Unit 21 Electromagnetism
21.1 Magnetic effect of a current
Experiment 21.1 P415
- Demonstrating the magnetic effect of a current – Oersted’s Experiment
Definition: A current-carrying conductor produces a magnetic field around it.
- Magnetic field pattern around a straight wire
·magnetic field lines near the wires are closer together → the magnetic field is stronger closer to
the wire.
·the magnetic field strength decreases with increasing distance from the wire.
·the strength of the magnetic field also depends on the magnitude of the current passing through
the wire.
·the larger the current, the greater the magnetic field strength.
Definition: The magnetic field of a long, straight current-carrying wire is stronger:
1. when it is closer to the wire;
2. when a larger current flows through the wire.
- Magnetic field pattern around a flat coil
With a flat coil, the magnetic field at the centre of the coil is stronger, that is magnetic field lines
are closer.
Definition: There are two ways to increase the magnetic field strength at the centre of the flat
coil:
1. increase the current;
2. increase the number of turns of the coil.
- Magnetic field pattern of a solenoid
used as an electromagnet
Increasing the number of turns on flat coil → solenoid
↑
The resulting magnetic field pattern of a solenoid resembles that of a bar magnet → solenoid has
two poles and it acts like a bar magnet.
The magnetic field is stronger inside the solenoid. The parallel field lines also show that the
magnetic field strength is almost uniform in the solenoid. The magnetic field in a solenoid can be
made stronger or weaker.
Definition:
The magnetic field strength in a solenoid can be increased by:
1. increasing the current;
2. increasing the number of turns per unit length of the solenoid;
3. Placing a soft iron core within the solenoid. The soft iron core concentrated the magnetic
field lines, thereby increasing the magnetic field strength.
- Uses of electromagnets
A solenoid can be made into a strong electromagnet by placing a soft iron bar in it and passing a
current through the coil.
Electromagnets are used in the following devices:
1. Circuit breaker
2. Magnetic relay
3. Electric bell
4. Magnetic Resonance Imaging (MRI)
·Circuit breaker
A safety device that switches off the electric supply when excessive current flows through the
circuit. It works because of an electromagnet inside it.
When the current is within the limit, the solenoid magnetic field is not strong enough to attract the
soft ion latch. The interrupt point remains closed and current flow normally through the circuit.
When there is a sudden surge in current （e.g. more than 20A）such as a short circuit or
overloading, the solenoid becomes a very strong electromagnet due to the larger current. It is then
able to attract the soft iron latch, releasing the spring, which pushes the safety bar outward. The
interrupt point is now open and the circuit is witched off.
After the fault is repaired, the circuit breaker can be reset by pushing the button of the safety bar.
This would once again close the interrupt point.
·Magnetic relay
A relay is a device used to control the switch of another circuit. It uses an electromagnet to do so.
It has at least two circuits. The input circuit can be used to control the switches of other output
circuits without any direct electrical contact between them.
The input circuit supplies current to eh electromagnet. Only a very small current is needed. When
the current is switched on, the electromagnet attracts one end of the pivoted iron armature and
raises the other end.
The raised end of the armature (电枢) then closes the contacts in the second circuit. Thus, the
input circuit which works with a lower current or voltage controls another circuit which requires a
higher current or voltage (dangerous).
·Electric bell
The electromagnet forms the core (核心) of the electric bell. When the bell button is pushed down,
the circuit is closed and current flows. The electromagnet becomes magnetized, attracting the soft
iron armature. This causes the hammer to strike the gong.
As soon as the hammer moves towards the gong, the circuit is broken. The current stops flowing
and the electromagnet loses its magnetism. This allows the springy metal strip to pull back the
armature, connecting the contact at S and thus closing the circuit again. The cycle is repeated and
the bell rings continuously as long as the bell button is pressed down.
·Magnetic Resonance Imaging (MRI)
MRI is an increasingly popular method of medical imaging that provides vies of tissues in the
body. The patient is passed through a huge scanner containing a solenoid made of superconductors.
Superconducting solenoids have very little electrical resistance, so they can pass very large
currents without overheating. This means that very strong magnetic fields can be produced. These
strong magnetic fields cause atoms in the body to emit their own radio waves. The emitted radio
waves are then picked up by detectors and processed by computers to give an image of the tissues
in the human body.
