Transcript Nature of Electricity

Nature of Electricity
Nature of Current Electricity
To make things work
Need source
•1.5 V cell has a limited life ( use it then throw it away)
•240 Hydro Electric Cell virtually limitless
Nature of Current Electricity
To make things work
Need Thing
A device that converts energy into a different form of
energy
Heat, light, sound, motion, etc
Nature of Current Electricity
To make things work
Need Switch
 Interrupts the circuit
Wire
Carries the electrons to and from the thing
Why do we need two wires?
• the motion of electrons from high potential energy to low
potential energy =
• One wire carries electrons to the globe or CD player
from the source, the other returns them to the source
whereupon they are boosted back up to continue their
circulation
Law of Repulsion and Attraction
 Like charges repel and unlike charges attract.
 Negative electrons are pushed away from the negative terminal
and are attracted to the positive terminal
In a battery
how do
charges
flow?
Measuring Current
Current is represented by the symbol (I)
Current is measured in Amps
Current is measured using an Ammeter
which measures the number of electrons
passing through a circuit at a given point
Current
• One amp is equal to the flow of one
coulomb* of charge per second.
COPY
DIAGRAMS
1. A current of 2 A flows for 30 seconds
through a lamp.
How much charge has moved?
2. A current of 12 A flows for 20 minutes into
an electric cooker.
How much charge has the cooker used?
3. If 18400 C of charge flows through an air
conditioning unit every hour, what current
does it draw?
4. A current of 3 A flows into a television set.
For how long would it take 1500 C of charge
to flow through it?
What is VOLTAGE?
In order for current to flow something has
to make it flow =
Voltage
Voltage measures the energy available to
drive the flow of current.
Provided by an energy source like a battery
Measured in volts (by a voltmeter)
Potential Difference
The Voltage available to drive a flow of
current is called Potential Difference
There is a direct relationship between voltage and
current.
We can demonstrate this with the circuit below
which connects a cell and a lamp.
The cell has an output of 2 volts.
The lamp glows moderately.
If we replace the 2-volt cell with a 1-volt cell, the
lamp will glow dimly.
This is because a 1-volt cell provides half as much
energy as a 2-volt cell, so the lamp converts less
energy into heat and light.
But how was the current affected?
To find out how the current was affected we repeat
the experiment, adding an ammeter.
As before, the cell has an output of 2 volts and the
lamp glows moderately.
But this time we know that the current is 1 amp.
As before, we replace the 2-volt cell with a 1-volt cell and the
lamp glows dimly.
But what does the ammeter read?
The ammeter reads 0·5 amps.
The current is halved because a 1-volt cell provides
half as much energy as a 2-volt cell.
This time we connect a cell with an
output of 4 volts.
The ammeter reads 2 amps and
the lamp glows brilliantly.
Now we replace the 4-volt cell with a 2-volt
cell and the lamp glows moderately.
The ammeter reads 1 amp.
The current is halved because a 2-volt cell
provides half as much energy as a 4-volt
cell.
Voltage & Current Summary
Voltage is the energy that drives the flow of current.

Voltage is measured in volts with a voltmeter:


Voltage is represented by the symbol, V.

One volt is the energy required to drive a current of one
amp through a circuit with a resistance of one ohm.

An increase in voltage means an increase in energy and
therefore an increase in current.

A decrease in voltage means a decrease in energy and
therefore a decrease in current.
Resistance & Resistors
Resistance & Resistors
When walking, we don't normally notice
we are moving through air as it offers such
little resistance to our motion.
But compare moving through air with
moving through water which offers
significant resistance to our motion.
It's the same for a current moving through
a conductor...
Conductors
All conductors offer resistance to the flow of
current. This resistance is determined by the
conductor's atoms.
For example:
Silver and copper atoms offer negligible
resistance to an electric current because a
significant proportion of their electrons are free
to move from atom to atom.
Silver and copper, having negligible resistance,
are commonly in used as conductors.
• On the other hand...
Insulators
Glass and Plastic offer very considerable
resistance to an electric current because a
significant proportion of their electrons are not
free to move from atom to atom.
Glass and synthetic polymers, having very
considerable resistance, are commonly used as
insulators.
A conductor's length, temperature, and crosssectional area also affects its resistance.
Ohm's Law
In Voltage & Current we explained the
relationship between voltage and current.
In Resistance & Resistors we explained
the relationship between resistance and
current.
Here we explain the relationship between
all three.
The relationship between voltage,
resistance and current is expressed in
Ohm's Law which is named after the
physicist who discovered it.
Ohm's Law most simply translates to the
equation, V = IR, or:
Voltage = Current × Resistance
This is all, however, at a constant
temperature
• We can demonstrate the relationship
between voltage, current and resistance in
the circuit below which connects a cell, an
ammeter and a resistor.
• The cell provides a voltage of 1 volt.
The resistor has a value of 1 ohm.
The ammeter reads a current of 1 amp.
• If we replace the 1 ohm resistor with a 2
ohm resistor, what will the ammeter read?
• A 2 ohm resistor is twice as resistant as a 1 ohm
resistor, so it is twice as hard for the current to flow. The
resistor allows half of the 1 amp current to pass.
• By Ohm's Law:
• V = IR
• Solving for I:
•
• Now replacing the symbols with the values given:
•
• 0.5 = I
• I = 0.5 amps
Summary
The relationship between voltage, resistance
and current is expressed in Ohm's Law which
most simply translates to:
V = IR
or
Voltage = Current × Resistance
Which is true only at a constant temperature.