Define an electric field .

A region where an electric charge experiences a force .

Do like charges attract or repel?

Like charges (positive-positive or negative-negative) repel each other.

Do opposite charges attract or repel?

Opposite charges (positive and negative) attract each other.

For a point outside a uniform spherical conductor , how may its charge be treated?

The charge may be treated as a point charge located at the centre of the sphere.

If a spherical conductor is positively charged, the field lines are directed .......... the centre of the sphere.

If a spherical conductor is positively charged, the field lines are directed away from the centre of the sphere.

True or False? A uniform spherical conductor must be considered as an extended charge distribution rather than a point charge when calculating fields outside it.

False. For a point outside it, a uniform spherical conductor's charge may be treated as a point charge at its centre, so its field lines are identical to those of a point charge.

What do electric field lines represent?

The direction and magnitude of an electric field.

In a uniform electric field, how do the field strength and force on a test charge vary across the field?

The field lines are equally spaced , so the electric field strength is constant and the force on a test charge has the same magnitude and direction at all points.

In a radial electric field, how does the field strength change with distance from the charge?

The field lines spread further apart with distance, so the electric field strength decreases with distance from the charge.

Around a point charge, field lines are directed radially .......... if the charge is positive.

Around a point charge, field lines are directed radially outwards if the charge is positive.

For two charges of the same type placed near each other, what is true of the point midway between them?

There is a neutral point at the midpoint where the resultant electric force is zero .

How are field lines directed between two parallel plates , and how is the field described?

Field lines run from the positive to the negative plate and are equally spaced, giving a uniform electric field between the plates.

True or False? Electric field lines are allowed to cross where two fields overlap.

False. Field lines must never cross , and they must touch the surface of the source charge or plate.

Define electric field strength .

The electrostatic force per unit positive charge acting on the charge at that point.

Why must a positive test charge be used when defining electric field strength?

Because it determines the direction of the electric field: away from a positive charge and towards a negative charge.

The greater the separation between two charged parallel plates, the .......... the electric field between them.

The greater the separation between two charged parallel plates, the weaker the electric field between them.

State Coulomb's law .

The electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation.

What happens to the electrostatic force between two point charges when their separation doubles ?

The force reduces by a factor of (½)² = ¼ , since it follows an inverse square law .

For like charges, the product Qq is positive and F is positive, so the charges .......... .

For like charges, the product Qq is positive and F is positive, so the charges repel .

True or False? Coulomb's law can be used to calculate the electrostatic force between charges distributed on irregularly-shaped objects.

False. Coulomb's law only applies to charged spheres whose size is much smaller than their separation, so the point charge approximation is valid.

What happens to the electric field strength around a point charge when the distance r is doubled ?

The field strength decreases by a factor of four , since it follows an inverse square law with distance.

If a point charge is negative , the electric field strength points .......... the centre of the charge.

If a point charge is negative , the electric field strength points towards the centre of the charge.

True or False? Electric field strength around a point charge is constant regardless of distance from the charge.

False. Field strength in a radial field is not constant — it decreases by a factor of 1/ r ² as distance from the charge increases.

Define gravitational field strength .

The gravitational force per unit mass exerted on a point mass.

Define electric field strength .

The electric force per unit charge exerted on a small positive test charge.

How does the direction of a gravitational field compare to that of an electric field around a positive charge?

A gravitational field always points towards the mass, whereas an electric field points away from a positive charge (and towards a negative charge).

Gravitational forces are .......... attractive and cannot be repulsive.

Gravitational forces are always attractive and cannot be repulsive.

Name three key similarities between gravitational and electric fields.

Any three from: both have an inverse square law force between two point masses/charges; radial field lines around a point mass and negative point charge are identical; field lines in a uniform field are identical; both g and E have a 1/ r ² relationship in a radial field; both gravitational and electric potential have a 1/ r relationship; equipotential surfaces are spherical around a point mass/charge and equally spaced parallel lines in uniform fields.

True or False? Gravitational potential can be either positive or negative, just like electric potential.

False. Gravitational potential is always negative , whereas electric potential can be either negative or positive .

Define relative permittivity .

Relative permittivity ( ε r ) is the ratio of the permittivity of a material to the permittivity of free space . It has no units, as it is a ratio of two quantities with the same unit.