21.2 Force on Current-carrying Conductors
- Force on current-carrying conductor in a magnetic field——the motor effect
When you place a current-carrying wire in a magnetic field, the wire experiences a force. This
is called the motor effect.
The swing moves outwards when a current flows through the wire. Conclusion: a force acts on
the current-carrying wire when it is placed in a magnetic field. This force acts perpendicular to
both the direction of the current and the direction of the magnetic field. The direction of the
force is reversed when we reverse the direction of the:
1. current;
2. magnetic field.
Felming’s Left-Hand Rule
Forefinger – the direction of the magnetic field (N & S)
Second finger – direction of the current
Thumb – the direction of the motion of the wire, that is the direction of the force
- Why dose a current-carrying conductor experience a force when placed in a magnetic field?
P422+1
There is a stronger field on one side of the wire at A, since all the magnetic field lines are acting in
the same direction. At B, the magnetic field lines of the current oppose those of the magnet,
making the combined field weaker. A force then acts on the wire from the stronger field to the
weaker field.
- Force on a moving charge in a magnetic field
Current-carrying wire experiences a force when it is placed in a magnetic field.
The effect of moving electric charges in a magnetic field.
1. When a beam (束，梁) of positive charges, e.g. protons, enter the magnetic field region, the
beam is deflected upwards in a circular path. This is because the moving charges experience a
force perpendicular to tits direction of motion.
2. when a beam of electrons or negative charges, we follow the conventional current direction
which is opposite to that of electron flow.
3. the beam of positive charges or protons enter a region of a magnetic field which is directed
out of the plane of the paper towards you. The beam is now deflected downwards.
Refers to Textbook P424-425
- Force between two parallel current-carrying wires
When two current-carrying wires are placed parallel to each other, we would expect a force to
act on each wire.
When the current in each conductor flows in the opposite direction, the strips repel. However,
when both currents are in the same direction, they attract each other. We can summarise the forces
between two parallel current-carrying wires as:
Definition: Current is opposing directions cause repulsion. Currents in similar directions cause
attraction.
Explainment on this phenomenon by looking at the combined magnetic field due to both wires.
Combined magnetic fields of two parallel wires with currents in opposing directions and similar
directions respectively. The magnetic field pattern of a current-carrying wire can be found by
applying the right-hand grip rule.
Refers to textbook P425
21.3 Force on a Current-carrying Rectangular Coil in a Magnetic Field
These two forces produce a clockwise turning effect when viewed from P. To increase the turing
effect on the wire coil:
1. increase the number of turns on the wire coil;
2. increase the current in the coil.
Refer to textbook P426-427
Turning effect on a current-carrying coil in a magnetic field —— very important application –
the direct current (d.c.) motor. The d.c. motor is used commonly in (e.g. toy cars, DVDs and hard
disk drives.)
There is also the alternating current (a.c.) motor which is used in many electrical appliances (e.g.
electric fans, hair dryers and washing machines.)
- The d.c. motor
The purpose of a d.c. motor is to convert electrical energy into mechanical energy. The motor
consists of the following components:
1. Rectangular coil connected in series to a battery and rheostat
2. Permanent magnets
3. Split-ring commutator
4. Two carbon brushes
·the rectangular coil of wire ABCD is mounted on an axle that allows it to rotate about PQ.
·the ends of the wire are connected to a split-ring commutator XY.
·the commutator rotates with the coil.
·two carbon brushes press lightly against the commutator
How does a d.c. motor work?
1). When current flows through the coil ABCD, using Fleming’s left-hand rule
2). A downward force will act on side AB + an upward force on side CD
3). The coil thus rotates anticlockwise about axis PQ until it reaches the vertical position
4). The current is cut off because X + Y are both not in contact with the carbon brushes
5). The turning effect of the coil, however carries it past the vertical position
6). This reverses the current direction in the wire arm CD and now a down ward force acts on it
7). An upward force acts on wire arm AB
8). The coil continues to rotate in the anticlockwise direction
Purpose of the split-ring commutator → reverse the direction of the current in the coil every
half a revolution whenever the commutator changes contact form one brush to the other →
ensures that the coil will always turn in the same direction.
Insert a soft iron core or cylinder into the coil to
concentrate the magnetic field lines
Increase the number of turns or the current in the coil
increase the turning effect of the coil in
the motor.
→ it is common to see the coil in practical d.c. motors made up of hundreds of turns of wire with
a soft iron core at the centre.