Download Current Electricity

Current Electricity
Written by YJ Soon ([email protected])
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Electricity is a term that describes all the phenomena associated
with electrical charges, and an electric circuit is a complete
conducting path containing an energy supply and a load. In this
chapter, we’ll consider the fundamental measurements you can
make from a circuit – namely, current, voltage and resistance.
What are these things, really?
hellooooo
Electric current
Current is the rate of flow of charged particles.
Conventional current flow is defined as positive to negative, while electron flow is defined as
negative to positive. Why?
I=Q/t
I: Current, measured in Amperes (A)
Q: Charge, measured in Coulombs (C)
t: Time, measured in seconds (s)
Current is measured using an ammeter (see below), which is connected in series with the wire
whose current is being measured. Since ammeters have very low resistance, they do not affect
the current reading.
A
Why in series? Imagine if you’re trying to measure the rate of flow of water with a supermagical water-particle counter (SMWPC*). You’d have to stick this SMWPC into the stream
in order for the water particles to hit it and be counted, right? Similarly for the ammeter,
except it has to be in the path of the charges.
Voltage
*
The SMWPC is not in the syllabus.
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Voltage is more accurately known as either potential difference or electromotive force
(EMF**). When measuring voltage, you’re actually measuring the difference in potential
between two points of a circuit, that is, the difference between energy levels across those two
points. When there is a potential difference, current flows. Think of potential as the “amount
of energy”, if you will – high potential, high energy.
The potential difference, or voltage drop, across two points of an electric circuit is the
measure of the electrical potential energy lost by charge carriers, or work done in driving a
unit charge across these two points.
Hence, the equation:
V=W/Q
V: Voltage drop, measured in Volts (V)
W: Work done, measured in Joules (J)
Q: Charge, measured in Coulombs (C)
The EMF of a circuit, , is the work done in driving a unit charge around the entire circuit.
Note the difference between this and the potential difference.
=W/Q
EMF is provided by cells or batteries, pictured below. The positive end is longer. When
referring to the EMF of a cell, we are saying it has that much energy to drive charge around
the circuit.
When cells are connected in series (plus-minus-plus-minus-plus-minus-etc.*), the total EMF
of the cells is the sum of the individual cells. However, when they are connected in
opposition, the EMFs cancel out.
When identical cells are connected in parallel (see below), the total EMF of the cells is equal
to that of a single cell.
Why is it a bad idea to leave cells connected in parallel when not in use? Hint: do real cells
have identical EMF? What happens when there is a potential difference?
**
*
Not to be confused with the second-hand bookstore.
Try saying that ten times fast
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In order to measure potential difference, one would use a voltmeter. The voltmeter has to be
placed in parallel with (or across) the component being measured; it won’t affect the reading
because of its high resistance. Why? Imagine trying to measure water at two different levels.
You would have to place your super-magical water-height counter (SMWHC**) at both levels
in order to measure the height difference. Similarly, in order to measure the difference
between the potential levels at two points, your measuring device (in this case, the voltmeter)
would have to be connected across both points.
V
Yes, I can’t connect things properly in WordArt. Correction-tape over the excess wire above,
please.
Resistance
The resistance, R, of a component is the opposition provided by the component to the flow of
current through it. It is measured as the ratio of the potential difference across the component,
to the current across it.
R = V/ I
R: Resistance, measured in ohms ().
V: Potential difference, measured in Volts (V).
I: Current, measured in Amperes (A).
The resistance of an object is the property of that object itself, as measure in a circuit. The
resistivity, , however, is a function of the material that the object is made of, and is constant.
Otherwise, resistance varies directly as the length of the object and inversely as the crosssectional area.
R = ( L ) / A
Resistivity, measured in ohm-metres (m).
L: Length, in metres (m).
A: Cross-sectional area, in metres-squared (m2).
Ohm’s Law states that the current flowing in a conductor varies directly with the voltage drop
across the conductor. Hence, the current-voltage (I-V) graph is a straight line (see below),
but, as we’ll see later, only for constant temperature.
What happens when temperature increases? This is an issue because as you pass greater and
greater current through a conductor, the conductor heats up. As the conductor heats up,
resistance increases and hence the I-V graph curves away from the ideal. Why does it curve
this way and not the other? Think about the gradient of the graph and how it relates to
resistance.
**
The SMWHC is in the syllabus. Okay, not really.
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I-V characteristic at
constant temperature
I-V characteristic with
temperature increase
The graph showing constant temperature shows negative voltage and negative current. What
does this mean? What would the negative voltage – negative current graph look like for the
graph showing temperature increase?
end.
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