Coulomb forces

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Question statement

This question relates to Coulomb’s Law, which gives the force per unit charge due to the interaction between stationary charges.

When providing numerical answers you may express them using scientific notation. Express values to four significant figures and use the values of physical constants as provided in the course notes, and take the relative permittivity of air to be 1.0005.

Give any introductory information the student needs.

			Name	Type	Generated Value

eps0	number	0.000000000008854
epsair	number	1.0005
epsmat	number	6.2
force1	number	179.6650162516
force2	number	28.9927175419
g	number	9.8
mass	number	18.3331649236
q1	number	0.50
q2	number	1.0
r1	number	0.5

Automatically regenerate variables?

Name

Data type

Value

xxxxxxxxxx
 
8.854*10^-12

JME function reference

Description

Permittivity of free space in F/m.

Describe what this variable represents, and list any assumptions made about its value.

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Generated value: `number`

0.000000000008854

This variable doesn't seem to be used anywhere.

Parts

Force between two point charges Gap-fill
1. Gap force Number entry
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Effect of permittivity Gap-fill
1. Gap percentage Number entry
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Balancing gravitational force Gap-fill
1. Gap mass Number entry
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Ask the student a question, and give any hints about how they should answer this part.

Using Coulomb's Law, calculate the force on a {q1} $\mu$ C point charge due to the electric field from a {q2} $\mu$ C charge when they are separated by {r1} cm of air.

$|\vec{F}| =$ force Newtons.

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If you got the wrong answer, did you remember to

Convert all quantities to SI units?
Include the relative permittivity of air (1.0005)?
Square the distance?

The $E$ -field due to a point charge is

$\displaystyle{q\over 4 \pi \varepsilon_0 \varepsilon_r r^2}$

radially away from the point charge. The force experienced in an electric field is generally $\vec{F}=q\vec{E}$ , so the force on a point charge, $q_1$ due to another point charge, $q_2$ has a magnitude of

$\displaystyle q_1 {q_2\over 4 \pi \varepsilon_0 \varepsilon_r r^2}$ .

Since typically $\varepsilon_r\ge1$ , any material between the two charges tends to reduce the force relative to the point charges in vacuum. It is important to note that the question asks for the percentage by which the force is reduced, not the percentage of the original force that remains.

The force due to gravity is $mg$ towards the centre of the Earth, so one can calculate the force due the electric field and work out the mass that when mulitplied by $g$ has the same magnitude.

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This question is used in the following exams:

Tutorial questions - electrostatics by Jon Goss in EEE1001 & PHY1023.
Week 3 tutorial by Jon Goss in EEE1001 & PHY1023.
Stage 2 EM Coulomb by Jon Goss in Jon's workspace.
All electromag by Jon Goss in EEE1001 & PHY1023.
TEST by Jon Goss in Jon's workspace.

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Coulomb forces

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Taxonomy: mathcentre

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Jon Goss 4 years, 3 months ago

Jon Goss 5 years, 5 months ago

Jon Goss 5 years, 5 months ago

Jon Goss 5 years, 5 months ago

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