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Electricity and Magnetism

In this unit we will study electricity, electric fields. From there we will develop this so that we can understand what electrical currents are, and how they can be used to make electrical circuits.

Next will be magnetism, and the phenomenon of electromagnetic induction.

All aspects of electricity and magnetism depends on one of the four fundamental forces of nature – the electromagnetic force.

There are 2 types of electric charges

What is an electric field?

“An electric field is said to exist anywhere a force is felt on a positive test charge. Placing a positive test charge in a field and observing its path creates a segment of a “field map.” The movie below demonstrates the tracing of the path of a positive test charge.”

When you have traced the paths of that test charge many times in many different places, you start to get an idea of what the field map looks like. The completed field map is shown below.

When you have traced the paths of that test charge many times in many different places, you start to get an idea of what the field map looks like. The completed field map is shown below.

Lines have a direction to them.

Direction represents the motion of the positive (+) test charge, when placed at different points around the field.

Field lines never cross each other.

The closer the field lines are to each other, the greater the field’s strength.

From ThePhysicsClassroom website:

App: Electric Field Lines Interactive


What is a compass? How do we use one?

The simplest compass is a magnetized metal needle mounted in such a way that it can spin freely. (You can make one yourself by magnetizing an ordinary needle, placing it carefully on a slice of cork, and letting the cork float in a tray of water.)

Left to its own devices, the needle turns until one end points north and the other south.

You can usually figure out which end is which from the position of the Sun in the sky, remembering that the Sun rises in the east and sets in the west. So if you’re looking down on the floating needle at about noon, with the eye on the left and the point on the right, and the Sun in front of you, you know the point is indicating north.



One end of the metal needle always points to the North Pole’ the other always points to the South Pole.


What is pulling the metal needle in the compass?

There must be something inside the Earth itself which creates a magnetic field!


The red pointer in a compass is attracted by Earth’s own magnetism (sometimes called the geomagnetic field—”geo” simply means Earth).

As English scientist William Gilbert explained about 400 years ago, Earth behaves like a giant bar magnet with one pole up in the Arctic (near the north pole) and another pole down in Antarctica (near the south pole).

Earth’s magnetic field is actually quite weak compared to the “macho” forces like gravity and friction that really dominate our lives.

For a compass to be able to show up the relatively tiny effects of Earth’s magnetism, we have to minimize the effects of these other forces.

That’s why compass needles are lightweight (so gravity has less effect on them) and mounted on frictionless bearings (so there’s less frictional resistance for the magnetic force to overcome).


– – – – –

“Where the Earth’s magnetic field comes from, Chris Rowan

The Earth’s magnetic field may approximate to a simple dipole, but explaining precisely how that dipole is generated and maintained is not simple at all. The field originates deep in the Earth, where temperatures are far too high for any material to maintain a permanent magnetisation.

The dynamism that is apparent from the wandering of the magnetic poles with respect to the spin axis (secular variation), and the quasi-periodic flips in field polarity, also suggest that some process is actively generating and maintaining the geomagnetic field. Geophysicists therefore look to the most dynamic region in the planetary depths, the molten outer core, as the source of the force that directs our compass needles…


How a planet becomes a magnet

Earth’s magnetic field single-handedly protects life on this planet from a deadly case of solar wind-burn, By Bernie Hobbs



App: The solar wind and Earth’s magnetic field


Learning about Earth’s magnetic field: ESA’s Swarm mission



How do magnets work?


Bar magnets Poles

What is a magnetic field?

We are surrounded by magnetic fields all the time, but our bodies don’t feel them.  Our nerves don’t detect magnetism.  So how can we see these invisible magnetic lines of force? How do we know that they are real?

Bar magnet surrounded by compasses



“A magnetic field consists of imaginary lines of flux coming from moving or spinning electrically charged particles. Examples include the spin of a proton and the motion of electrons through a wire in an electric circuit.”


The Earth’s interior generates a magnetic field

This field reaches out into space



This magnetic field protects us from some types of radiation

Earth’s North geographic pole has a South magnetic field

Why?  Because the “north” pole of a compass – by definition – is pulled to a “south” magnetic pole.  So if we hold a compass in our hands, and call the part pointing toward’s the land of the Polar bears “north”, then we have to call the place attracting it south.

Earth North Geographic Pole South Magnetic Pole

Learning Standards

Massachusetts 2016 Science and Technology/Engineering (STE) Standards

7.MS-PS2-5. Use scientific evidence to argue that fields exist between objects with mass, between magnetic objects, and between electrically charged objects that exert force on each other even though the objects are not in contact.

7.MS-PS3-2. Develop a model to describe the relationship between the relative positions of objects interacting at a distance and their relative potential energy in the system. {Examples could include changing the direction/orientation of a magnet.}

HS-PS2-1. Analyze data to support the claim that Newton’s second law of motion is a mathematical model describing change in motion (the acceleration) of objects when acted on by a net force….{forces can include magnetic forces}

HS-PS3-5. Develop and use a model of magnetic or electric fields to illustrate the forces and changes in energy between two magnetically or electrically charged objects changing relative position in a magnetic or electric field, respectively.

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