Saturday X-tra
X-Sheet: 20
Electrodynamics
Key Concepts
This lesson focuses on the following:
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Magnetic field
Current-carrying conductors in a magnetic field
Electromagnetic induction
Alternating current
Direct current
Terminology & definitions
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Magnetic field – a region in space around a magnet where a magnetic substance
will experience a force.
An electric motor – an electrical device that converts electrical energy into
chemical energy.
Generator – an electrical device that converts mechanical energy into electrical
energy.
Direct current – current that moves in the same direction.
Alternating current – current that continuously change its direction in a periodic
manner.
Fleming’s Left Hand Motor Rule – a method used to determine the direction of
the current and/or the direction of the force experienced by a current-carrying
conductor placed in a magnetic field.
X-planation of key concepts and terminologies
A current carrying conductor creates a magnetic field around itself. A cross section of
a conductor carrying current into the page can be illustrated as follows: Note that the
direction of the magnetic field is clockwise in this situation.
A cross section of a conductor carrying current out of the page can be illustrated as
follows: Note that the direction of the magnetic field is anticlockwise in this situation.
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The diagram below shows the magnetic field around a conductor, L. In this diagram
the arrow indicates the direction of current in the conductor and the crosses and dots
indicate the direction of the magnetic field. The number of crosses and dots decreases
as you move away from the conductor. This means that as you move away from the
conductor, the magnetic field strength weakens (decreases).
When a current-carrying conductor is placed inside a magnetic field, its generated
magnetic field interacts with the magnetic field due to the permanent magnets. The
conductor will then experience either an upward or downward force. The force will
have a turning effect on the conductor if it is curled into a coil. This is the principle
used in the making of motors.
A simple direct current motor will consist of two permanent magnets with a coil placed
inside the magnetic field. The coil is connected to a split ring connected to an external
power supply through brushes.
When the current is supplied through the brushes, the coil will experience a turning
force. A break in power supply is experienced when the split (open) section of the split
ring commutator reaches the brushes. At this point there is no current in the coil.
However, the angular momentum of the rotating coil carries the rotation forward. The
section of the split ring that was in contact with the positive terminal of the battery now
comes in contact with the negative terminal of the battery. So the current moves in the
opposite direction around the coil. This results in a steady movement of the coil in a
specific direction (clockwise or anticlockwise).
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The speed of the motor can be increased by any or all of the three things below:
Increasing the number of coils of the conductor
Increasing the current inside the conductor
Increasing the strength of the magnets
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When a conductor turns inside a magnetic field, an electromotive force is induced into
the conductor causing charges to move. The moving charges constitute an electric
current. This is the principle used in the making of electric generators.
A DC generator is actually the same as the DC motor explained above. However, the
current-carrying conductor inside the magnetic field is now turned mechanically, and
the output on the brushes is an emf induced onto the conductor. The current
generated moves in one direction but generally increases from zero to a maximum,
and then drops back to zero according to the following graph:
V
(V)
t (s)
An AC generator consists of two slip rings. Each slip ring is connected to one end of
the coil. The slip ring commutator is continuously in contact with the brushes. The
induced emf moves in an alternating direction. It starts from zero to a maximum,
returns and moves past zero to a minimum, and back to zero according to the
following graph:
V
(V)
t (s)
The AC voltage generated from power stations has a peak voltage and, therefore,
peak current as well. However, what is received for use in households, for example, is
the average voltage and average current called the root mean square (rms) values.
The root mean square (rms) for voltage and current are found using the following
formulae:
I rms =
I max
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for current where Irms is the root mean square current and Imax is the peak
current.
Vmax
for voltage where Vrms is the root mean square voltage and Vmax is the peak
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voltage.
Vrms =
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X-ample Questions
1. Determine the direction in which the conductor will move in the diagrams below:
a.)
b.)
2. Determine the direction of the magnetic field and the current if you require the
conductor to move to the left.
a.)
b.)
3. Show on the diagram below how you could get the coil to move in an anticlockwise
direction.
4. Write down the difference between
4.1
(a) a motor and a generator
(b) a dc and ac generator
4.2 How would you increase the induced emf in a conductor rotating within a magnetic
field?
4.3 How would you increase the speed of rotation of a motor?
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5. How would you increase the induced emf in a conductor rotating within a magnetic
field?
6. In South Africa voltage is delivered into our homes at 240 V. Calculate the
maximum voltage at which electricity is supplied at our homes.
X-ercise
1. A simplified sketch of a generator is shown below.
(Adapted from Physical Sciences P1: February/March 2010)
1.1 Is the output voltage AC or DC? Give a reason for your answer.
1.2 State TWO effects on the output voltage if the coil is made to turn faster.
1.3 What is the position of the coil relative to the magnetic field when the output voltage
is a maximum?
1.4 In South Africa, the major source of electricity is coal-driven generators. Recently
society has become concerned about fossil fuels (like coal) as the primary source of
electrical energy. Some business people have proposed that government should invest
in windmills as an alternative source of energy.
State ONE advantage and ONE disadvantage of using windmills over coal-driven
generators in supplying energy.
2. Lights in most households are connected in parallel, as shown in the simplified
circuit below. Two light bulbs rated at 100 W; 220 V and 60 W; 220 V respectively are
connected to an AC source of rms value 220 V. The fuse in the circuit can allow a
maximum current of 10 A.
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2.1 Calculate the peak voltage of the source.
2.2 Calculate the resistance of the 100 W light bulb, when operating at optimal conditions.
2.3 An electric iron, with a power rating of 2 200 W, is now connected across points a and
b. Explain, with the aid of a calculation, why this is not advisable.
Answers
1.1
AC – alternating current
A separate slip ring connected to each wire.
1.2 Increase in peak (or rms) voltage
Increase in frequency
1.3 The plane of the coil is parallel to the magnetic field.
1.4 Advantage
Less environmental pollution (noise, gases, etc.)
Disadvantage
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Will not operate in absence of wind.
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Many windmills needed to generate sufficient electricity –
unsightly appearance in environment.
2.1 Vmax = 311,13 V
2.2 R = 484 Ω
2.3 Irms = 10 A
The iron draws a current of 10 A. Therefore, together with the lights
the total current will exceed 10 A and the fuse wire will blow and the
current will stop.
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