Summary and Review of Electromagnetism
1. Oersted discovered the basic principle of electromagnetism, which is: Whenever electrons move through
a conductor, a magnetic field is created in the region of the conductor.
2. The magnetic field of a straight conductor is a series of concentric circular field lines. The distance
between these lines increases with the distance from the conductor. The direction of the field is given by
the left-hand rule: If the left thumb gives the direction of electron flow, the curled left fingers give the
direction of the magnetic field.
3. The magnetic field of a long coil or solenoid is a series of straight, equally spaced field lines in the core of
the coil. The direction of the field is given by the left-hand rule: If the curled left fingers give the direction
of the electron flow through the coil, the left thumb points in the direction of the magnetic field inside the
coil.
4. The strength of the magnetic field of a coil depends on (a) the amount of current in the coil, (b) the
number of loops per unit length in the coil, (c) the permeability of the core material.
5. Permeability refers to the ability of a material to change the magnetic field strength in a region of space.
6. The attraction of a ferromagnetic substance towards the core of a current-carrying coil may be used to
explain the operation of (a) a lifting electromagnet, (b) a relay, and (c) an electric bell.
7. The motor principle states: A current-carrying conductor that cuts across external magnetic field lines
experiences a force perpendicular to both the magnetic field and the current. The direction of the force is
given by the left-hand rule: If the left fingers give the direction of the external magnetic field, and the left
thumb gives the direction of the electron flow, the force is in the direction in which the palm faces.
8. The force on a conductor in a magnetic field may be used to explain the operation of (a) a moving-coil
speaker, (b) a moving coil galvanometer, and (c) a DC motor.
Review
1. (a) State Oersted's basic principle of electromagnetism.
(b) Sketch the magnetic field around a straight vertical wire that has charge moving down through it. If a
compass were placed immediately in front of this wire, in which direction would it point?
2. (a) Sketch the magnetic field of a current-carrying coil, showing the direction of the field lines in the core
and marking the magnetic polarities of each end of the coil.
(b) Draw a sketch of an experimental arrangement that could be used to magnetize a bar of iron so that
it would have an S-pole at each end and an N-pole in the middle.
3. State the effect of each of the following changes, considered separately, on the strength of the magnetic
field of a coil.
(a) The current flowing through the coil is tripled.
(b) The number of loops per unit length in the coil is halved.
(c) The core material, with a permeability of 1000, is replaced by a material with a permeability of 4000.
(d) All of the three previous changes are made together.
4. (a) Why is soft iron used rather than steel as a core material for the electromagnet in an electric bell?
(b) Draw a sketch of an electromagnetic relay designed to turn a light bulb off when current flows
through the control circuit.
5. Compare the magnetic field strength of two coils, one having 100 turns and carrying a current of 5.0 A,
the other having 50 turns and carrying 20 A of current, if they both have the same length.
6. (a) What direction would a current-carrying coil assume if suspended freely in the Earth's magnetic
field? Why?
(b) Explain how to determine the direction of charge flow through a simple circuit, using a compass.
(c) A compass needle is placed in its normal north-south direction above a horizontal conductor. In what
direction must charge move through the conductor in order for the compass needle not to move?
7. (a) State the motor principle and the right-hand rule for the direction of the force on a conductor in a
magnetic field.
(b) Make a fully labelled sketch of a simple DC motor, showing the field magnet polarity and the direction
of charge flow required to make the coil rotate in a counter-clockwise direction.
(c) A simple DC motor with two commutator segments is turned off, and the rotor comes to rest with the
brushes touching the insulation between the segments. What will happen if the motor is turned on
again? Why?
(d) How could you restart the motor?
8. Sketch a moving-coil speaker, labelling the four key parts and explaining the role of each in converting
pulses of electric current into vibrations that produce sound.
9. The diagram shows a rectangular coil of wire, WXYZ, between the poles of a magnet, and an axis about
which it is free to rotate. Charge flows through the coil in the direction shown.
(a) Which side of the coil will start to move into the page?
(b) In what position will the coil finally come to rest?
10. Using sketches of the magnetic fields around two parallel conductors, determine whether the conductors
will attract or repel each other when charge flows through them (a) in the same direction, and (b) in
opposite directions.
11. Draw a sketch to explain how a magnetic circuit-breaker might operate to limit the current flowing
through a conductor to 15 A.
12. Most practical DC motors employ an electromagnet as the field magnet. Draw a sketch showing this
electromagnet and the armature winding when the two are connected (a) in series, and (b) in parallel.
13. Copy each of the following diagrams in your notebook and add the requested information:
(a) Show the direction of charge flow and the direction in which each of the compasses points.
(b) Show the direction of charge flow and the direction in which each of the compasses points.
(c) Show the magnetic field of the magnet, the magnetic field of the wire, and the direction of the force
on the wire.
(d) Show the direction of charge flow through the wire.
(e) Determine the polarity of the magnet.
(f) Show the direction of charge flow through the brushes, commutator segments and coil, the polarity
of the armature, and the direction of rotation of the motor.