A 
 current is shared 
 between the 
 components 
2A 
2A 
1A 
1A 
copy the following circuits and fill in the missing ammeter readings. 
? 
? 
4A 
4A 
4A 
3A 
? 
? 
1A 
? 
3A 
1A 
1A 
measuring voltage 
The ‘electrical push’ which the cell gives to the current is called the voltage . It is measured in volts (V) on a voltmeter 
V 
Different cells produce different voltages. The bigger the voltage supplied by the cell, the bigger the current. 
measuring voltage 
Unlike an ammeter a voltmeter is connected across the components 
Scientist usually use the term Potential Difference (pd) when they talk about voltage. 
measuring voltage 
V 
This is how we draw a voltmeter in a circuit. 
SERIES CIRCUIT 
PARALLEL CIRCUIT 
V 
V 
measuring voltage 
V 
V 
V 
series circuit 
1.5V 
 voltage is shared between the components 
1.5V 
3V 
 voltage is the same in all parts of the circuit. 
3V 
parallel circuit 
3V 
3V 
measuring current & voltage 
copy the following circuits on the next two slides. 
complete the missing current and voltage readings. 
remember the rules for current and voltage in series and parallel circuits. 
measuring current & voltage 
V 
V 
6V 
4A 
A 
A 
a) 
measuring current & voltage 
V 
V 
6V 
4A 
A 
A 
A 
b) 
answers 
3V 
3V 
6V 
4A 
4A 
6V 
6V 
6V 
4A 
4A 
2A 
2A 
4A 
a) 
b)