Download electrical circuits

ELECTRICAL CIRCUITS
S.MORRIS 2006
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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 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.
cell
wires
switch
lamp
To make the circuit, these components are connected
together with metal connecting wires. Wires are
conductors since they conduct electricity.
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
Scientists usually draw electric circuits using symbols;
cell
lamp
switch
wires
circuit diagrams
In circuit diagrams components are represented by
the following symbols;
cell
ammeter
battery
voltmeter
switch
motor
lamp
buzzer
resistor
variable
resistor
types of circuit
There are two types of electrical circuits;
SERIES CIRCUITS
PARALLEL CIRCUITS
SERIES CIRCUITS
The components are connected end-to-end, one
after the other.
They make a simple loop for the current to flow
round.
If one bulb ‘blows’ it breaks the whole circuit and
all the bulbs go out.
PARALLEL CIRCUITS
The components are connected side by side.
The current has a choice of routes.
If one bulb ‘blows’ there is still be a complete circuit to
the other bulb so it stays alight.
Opened and Closed Circuits
• If a switch in a circuit is closed and allows
for the flow of electricity, it’s called a
“closed circuit”
• If you open the switch and stops the flow
of electricity, it’s called an “open circuit”
measuring current
Electric current is measured in amps (A) using
an ammeter connected in series in the circuit.
A
measuring current
This is how we draw an ammeter in a circuit.
A
A
SERIES CIRCUIT
PARALLEL CIRCUIT
measuring current
SERIES CIRCUIT
• current is the same
at all points in the
circuit.
2A
2A
2A
PARALLEL CIRCUIT
• current is shared
between the
components
2A
2A
1A
1A
copy the following circuits and fill in the
missing ammeter readings.
3A
?
4A
?
3A
1A
?
4A
?
4A
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
measuring voltage
Different cells produce different voltages. The
bigger the voltage supplied by the cell, the bigger the
current.
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
This is how we draw a voltmeter in a circuit.
V
SERIES CIRCUIT
V
PARALLEL CIRCUIT
measuring voltage
V
V
V
V
series circuit
• voltage is shared between the components
3V
1.5V
1.5V
parallel circuit
• voltage is the same in all parts of the circuit.
3V
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
a)
6V
4A
A
V
V
A
measuring current & voltage
b)
4A
6V
A
V
A
V
A
answers
a)
b)
4A
6V
6V
4A
6V
4A
4A
3V
2A
3V
4A
6V
2A
Resistors
• Resistors “resist” the flow of electrons in the
circuit. Electrons strike a lot of particles and
emit as heat. When electronics get warm, it’s
the resistance in the circuit heating up.
• Without them, the current would flow
uncontrolled and short the circuit.
• As resistance gets bigger, current gets smaller.
You can control current this way.
Symbol
Measuring Resistance
• The unit for a resistor is called an ohm
• For example: A wire with 1 ohm of resistance powered
by 1V of pressure, will push 1 amp of current through the
circuit.
• Ohmmeters are used to measure resistance
• Galvanometers measures very small currents
1V
1 ohm
1 Amp
Ohm’s Law
• Georg Ohm discovered the relationship
between voltage, current, and resistance
Down
• As resistance goes up, current goes ____
Up
• As voltage goes up, current goes ___
Up
• As resistance goes down, current goes __
Down
• As voltage goes down, current goes ____
Calculating Resistance
• If a car battery at 12V supplying 1 amp of
current into a car light, what is the
resistance of the light?
V =Ix R
___
12 V = 12 ohms
1A
Calculating Current
• If you stack two D-cell batteries together,
you get (1.5V+1.5V = 3V). How much
current is going to flow through a 10 ohm
wire?
V =Ix R
___
3V
= 0.3 Amps
10 ohms
Power
• Power (heat) in circuits is measured in Watts.
Power = Voltage x Current
P=VxI
• For example, 100V x 1 Amp = 100 Watts
• What’s the current of a 100W light bulb?
P = VxI
100W/120V = 0.83 ohms
Energy
• Energy is Watts over time
Energy = Power (kW) x Time (hours)
E=PxT
• For example, a 1000W hair dryer running for 1 hour uses
1 kilowatt hour.
• How many kWh will 10 100W light bulbs burn in 30
minutes?
Turn off those lights!
100W * 10 bulbs = 1000W
E = 1kW * 0.5 hours
E = 0.5kW hours