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Gravity
OCR G485 A2
Terms in this set (45)
How does an object become charged?
When one uncharged material is rubbed against another. The friction between them strips electrons off one material and onto the other. The material that has gained electrons is now negatively charged; the other is now positively charged.
What two types of charge are there?
Positive and negative.
What do opposite charges do?
Attract.
What do like charges do?
Repel.
How can a charged object attract an uncharged object?
Via electrostatic induction.
How can matter be modelled?
Matter can be modelled as consisting of negatively charged electrons, positively charged protons, and neutral neutrons.
What does the magnitude of charge in a proton equal?
That of an electron.
What does the magnitude of charge in an electron equal?
That of a proton.
What is special about the number of protons and electrons in an uncharged object?
They are the same.
Why is it that the electrons are involved in moving in attraction and repulsion?
Electrons, being on the outside of atoms, are less strongly held within a material than the protons.
What do charged objects create?
Electric fields around them.
How can we detect an electric field?
We need to put something in it that will respond to the field.
What are two ways of detecting an electric field?
1) Use a charged strip of gold foil placed between charged metal plates. The foil will be attracted to the opposite charge.
2) Use grains of a material such as semolina - the material lines up in the electric field.
How can we represent electric field patterns?
By using electric field lines.
What two things do the field lines indicate?
1) The direction of the field.
2) How strong the field is.
In which way does the electric field travel?
From positive to negative.
What do closely bunched lines in an electric field indicate?
A strong electric field.
How does the strength of a uniform field act?
It is the same at all points.
In which direction do radial fields act?
They spread outwards in all directions.
Define electric field strength at a point.
The electric field strength at a point is the force per unit charge exerted on a positive charge placed at that point.
What is the equation using force and charge that describes electric field strength?
E = F/Q
What are the units of electric field strength?
NC-1 and Vm-1.
How can you set up a uniform electric field?
You can set up a uniform field between two parallel metal plates by connecting them to the terminals of a power supply.
Between two parallel metal plates, what does the strength E between the plates depend upon?
The p.d. V between the plates and the separation d of the plates.
As EαV, and Eα1/d, what is the equation encompassing this?
E = - V/d
Why is E = - V/d negative?
V increases in the opposite direction that F acts.
As work done = energy transformed, how can we express this in terms of formulae?
Fd = VQ
How can we calculate the force F on a charge Q in a uniform field between two parallel plates?
We must combine the general equation for field strength E=F/Q with the equation for the magnitude of the electric field, E=V/d.
Doing this, we get F = QE = QV/d.
If the magnitude of charge on an electron is e, what is the force experienced by the electron between charged parallel plates?
F= eV/d.
What is the motion of an electron in a uniform electric field?
The motion of an electron in a uniform electric field is equivalent to a ball being thrown horizontally in the Earth's gravitational field - a parabolic shape.
Why is the motion of an electron in a uniform electric field that of a parabolic shape?
The electron is repelled by the negatively charged plate and attracted by the positively charged plate. The force on the electron is the same at all points.
Define Coulomb's law.
Any two point charges exert an electrical force on each other that is proportional to the product of their charges and inversely proportional to the square of the distance between them.
How does Newton's third law apply to two point charges Q and q, separated by a distance r?
The point charges interact with each other and, therefore, exert equal but opposite forces F on each other. Double the separation = ¼ of the force.
As F α Qq and F α 1/r^2, and. therefore, F α Qq/r^2, what equation does this result in?
F = kQq/r^2, where k=1/4πε0, where ε0=permittivity of free space.
Hence, the Coulomb's law equation is F=Qq/4πε0r^2.
When F is negative, how can we interpret this?
We interpret this as an attraction, as it means a positive and negative charge is involved.
How can we use Coulomb's law in uniformly charged spheres?
As they behave as though all of their charge was concentrated at the centre, we can measure the centre-to-centre distance between them.
How do we find the field strength near a point charge?
We have to imagine a small positive test charge placed in the field and determine the force per unit charge on it.
What is the equation for finding the electric field strength in a radial field?
As E= force/test charge:
E = Qq/4πε0r^2q.
E = Q/4πε0r^2/
Why is electrical field strength a vector?
Because force is a vector.
Where does the origin of gravitational fields and of electrical fields arise?
<b>Gravitational field -</b> from masses.
<b>Electric field -</b> from electric charges.
Describe the vector forces, if any, in gravitational and electric fields.
<b>Gravitational field -</b> only gravitational attraction, no repulsion.
<b>Electric field -</b> both electrical attraction and repulsion are possible.
Define field strength for gravitational and electric fields.
<b>Gravitational field:</b>
Field strength g=F/m.
Field strength is force per unit mass.
<b>Electric field:</b>
Field strength E=F/Q.
Field strength is force per unit charge.
What are the units of force and field strength in gravitational fields and electric fields?
<b>Gravitational field -</b> F in N, g in Nkg-1 or ms-2.
<b>Electric field -</b> F in N, E in NC-1 or Vm-1.
What are field lines like in uniform gravitational and uniform electric fields?
<b>For uniform gravitational fields:</b>
Parallel gravitational field lines.
g=constant.
<b>For uniform electric fields:</b>
Parallel electric field lines.
E=V/d=constant.
What are spherical gravitational and electric fields like?
<b>For spherical gravitational fields:</b>
Radial field lines.
<b>Force given by Newton's law:</b>
F=-GMm/r^2
<b>Field strength is therefore:</b>
g=-GM/r^2
Force and field strength obey an inverse square law with distance.
<b>For spherical electric fields:</b>
Radial field lines.
<b>Force given by Coulomb's law:</b>
F=Qq/4πε0r^2
<b>Field strength is therefore:
</b>E=Q/4πε0r^2
Force and field strength obey an inverse square law with distance.
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