Transcript Current electricity

Current Electricity
Electrical Circuits
Waterfalls and Circuits:
More in common than you think
Electrical Circuits can be thought of as a waterfall
Voltage (V)
Height of Waterfall
Measured in volts (V)
Current (I)
Amount of water falling at one point at any moment
Measured in amperes (A)
Resistance (R)
Constriction of water
Measured in ohms ()
Simple Electric Circuit
Voltage (V)
Battery of the system
Measured in volts
Current (I)
Component using Electricity
Measured in amperes
Resistance (R)
Device inhibiting the flow of
electrons
Measured in ohms
Current
Current is a flow of charge (the charge is
usually electrons) flows from positive to
negative
The unit of current is the amp (the symbol for the
amp is A).
The symbol for current is I.
Current is measured with an ammeter or
multimeter.
For current to flow we need two
things:
1. There has to be a complete circuit.
2. There has to be a source of potential
difference (power supplies and batteries
both act as a source of potential difference).
Voltage
Potential difference
(commonly called ‘voltage’)
Current will flow between two points if there is a
potential difference between the two points.
This is a bit like saying that water will flow between
two points if there is a height difference between the
two points.
In an electric circuit current flows from the positive
end of the battery to the negative end.
The positive end is represented with a long solid line,
and the negative end is represented with a short
solid line
Another way of thinking about
potential difference is that it provides
the ‘push’ to move the electrons
around a circuit.
The unit of potential difference is the volt (the
symbol for the volt is V)
The symbol for potential difference is V.
Potential difference is measured with a voltmeter
or multimeter.
Resistance
Resistance opposes the movement of
electrons around a circuit.
The unit of resistance is the ohm (the symbol for
the ohm is Ω).
The symbol for resistance is R.
Resistance is measured with an ohmmeter or
mulitmeter.
Summary
Quantity Symbol
Current
I
Unit Symbol Measured Sym
with
bol
Amps
A
Ammeter
Potential
difference
V
Volts
V
Voltmeter
Resistance
R
Ohms
Ω
Ohmmete
r
Making circuits
• Requires a full complete “circuit”
• Requires a potential difference (Voltage)
• Requires a resister to reduce the flow of
current in a circuit
• Remember current flows from + to –
• Always double check circuit before turning
circuit on.
Electrical Circuits
An electric current is a flow of electric charge
Conductors are substances which allow
current to flow through them freely
Conductors conduct electrical current very
easily because of their free electrons. Most
metals are good conductors of electrical
current.
Electrical Circuits
Insulators are substances which do not allow
electrical current to flow through them.
Exp: To test electrical conduction in a variety of
materials, and classify each material as a
conductor or insulator.
Method:
1.
Set up the apparatus as shown in the
diagram.
2.
Place the substance you want to test
between the clips and see if the bulb lights.
3.
Repeat with different substances and
draw up a table of conductors and insulators.
Result:
Conductor
Insulator
Conclusion:
If the bulb lights the material is a
conductor. If the material does not light the
material is an insulator. Some materials are
better conductors that others.
How do we test for V, I, and R
in a Simple Circuit?
Device:
Digital Multimeter
Mathematically:
Ohm’s Law
Very Important Rule
Ohm’s Law
V = Voltage
I = Current
R = Resistance
To solve for V
V=I•R
To solve for I
I=V/R
To solve for R
R=V/I
Exp: Set up a simple circuit, use appropriate instruments
to measure current, potential difference (voltage) and
resistance, and establish the relationship between them.
Method:
1.
Set up the apparatus as shown in the diagram.
2. Read the current from the ammeter.
3. Read the voltage from the voltmeter.
4. Now connect an ohmmeter from A to B and
read the resistance.
5. Adjust the variable resistor to give a slightly
larger current, note voltage and resistance
now.
6. Repeat a number of times.
Result:
Current
Voltage
Conclusion: Current = Voltage
Resistance
This is know as Ohm’s Law.
Resistance
Current
Voltage
Resistance
OP 50
Check Your Understanding
For each of the following pictures, determine what the
following values are for when the light bulb is lit:
Voltage
18 V
18
R=
=6
3A
Current
3
Resistance
#1
http://jersey.uoregon.edu/vlab/Voltage/
#2
36
R=
=6
3
#3
54
R=
= 18 
3
#4
72
R=
= 24 
3
1.
