Physical Science
Chapter 7 Notes
Section 1 - Magnetism
1. Magnets
• Magnetism – the properties and interactions of magnets
• Magnetic force – the force exerted by a magnet on another magnet
⇒ This force can attract or repel depending on the orientation of the magnet(s)
⇒ The strength of the force increases as the magnets get closer together
2. Magnetic field – the area around a magnet where the magnetic force is felt
• The field is strongest close to the magnet and weakens with distance
• The field forms curved lines called magnetic field lines
• Magnetic poles – a region of space that surrounds a magnet and exerts a force on other
magnets and objects made of magnetic materials
⇒ All magnets have north and south poles
⇒ Magnetic field lines start at the north pole and end at the south pole
• Two magnets can either attract of repel each other
⇒ When the magnets are aligned so that like poles are adjacent, the magnetic force will
push the magnets apart
⇒ When the magnets are aligned so the unlike poles are adjacent, the magnetic force will
pull the magnets together
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3. Magnetic Materials
• Only a few metals such as iron, cobalt, or nickel are attracted to magnets or can be made into
permanent magnets
• Magnetic domains – groups of atoms within a metal with aligned magnetic poles
⇒ Temporary magnet – occurs when the magnetic domains of the atoms in a metal are
aligned. This alignment occurs when the object is placed near a permanent magnet. The
alignment is lost due to the random motion of the atoms.
• Permanent magnet – made by placing a piece of magnetic material, iron, cobalt, or nickel, in
a strong magnetic field. The aligned magnetic domains add together and create a magnetic
field inside the material that may be several thousand times larger than the field outside the
material
Section 2 – Electricity and Magnetism
1. Electric Current and Magnetism
• Passing electric current through a wire creates a magnetic field.
⇒ The field forms a circular pattern around the wire
⇒ The direction of the current of the field depends on the direction of the current
⇒ The strength of the field depends on the amount of current flowing through the wire. No
current, no magnetic field
2. Electromagnetism
• Electromagnetic force – the attractive or repulsive force between electric charges and
magnets
• Electromagnetism – the interaction between electric charges and magnets
3. Electromagnet – a temporary magnet made by placing a piece of iron inside a current-carrying
coil of wire
• Behaves like an ordinary magnet while current is flowing through the wire
• Has north and south poles
• Galvanometer – a device that uses an electromagnet to measure electric current
4. Electric motors
• Electric motor – a device that changes electrical energy to mechanical energy
⇒ Contains an electromagnet that is free to rotate between the poles of a permanent, fixed
magnet
Section 3 – Producing Electric Current
1. From mechanical to electrical energy
• Electromagnetic induction – the production of an electric current by moving a loop or wire
through a magnetic field, or moving a magnet through a wire loop
• Generator – produces electric current by rotating a coil of wire in a magnetic field
• Turbine – a large wheel that turns when pushed by water, wind, or steam
2. Direct and Alternating currents
• Direct current – electric current flows through the circuit in only one direction
• Alternating Current – the direction of the current alternates (changes) in a regular pattern
⇒ In North America, generators produce alternating current at a frequency of 60 Hz. The
current changes direction twice each second
3. Transformers
• Transformer – a device that increases of decrease the voltage of an alternating current
⇒ Made of two coils of wire called the primary and secondary coils. The coils are wrapped
around the same iron core
⇒ As alternating current passes through the primary coil it changes the direction of the
magnetic field of the iron core which in turn induces an alternating current in the
secondary coil
⇒ Step-up transformer – when the secondary coil has more turns of wire than the primary
coil, the voltage leaving the transformer is greater than the voltage entering the
transformer
⇒ Step-down transformer – when the secondary coil has fewer turns of wire than the
primary coil, the voltage leaving the transformer is less than the voltage entering the
transformer
⇒ Typically, electrical energy is transmitted through power lines at very high voltages (up to
750,000 V) to reduce energy loss due to friction in the power line. Before the current
enters a house to be used it must pass through a step-down transformer.