Download Electric currents

Electric currents
Static Electricity
Static electricity is when charge “builds up” on an object and
then stays “static”. How the charge builds up depends on what
materials are used:
-
+
-
+
-
+
+
-
-
+
-
+
-
+
-
+
+
-
-
+
Static Electricity
+
+
-
-
+
-
-
-
-
-
-
-
Static shock
Measuring Charge
 The charge on an electron is very small, so we measure
charge using units called “coulombs” (C).
 One electron has a charge of 1.6 x 10-19 C.
 Charge can be measured using a coulombmeter, and they
usually measure in nanocoloumbs (1nC = 10-9 C).
 For example, a charged polythene rod may carry a charge of
a few hundred nanocoulombs
Electric Current
Electric current is a
flow of negatively
charged particles (i.e.
electrons). We call
them “charge
carriers”
+
e-
-
e-
Note that
electrons go
from negative
to positive
Define electric current.
It is sufficient for students to know that
current is defined in terms of the force
per unit length between parallel
current-carrying conductors.
Answers
Calculating Charge (Q)
By definition, current is the rate of flow of charge. In other words, its
how much charge flows per second. One amp (1 A) is equal to one coulomb
per second (1 Cs-1). Charge and current are related by the equation:
Current = rate of flow of charge
I = Δq
Δt
1. A battery supplies 10 C over a period of 50 seconds. What
is the current?
2. Another battery is connected for 2 minutes and provided a
current of 0.4 A. How much charge flowed?
3. A car battery has a capacity of 24 Ah (amp hours). If it
provides a current of 48A how long can it be used for?
How much charge (in coulombs) does it contain?
Conventional Current
As we said, technically electrons go from negative to positive.
However, we usually talk about “conventional current” and we
say that current moves from positive to negative:
+
-
How long will it take for the
electrons to complete 1 circuit?
Electron
Drift
What happens inside a conducting material?
model of a metal wire could help:
Electrons
The following
Ions
At normal temperatures, with no current flowing, electrons
hurtle around continuously. They collide with ions but because
their movement is random there is no net energy transfer.
Electron
Drift
Now apply a voltage:
Negative
Electrons
Ions
Positive
This time we can see that the electrons are accelerated from
negative to positive. This movement is superimposed on top of
the random velocities and is responsible for electrical effects.
What does an electric current look
like?
Download
Run Now!
Conductors…..
How many free electrons?
Hyperlink
Basic ideas…
Electric current is when electrons start to flow around a
circuit. We use an _________ to measure it and it is
measured in ____.
Potential difference (also called _______) is
how big the push on the electrons is. We use a
________ to measure it and it is measured in
______, a unit named after Volta.
Resistance is anything that resists an electric current. It is
measured in _____.
Words: volts, amps, ohms, voltage, ammeter, voltmeter
Voltage
Earlier on we said that current is when electrons move:
+
-
e-
“Voltage” is the energy that
allows the electrons to move. For
e
electrons to move there must be a “voltage difference”,
sometimes called a “potential difference” (p.d.). A higher p.d.
means a stronger push, which causes an increase in current.
Define electric potential difference
The potential difference between 2 points in
a circuit is……..
(1 volt = 1 joule per coulomb)
Resistance
Resistance is anything that will
RESIST a current. It is measured
in Ohms, a unit named after me.
Define resistance.
Students should be aware that R = V/I is a general
definition of resistance. It is not a statement of Ohm’s
law. Students should understand what is meant by
resistor.
Georg Simon Ohm
1789-1854
The resistance of a component can be
calculated using
Resistance
(in )
=
V
Voltage (in V)
Current (in A)
I
R
Resistance
Resistance is anything that opposes an electric current.
Resistance (Ohms, ) =
Potential Difference (volts, V)
Current (amps, A)
What is the resistance of the following:
1) A bulb with a voltage of 3V and a current of 1A.
2) A resistor with a voltage of 12V and a current of 3A
3) A diode with a voltage of 240V and a current of 40A
4) A thermistor with a current of 0.5A and a voltage of
10V
Resistivity
Apply the equation for resistance in
the form
where ρ is the resistivity of the
material of the resistor.
What effect does doubling the diameter have on the
resistance?
Compare ohmic and non-ohmic
behaviour.
For example, students should be able to draw
the I–V characteristics of an ohmic resistor and a
filament lamp.
Now determine the I-V graphs for
a resistor and a bulb.
A
V
Current-voltage graphs
Consider a resistor:
I
R
V
Current increases in
proportion to voltage
V
Resistance stays
constant
Current-voltage graphs
Now consider a bulb:
I
R
V
As voltage increases the
bulb gets hotter and
resistance increases
V
Resistance increases as
the bulb gets hotter
Current-voltage graphs
Now consider a diode:
I
Now consider a thermistor:
I
V
A diode only lets
current go in the
“forward”
direction
V
Resistance decreases as the
(“negative-temperaturecoefficient”) thermistor
gets hotter
LDR and Thermistor
How does their
resistance vary with
light and heat?
Two simple components:
1) Light dependant
resistor – resistance
DECREASES when light
intensity INCREASES
Resistance
2) Thermistor –
resistance DECREASES
when temperature
INCREASES
Resistance
Amount of light
Temperature
Current in a series circuit
If the current
here is 2
amps…
The
current
here will
be…
The current
here will
be…
And the
current
here will
be…
In other words, the current in a series
circuit is THE SAME at any point.
