Activity: Electric fields

Aims

• Introduce the concept of charge.
• Observe the electric force.
• Explain simple electrostatics experiments in terms of moving charge.
• Introduce Coulombs law.
• Define electric field strength.
• Draw field lines for point charges, spheres of charge and parallel plates.

Electric charge

We now use electricity to power many devices but the idea of charge was originally developed to explain static electricity. Take a balloon and rub it on your head and put it on the table then hold your head next to the balloon your head should attract the balloon (if you are bald this probably won't work although I have never tried it). Take two rubbed balloons and they should repel. If it doesn't work there will be some complicated reason why but maybe its better to use this simulation.

The effect is similar to gravity but unlike gravity there is a repulsive force as well as attractive. This is explained in terms of a property of matter called charge.

• What can you say about the amount of + and - charge in neutral matter?
• Why does the balloon become charged when rubbed?
• Do two positives attract or repel?
• Is there a force between neutral bodies?
• Why does a balloon stick to the wall even though the wall is not charged?
• In these experiments why do the - charges move around not the + ones?
Atomic charge

So far in this course we have used the idea of atoms to describe the properties of matter (particularly gases) and the propagation of sound but we haven't dealt with the particles that make up the atom, this will be done in the atomic and nuclear physics topic but for the moment it is enough to say that if matter contains charge and matter is made of atoms then atoms must contain charges. Here is a simple model. Unit of charge

The unit of charge is the Coulomb.
The smallest amount of charge that exists is 1.6 x 10 -19 C, this is called the fundamental unit of charge and is the charge associated with the components of the atom. Electrons have a charge of -1.6 x 10-19 C

• How many electrons would it take to make a charge of -1 C?

Electric field

The electric effect is similar to gravity as a force is experienced over a distance. We used the concept of a field to model gravitation and can use the same idea here.

An electric field is a region of space where a body experiences a force due to its charge.

Coulombs law

Coulombs law is the electrical equivalent of Newton's universal law of gravity and is used in the same way to define quantities like field strength and potential for point charges.

The force experienced by two point charges is directly proportional to the product of their charge and inversely proportional to their separation squared • Given that the constant of proportionality is k write an equation for the force experienced by the charges shown here.
• What are the units of the constant?

Note that spheres of charge create the same field around them as if the charge was at a point in the centre. • Given that the constant k = 9 x 109 Nm2C-2 calculate the force experienced by a 10-6 C charge placed 1m from the centre of a sphere with charge 2 C.

Field strength (E)

The force per unit charge experienced by a small positive test charge placed at the point.

• Is field strength a vector or a scalar?
• What are the units of field strength?
• Calculate the field strength for the position of the small charge in the previous example.

Field lines

Field lines are drawn to show the direction and strength of the field (as in gravitational fields). They show the direction of the force experienced by a small positive charge placed in the field.

The following simulation can be used to plot them.

• Move the charge around in the simulation below to see how the field varies around the following arrangements of charge then sketch the field line patterns for each. Note that the "show E field button" shows the direction at different points but this is not the way we represent the field. You should draw continuous lines that start on the charges.

• Create a uniform field between a line of + charges and a parallel line of - charges. You could try making your own version of this in GeoGebra Electric fields simulation (GeoGebra).

Electric field problems

Electric fields multiple choice

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