In a dielectric, aligned polar molecules produce their own electric field that .......... the field from the capacitor plates.

In a dielectric, aligned polar molecules produce their own electric field that opposes the field from the capacitor plates.

True or False? Inserting a dielectric between charged capacitor plates decreases the capacitance.

False. The dielectric's opposing field reduces the potential difference between the plates, which increases the capacitance.

How does a charged particle move if it is initially stationary in a uniform electric field?

It moves parallel to the field lines, either along or against them depending on the sign of its charge.

What path is followed by a charged particle that enters a uniform electric field travelling perpendicular to the field lines?

A parabolic trajectory , since the particle experiences a constant electric force perpendicular to its initial velocity.

In a uniform electric field between charged plates, towards which plate is a positively charged particle deflected?

Towards the negative plate (a negatively charged particle is deflected towards the positive plate).

Name three properties of a charged particle that affect the amount of deflection it undergoes in a uniform electric field.

Mass (greater mass gives smaller deflection), charge (greater charge gives greater deflection) and speed (greater speed gives smaller deflection).

True or False? An uncharged particle, such as a neutron, is deflected when it passes through a uniform electric field.

False. An uncharged particle experiences no force in an electric field, so it travels straight through undeflected .

A LevelPhysicsOCRFlashcards6. Particles & Medical PhysicsElectric Fields

Electric Fields (OCR A Level Physics): Flashcards

Exam code: H556

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Define an electric field.

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Cards in this collection (45)