Kirchoff’s First Law
“The sum of the currents leaving a point is the
same as the sum of the currents entering that
point.”
Gustav Kirchoff
(1824-1887)
For example:
6A
If the current
through here is 4A...
…and the current
through here is 2A…
… then the
current here
will be 6A
What happens if you have a
choice?
Current in a parallel circuit
A PARALLEL circuit is one where the current has a “choice
of routes”
Here comes the current…
Half of the current
will go down here
(assuming the bulbs
are the same)…
And the rest will
go down here…
Current in a parallel circuit
If the
current
here is 6
amps
The current
here will be…
The current
here will be…
And the
current here
will be…
The current
here will be…
What is the current in each bulb?
3A
6A
More basic ideas…
If a battery is added
the current will
________ because
there is a greater
_____ on the electrons
so they move ______
If a bulb is added the
current will _______
because there is
greater ________ in
the circuit, so the
electrons move _____
Words – faster, decrease, slower, increase, push, resistance
Voltage in a series circuit
If the voltage
across the
battery is 6V…
V
…and these
bulbs are all
identical…
…what will the
voltage across
each bulb be?
V
V
2V
Voltage in a series circuit
If the voltage
across the
battery is 6V…
…what will the
voltage across
two bulbs be?
V
V
4V
Voltage in a parallel circuit
If the voltage across
the batteries is 4V…
What is the
voltage here?
4V
V
And here?
V
4V
In a SERIES circuit:
Summary
Current is THE SAME at any point
Voltage SPLITS UP over each component
In a PARALLEL circuit:
Current SPLITS UP down each “strand”
Voltage is THE SAME across each”strand”
An example question:
6V
A3
3A
A1
V1
A2
V2
V3
An example question:
Ammeter
reads 2A
A
V
Voltmeter
reads 10V
1) What is the resistance across
this bulb?
2) Assuming all the bulbs are the
same what is the total resistance
in this circuit?
Solve problems involving potential
difference, current and resistance.
10V
A3
3A
A1
V1
A2
V2
V3
An example
Calculate the missing values (from A-level June 2006)
6V
A
?
4Ω
?
?
R
A
?
V
15Ω
0.24A
A
More examples…
3A
6V
12V
3A
2A
4V
2V
1A
What is the
resistance of
these bulbs?
I
Resistors in Series
“In a series circuit current stays
the same but voltage splits up”
R1
V1
VT = V1 + V2
VT = IRT
VT
R2
V2
But V1 = IR1 and V2 = IR2
IRT = IR1 + IR2
R T = R1 + R2
Resistors in Parallel
IT
I1
“In a parallel circuit voltage stays
the same but current splits up”
IT = I1 + I2
I2
IT = V
R1
R2
RT
V
V = V + V
RT
IT
R1
R2
1 =
1 +
1
RT
R1
R2
Example questions
Calculate the equivalent resistance:
1)
10Ω
40Ω
2)
20Ω
10Ω
3)
100Ω
100Ω
20Ω
20Ω
4)
100Ω
50Ω
50Ω
Question 1
Question 2
Question 3
Question 4
Question 5
20s?
Calculate the total resistance of the circuit.
Calculate the current I.
Calculate the voltage across the points A and B.
Calculate the current I1.
How much charge will pass through the cell in
Each of the resistors in the circuit below has a resistance R
Ohms. What is their total resistance (in terms of R).
Calculate the total resistance of the four resistors in the
circuit shown below
Calculate the total resistance of the
same four resistors when a wire of
very low resistance is connected
across points X and Y.
Describe a real life situation for……
Describe the use of ideal ammeters
and ideal voltmeters.
Ideal ammeters have ……. resistance and
are connected in ………… .
Ideal voltmeters have ……. resistance and
are connected in ………… .
Emf and internal resistance
The Emf of a cell is the total energy
required to move each coulomb of
charge around the complete circuit.
Measuring Emf and internal
resistance
Apparatus required:
Cell
Two multimeters (or an ammeter and voltmeter)
Rheostat (approx. 20 Ω)
Leads
Circuit:
A
V
Procedure
Start with the rheostat on its maximum resistance. Record V and I. Gradually
reduce the rheostat to its lowest resistance (zero) measuring V and I a minimum of
7 times over the range. Don’t leave the circuit connected for long when the
resistance is low (current high) because this will run the cell down quickly.
Plot a graph of V against I. Determine the Emf and r
Analysis
V
E
=r
I
The electronvolt
• What is an electronvolt?
• What are the units of the the electronvolt?
• Why do we need to use the electronvolt?
• How are electron beams produced?
• How fast do electrons move in an
electron beam?
Define the electronvolt
Click to play
Summary of units and symbols
Quantity
Symbol
SI unit
Comments
Equations
potential
difference
V
volt (V)
also use mV and
kV
V=W/Q
charge
Q
coulomb
(C)
current
I
amp (A)
also use mA
energy
E or W
joule (J)
also use kJ, MJ
power
P
watt (W)
also use mW, kW,
MW
P = W / t, P = I V
= V2 / R = I 2 R
resistance
R
ohm ()
also use k
R=V/I
time
t
second (s)
Q = It