Electric currents

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Electric currents
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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


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Static Electricity
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Static shock
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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

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Electric Current
Electric current is a flow of negatively charged
particles (i.e. electrons). We call them charge
carriers
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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.
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Answers
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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

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?

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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
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How long will it take for the electrons to
complete 1 circuit?
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Electron Drift
What happens inside a conducting material? The
following model of a metal wire could help
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.
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Electron Drift
Now apply a voltage
Negative
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.
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What does an electric current look like?
Download Run Now!
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Conductors..
How many free electrons?
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Hyperlink
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Basic ideas
Words volts, amps, ohms, voltage, ammeter,
voltmeter
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Voltage
Earlier on we said that current is when electrons
move
Voltage is the energy that allows the electrons
to move. For 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.
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Define electric potential difference

• The potential difference between 2 points in a
  circuit is..
• (1 volt 1 joule per coulomb)

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Resistance
Define resistance.
Students should be aware that R V/I is a
general definition of resistance. It is not a
statement of Ohms law. Students should
understand what is meant by resistor.
The resistance of a component can be calculated
using
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Resistance
Resistance is anything that opposes an electric
current.

• What is the resistance of the following
• A bulb with a voltage of 3V and a current of 1A.
• A resistor with a voltage of 12V and a current of
  3A
• A diode with a voltage of 240V and a current of
  40A
• A thermistor with a current of 0.5A and a voltage
  of 10V

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Resistivity
Apply the equation for resistance in the
form where ? is the resistivity of
the material of the resistor.
What effect does doubling the diameter have on
the resistance?
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Compare ohmic and non-ohmicbehaviour.
For example, students should be able to draw the
IV characteristics of an ohmic resistor and
a filament lamp.
Now determine the I-V graphs for a resistor and a
bulb.
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Current-voltage graphs
Consider a resistor
Current increases in proportion to voltage
Resistance stays constant
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Current-voltage graphs
Now consider a bulb
As voltage increases the bulb gets hotter and
resistance increases
Resistance increases as the bulb gets hotter
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Current-voltage graphs
Now consider a diode
Now consider a thermistor
Resistance decreases as the (negative-temperature
-coefficient) thermistor gets hotter
A diode only lets current go in the forward
direction
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LDR and Thermistor
How does their resistance vary with light and
heat?
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Two simple components
2) Thermistor resistance DECREASES when
temperature INCREASES
1) Light dependant resistor resistance
DECREASES when light intensity INCREASES
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Current in a series circuit
In other words, the current in a series circuit
is THE SAME at any point.
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Kirchoffs First Law
The sum of the currents leaving a point is the
same as the sum of the currents entering that
point.
For example
6A
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What happens if you have a choice?
         
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Current in a parallel circuit
A PARALLEL circuit is one where the current has a
choice of routes
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Current in a parallel circuit
If the current here is 6 amps
The current here will be
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What is the current in each bulb?
3A
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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
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Voltage in a series circuit
2V
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Voltage in a series circuit
4V
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Voltage in a parallel circuit
4V
4V
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Summary
In a SERIES circuit 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
eachstrand
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An example question
6V
A3
3A
A1
V1
A2
V2
V3
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An example question

1. What is the resistance across this bulb?
2. Assuming all the bulbs are the same what is the
   total resistance in this circuit?

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Solve problems involving potentialdifference,
current and resistance.
10V
A3
3A
A1
V1
A2
V2
V3
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An example
Calculate the missing values (from A-level June
2006)
6V
?
?
A
?
A
R
?
4O
V
0.24A
A
15O
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More examples
6V
12V
What is the resistance of these bulbs?
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Resistors in Series
In a series circuit current stays the same but
voltage splits up
VT V1 V2
VT IRT
But V1 IR1 and V2 IR2
IRT IR1 IR2
RT R1 R2
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Resistors in Parallel
In a parallel circuit voltage stays the same but
current splits up
IT I1 I2
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Example questions
Calculate the equivalent resistance
40O
1)
10O
2)
20O
10O
20O
100O
50O
100O
3)
4)
20O
100O
50O
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Question 1  Calculate the total resistance of the
circuit. Question 2  Calculate the current
I. Question 3  Calculate the voltage across the
points A and B. Question 4  Calculate the current
I1. Question 5  How much charge will pass through
the cell in 20s?
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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.
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Describe a real life situation for
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Describe the use of ideal ammetersand ideal
voltmeters.

• Ideal ammeters have . resistance and are
  connected in .
• Ideal voltmeters have . resistance and are
  connected in .

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Emf and internal resistance
                                                               
The Emf of a cell is the total energy required to
move each coulomb of charge around the complete
circuit.
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Measuring Emf and internal resistance
Apparatus required Cell Two multimeters (or an
ammeter and voltmeter) Rheostat (approx. 20
O) Leads Circuit 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. Dont 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
V
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Analysis
V
E
r
I
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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?

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Define the electronvolt
Click to play
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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)   Q It
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 I2 R
resistance R ohm (W) also use kW R V / I
time t second (s)