Download Coulomb`s Law

Coulomb's Law
Introduction:
The force between two electrically charged objects depends on the size of their charges and on their
separation. This experiment investigates the effect of the separation on the size of the force.
Theory:
If a charged object is suspended from two angled threads, another object of like charge can push it away.
The size of the push can be measured from the amount of deflection of the suspended object.
Resolving forces: T sin θ = F, T cos θ = mg and
F = mg tan θ, but tan θ = d/L, so F = (mg/L)d. That
is F is proportional to d.
θ
T, tension by string
on object
F, electric force by
other charge
The aim of the experiment is to investigate how
d varies with r.
L
d
r
Both 'd' and 'r' are going to be small distances, so to
effectively measure, the method
of 'shadow puppets' is used.
mg, force by earth on object
A light globe is placed on one
side of the apparatus to cast a
shadow of the two charged
objects on a screen or a distant
wall. The spacing between the
shadows will be proportional to
'd' and 'r'.
Light globe
d
r
'D' is proportional to 'd' and 'R' is
proportional to 'r', so a graph of
R against D is effectively a
graph of Force against
separation.
D
R
Procedure:
1.
Suspend a light sphere with a conducting surface from two angled threads so it swings in only one
direction. Ensure the sphere is insulated.
2.
Charge the sphere either by:
i)
induction, where a charged sphere is brought near the suspended sphere and the opposite of
the suspended sphere is quickly earthed, or by
ii) contact, where a charged sphere touches the suspended sphere and shares charge.
3.
Bring up a like charged sphere on a insulated stand near the suspended sphere.
4.
Measure D and R as the sphere on the stand is moved towards the suspended sphere.
Analysis:
5.
Draw a graph of D against R and describe the shape of your graph.
6.
What kind of relationship does your graph suggest? Plot a graph to check this.
Questions:
7.
Some charge may leak away through the thread and the insulating stand, introducing a source of
error. How can you test to see if charge has leaked away?
8.
How could you use this apparatus to investigate the effect of halving the charge?
Electric Potential and Electric Fields
Introduction:
The electric field and electric potential are related. This experiment investigates the relationship to two
experimental setups, i) two straight-edged electrodes across a conducting sheet , ii) two small circular
electrodes.
Equipment:
One sheet of teledeltos paper, two sheets of paper, masonite board, G clamps, galvanometer, two coins,
two straight edged plates, two batteries, a wire with a metal pointer on one end, a voltmeter and rheostat.
Procedure:
1. Connect up the equipment as in the diagram on
the side. Check the battery connections are as
in the diagram.
2. Place the pointer, P, at any spot on the paper.
Adjust the contact on the rheostat until the
galvanometer reads zero.
V
P
G
Teledeltos paper above
white paper on board.
Question
3. What is the reading on the voltmeter? How
does the value of the electric potential at the point P compare with this reading. Explain your answer.
4.
5.
6.
7.
8.
9.
Push the pointer down on the paper to make an indentation on the white paper underneath.
Move the pointer to another spot where the galvanometer again reads zero. Mark this point as well.
Repeat this for two other spots.
Now move the contact of the rheostat so that the voltmeter reading is 0.5 V.
Find four spots where the galvanometer reads zero and mark these spots.
Repeat steps 6 and 7 for increments of 0.5 V.
Remove the white paper and draw a line through the set of marks produced for each voltmeter setting.
These are lines of equal electric potential. For each set of marks write down at the end of the line,
their common voltage reading.
Question
10. Describe what you see.
11. Draw a line across your sheet through the middle from one plate to the other.
12. Along this line, measure the separation of the lines of equal electric potential and use these values to
calculate the strength of the electric field at different points from plate to plate.
Use Electric Field Strength , E = V/ d
13. Record your values for the electric field strength
Question:
14. Comment on these values.
15. Repeat the full procedure with two coins in place of the two plates.
Question:
16. An alternative method is in the figure on the right. It is not as
accurate as the circuit above. Suggest a limitation of this circuit.
P
V
Electric Potential and Electric Fields in a wire
Introduction:
The electric field and electric potential are related. This experiment investigates the relationship along a
wire.
Equipment:
A metre ruler with length of nichrome wire stretched along its length, a battery, two voltmeters and a solid
metal contact.
Procedure:
1. Connect up the equipment as in the diagram on the side.
2. Move the contact, P, along the wire. Record the voltmeter
reading every 5 cms.
3. Draw a graph of Electric potential, V against length from the
left end of the wire.
Questions
4. What does the gradient of the graph at each point represent.
5. Calculate the value of the electric field strength inside the wire.
P
V