Define an electric field.
A region where an electric charge experiences a force.
Do like charges attract or repel?
Like charges (positive-positive or negative-negative) repel each other.
Do opposite charges attract or repel?
Opposite charges (positive and negative) attract each other.
For a point outside a uniform spherical conductor, how may its charge be treated?
The charge may be treated as a point charge located at the centre of the sphere.
If a spherical conductor is positively charged, the field lines are directed .......... the centre of the sphere.
If a spherical conductor is positively charged, the field lines are directed away from the centre of the sphere.
True or False?
A uniform spherical conductor must be considered as an extended charge distribution rather than a point charge when calculating fields outside it.
False.
For a point outside it, a uniform spherical conductor's charge may be treated as a point charge at its centre, so its field lines are identical to those of a point charge.
What do electric field lines represent?
The direction and magnitude of an electric field.
In a uniform electric field, how do the field strength and force on a test charge vary across the field?
The field lines are equally spaced, so the electric field strength is constant and the force on a test charge has the same magnitude and direction at all points.
In a radial electric field, how does the field strength change with distance from the charge?
The field lines spread further apart with distance, so the electric field strength decreases with distance from the charge.
Around a point charge, field lines are directed radially .......... if the charge is positive.
Around a point charge, field lines are directed radially outwards if the charge is positive.
For two charges of the same type placed near each other, what is true of the point midway between them?
There is a neutral point at the midpoint where the resultant electric force is zero.
How are field lines directed between two parallel plates, and how is the field described?
Field lines run from the positive to the negative plate and are equally spaced, giving a uniform electric field between the plates.
True or False?
Electric field lines are allowed to cross where two fields overlap.
False.
Field lines must never cross, and they must touch the surface of the source charge or plate.
Define electric field strength.
The electrostatic force per unit positive charge acting on the charge at that point.
What is the equation for electric field strength, E, in terms of force, F, and charge, Q?
$E = \frac{F}{Q}$
Why must a positive test charge be used when defining electric field strength?
Because it determines the direction of the electric field: away from a positive charge and towards a negative charge.
What is the equation for electric field strength between two parallel plates?
$E = \frac{V}{d}$ where V is the potential difference between the plates and d is their separation.
The greater the separation between two charged parallel plates, the .......... the electric field between them.
The greater the separation between two charged parallel plates, the weaker the electric field between them.
True or False?
The equation $E = \frac{V}{d}$ can be used to find the electric field strength around a point charge.
False.
This equation only applies to the uniform field between parallel plates; a point charge produces a radial field, where $E = \frac{F}{Q}$ or $E = \frac{Q}{4 π ϵ_{0} r^{2}}$ must be used instead.
State Coulomb's law.
The electrostatic force between two point charges is proportional to the product of the charges and inversely proportional to the square of their separation.
What is the equation for the electrostatic force, F, between two point charges Q and q?
$F = \frac{Q q}{4 π ϵ_{0} r^{2}}$
What happens to the electrostatic force between two point charges when their separation doubles?
The force reduces by a factor of (½)² = ¼, since it follows an inverse square law.
For like charges, the product Qq is positive and F is positive, so the charges ...........
For like charges, the product Qq is positive and F is positive, so the charges repel.
What is the value of ε₀, the permittivity of free space?
$ϵ_{0} = 8.85 \times 10^{- 12} C^{2} N^{- 1} m^{- 2}$
True or False?
Coulomb's law can be used to calculate the electrostatic force between charges distributed on irregularly-shaped objects.
False.
Coulomb's law only applies to charged spheres whose size is much smaller than their separation, so the point charge approximation is valid.
What is the equation for the electric field strength, E, at distance r from a point charge Q?
$E = \frac{Q}{4 π ϵ_{0} r^{2}}$
What happens to the electric field strength around a point charge when the distance r is doubled?
The field strength decreases by a factor of four, since it follows an inverse square law with distance.
If a point charge is negative, the electric field strength points .......... the centre of the charge.
If a point charge is negative, the electric field strength points towards the centre of the charge.
Why can the equation $E = \frac{Q}{4 π ϵ_{0} r^{2}}$ only be used around a point charge?
Because it describes a radial field, which is only produced by a point charge (or a charged sphere treated as one).
True or False?
Electric field strength around a point charge is constant regardless of distance from the charge.
False.
Field strength in a radial field is not constant — it decreases by a factor of 1/r² as distance from the charge increases.
Define gravitational field strength.
The gravitational force per unit mass exerted on a point mass.
Define electric field strength.
The electric force per unit charge exerted on a small positive test charge.
How does the direction of a gravitational field compare to that of an electric field around a positive charge?
A gravitational field always points towards the mass, whereas an electric field points away from a positive charge (and towards a negative charge).
Gravitational forces are .......... attractive and cannot be repulsive.
Gravitational forces are always attractive and cannot be repulsive.
Name three key similarities between gravitational and electric fields.
Any three from: both have an inverse square law force between two point masses/charges; radial field lines around a point mass and negative point charge are identical; field lines in a uniform field are identical; both g and E have a 1/r² relationship in a radial field; both gravitational and electric potential have a 1/r relationship; equipotential surfaces are spherical around a point mass/charge and equally spaced parallel lines in uniform fields.
True or False?
Gravitational potential can be either positive or negative, just like electric potential.
False.
Gravitational potential is always negative, whereas electric potential can be either negative or positive.
Define relative permittivity.
Relative permittivity (ε_r) is the ratio of the permittivity of a material to the permittivity of free space. It has no units, as it is a ratio of two quantities with the same unit.
What equation gives the capacitance of a parallel plate capacitor containing a dielectric of relative permittivity ε_r?
$C = \frac{ε_{r} ε_{0} A}{d}$
Where A is the cross-sectional area of one plate and d is the plate separation.
In a dielectric, aligned polar molecules produce their own electric field that .......... the field from the capacitor plates.
In a dielectric, aligned polar molecules produce their own electric field that opposes the field from the capacitor plates.
True or False?
Inserting a dielectric between charged capacitor plates decreases the capacitance.
False.
The dielectric's opposing field reduces the potential difference between the plates, which increases the capacitance.
How does a charged particle move if it is initially stationary in a uniform electric field?
It moves parallel to the field lines, either along or against them depending on the sign of its charge.
What path is followed by a charged particle that enters a uniform electric field travelling perpendicular to the field lines?
A parabolic trajectory, since the particle experiences a constant electric force perpendicular to its initial velocity.
In a uniform electric field between charged plates, towards which plate is a positively charged particle deflected?
Towards the negative plate (a negatively charged particle is deflected towards the positive plate).
Name three properties of a charged particle that affect the amount of deflection it undergoes in a uniform electric field.
Mass (greater mass gives smaller deflection), charge (greater charge gives greater deflection) and speed (greater speed gives smaller deflection).
True or False?
An uncharged particle, such as a neutron, is deflected when it passes through a uniform electric field.
False.
An uncharged particle experiences no force in an electric field, so it travels straight through undeflected.

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