A resistor has a voltage across it of 12 volts and a
current through it of 2.5 amperes.
Calculate the resistance of the resistor.
resistance =
voltage
current
12
=
2.5
= 4.8 
V
I R
2. A voltage of 6 volts is across a resistor where the
current is 0.5 amperes.
What is the value of the resistor?
resistance =
voltage
current
6
=
0.5
= 12 
V
I R
3. Calculate the resistance of a component when a
voltage of 24 V causes a current of 0.1 amperes.
resistance =
voltage
current
24
=
0.1
= 240 
V
I R
4. If a current of 2 amperes exists through a lamp when
it has 12 volts across it, what is the resistance of the
lamp?
resistance =
voltage
current
12
=
2
= 6
V
I R
5. A torch bulb is marked "6 V, 0.25 A".
Calculate the resistance of the bulb.
resistance =
voltage
current
6
=
0.25
= 24 
V
I R
6. A resistor is placed in a circuit. The voltage across it
and current through it are measured as 12 volts and
0.02 amperes.
The resistor is now removed from the circuit and an
ohmmeter connected across it.
What is the reading on the ohmmeter?
resistance =
voltage
current
12
=
0.02
= 600 
V
I R
7.
Find the values of the resistors in the circuit
below.
2V
3V
5V
V
V
V
R1
R2
R3
0.25 A
V
2
R1 =
=
= 8
I
0.25
R2
R3
V
3
=
=
= 12 
I
0.25
V
5
=
=
= 20 
I
0.25
8. A resistor of resistance 50  is placed in a circuit.
The voltage across it is measured as 12 volts.
What is the current in the resistor?
current =
voltage
resistance
12
=
50
= 0.24 A
V
I R
9. A 15  resistor is connected to a 9.0 V battery.
What is the current in the resistor?
current =
voltage
resistance
9
=
15
= 0.60 A
V
I R
10. A 25  resistor is connected to a battery.
The current in the resistor is 0.25 A.
What is the voltage of the battery?
voltage = current x resistance
= 0.25 x 25
= 6.25 V
V
I R
11. A 480  resistor is connected to a battery.
The current in the resistor is 26 mA.
What is the voltage of the battery?
voltage = current x resistance
= 0.026 x 480
= 12.48 V
V
I R
Electric Circuits
• A circuit is a path where a current can flow
• If the flow is to be continuous, there can be
no gaps in the path
• A closed circuit is required for a current to
flow
• If the circuit is broken it is called an open
circuit
A circuit is the path that is
made for an electric current.
Electric Circuits
• There are two ways to connect multiple
devices to a voltage source
• One is called series
• The other is called parallel
• Each has unique properties which we now
examine
Series Circuit
A circuit that only has one path for
current to flow through is called a series
circuit.
Parallel Circuits
A type of circuit that has more than one path for
current is called a parallel circuit.
If one part of the path is
removed, the current continues
to flow through the other paths
of the circuit.
Series Circuits
RTotal = R1 + R2 + R3
Series Circuits
- A single pathway through the circuit
- The current is the same everywhere in the
circuit
- Each device provides resistance and total
resistance is the sum of the devices
- Voltage divides among the devices
Series Circuit Calculation
10 ohm
20 ohm
30 ohm
RTotal = R1 + R2 + R3
12 Volt
Parallel Circuits
Parallel Circuits
- Each device connects to the voltage source
- Voltage is the same across each device
- Current from source divides into devices
- Total current is the sum of device currents
- Current in each device is just V/R
- Add devices, lower total resistance
In a series circuit:
RTotal = R1 + R2 + R3
In a parallel circuit
Electric Power
- Moving charges do work
- We can heat the filament in a light bulb
- We can turn the rotor in a motor
- The rate at which work is done is power
- Electric Power = current x voltage
- Units are watts = joules/sec = amps x volts
Electric Power
Electric Power = current × voltage
P=I×V
Power Calculation
10 ohm
20 ohm
30 ohm
RTotal = R1 + R2 + R3
= 10 + 20 + 30
= 60 Ω
12 Volt
voltage
current =
resistance
12
=
60
= 0.20 A
V
I R
Find the power of the circuit
Current = 0.5A
Resistance = 50 Ohms
Alternating Current (a.c.) and Direct Current (d.c.)
Direct current is current which
flows in one direction only;
alternating current is current
which constantly changes
direction.
Mains Electricity
Electricity which comes through the sockets
in your house is referred to as ‘mains’
electricity’.
It changes direction 50 times per second and
so is called alternating current (a.c.).
‘Mains’ voltage is 230 volts
Fuses
- Limit the current that runs through wires in
your house
- These wires have some resistance
- Energy loss converts to heat
- Hot wires can start a fire
- Limit the current with a fuse or circuit breaker
A fuse is a deliberate weak link
in a circuit which will break
(melt) if the current exceeds a
preset value
•The neutral wire is blue.
The earth wire is yellow or green.
The live wire is brown or red.
The fuse is on the live wire.
In an electric circuit current flows
from the positive end of the
battery to the negative end.
The positive end is represented
with a long solid line, and the
negative end is represented with a
short solid line
Diode
A diode is a device that allows current to
flow in one direction only
The arrow indicates the direction in which
current can flow.
Image
Symbol
Used to change AC current to DC
current in circuits
Also to protect circuits in DC from batteries
placed incorrectly
Simple series circuits using diodes
In the diagram on the right, current would
normally flow in an anti-clockwise direction
(from the positive end to the negative end).
In this case however current will only flow
through part A, and not through part B
because the diode in part B is pointing in
the wrong direction.
Light Emitting Diodes (LEDs)
LEDs are similar to diodes but emit light
when current passes through them
They are very important in electronics
because they use very little electricity.
Image
Symbol
Simple series circuits using
LEDs
Note that in both circuits the current flows in
an anti-clockwise direction (can you
remember why?).
However the LED in the second circuit is
turned the wrong way around so no current
will flow in the second circuit and no light
will be emitted.
Light Dependent Resistors
(LDRs)
A LDR is a resistor whose resistance
decreases with increasing light intensity
Component
Symbol
To measure the resistance of a
LDR under varying degrees of
brightness
Connect the LDR to an ohmmeter or to a
multimeter set to read resistance and slowly
shield the LDR from light.
Notice that the resistance increases
Applications of the diode
Almost all electronic appliances have diodes
inside them which help turn alternating
current into direct current.
Applications of the LED
LEDs used to be used primarily as indicators
in electronic circuitry (e.g. as standby
indicators in televisions, radios etc) but
modern diodes can give out a lot of light
efficiently and so are now used in many
designs of flashlights.
Applications of the LDR
Textbooks often mention that LDRs are used
to switch on street lights when it gets dark,
yet this can seem confusing because the
resistance of the LDR is high when it is
dark so how can this be responsible for
current flowing through the street-light?
What actually happens is that this reduced
current is detected by a second circuit which
in turn uses that information to turn on the
1. What is the function of a diode in a circuit?
2. What is the function of a LED (Light Emitting
Diode)?
1. What do the letters LDR stand for?
1. Draw the symbol for (i) ldr, (ii) led
1. Give one everyday use of (i) an led, (ii) an ldr.
Electronics at home
Cost of electricity - the
kilowatt-hour
The kilowatt-hour is the unit electrical energy
used by electricity suppliers.
The photograph shows a kWh (kilowatt-hour)
meter.
This meter is connected into the electricity
consumer’s domestic circuit and it can
measure energy consumption in a selected
part of the circuit, the total energy used and
cost it.
The unit of electrical energy
used by electricity supply
companies is the kilowatt-hour
The number of kilowatt-hours
equals the number of kilowatts
multiplied by the number of
hours
kWh = kW × hours
Example
The ESB charges for electricity at a rate of 11
cent per kW h.
A hair-drier of power rating 1.5 kW is used
for 20 minutes each day.
(i) How many units of electricity are used?
(ii) What is the cost, in cent, of using the hairdrier for six days?
The power rating of various
appliances
Appliance
Washing machine
Microwave
Light-bulb
Lamp
Hair-dryer
Kettle
Television
Power (in watts)
2500
700
40
60
3000
600
kilowatts (kW)
2.5
.9
.04
.06
3
.6
Effects of an electric current
There are three effects of an electric current; a
heating effect, a magnetic effect and a
chemical effect
Effect
Demonstration
Everyday application
Heating
Effect
An electric current will cause a
light-bulb to heat up and emit
light
Electric kettle, electric fire
etc.
Magnetic An electric current will deflect a
Effect
magnetic compass
Electromagnets
Chemical
Electrolysis occurs when an
Effect
electric current splits water into
hydrogen and oxygen
Electroplating