Electric fields - A level Physics Edexcel

Define electric fields

A region in which a charge experiences a force

How do you work out the electric field?

Force per unit charge, the force which would act on a one coulomb charge at that position in the field

What can cause radial electric fields?

Charged particles (electrons, protons, particles and ions), charged spheres

The more densely packed the field lines the ________ the field

Stronger

On field line diagrams, what do the arrows show?

The direction of force on a positive charge placed in the field

What are stronger gravitational fields or electric fields?

Electric fields

What are on the axis of an inverse square law graph?

1/d^2 on the y axis and F on the x axis

What charge do alpha particles have?

2+, +2

You can put a charge anywhere in a uniform field and it will...

Experience the same force

Examples of point charges

Electrons, Protons, Van de graff generator, Positively charged sphere

How can you make a uniform field stronger?

By decreasing the plate separation and increasing the potential difference

What equations can you use for all types of electric fields?

E=F/q and F=kQq/r^2

What equation can you use in a uniform field?

E=V/d

What equation can you use in a radial field?

E=kQ/r^2

What is electrical levitation?

Where the point charge is hovering in equilibrium as Eq=W

If the charge between two parallel plates (eg Capacitor plates) is uniform a charge will...

experience the same force wherever it is placed between the plates.

What does the Electric field strength depend on?

The voltage between the plates and the distance between them.

Define capacitance

The ability of a body to hold on to electric charge/ the amount of electric charge for a given voltage.

What happens when a capacitor is charged?

Electrons move off one plate and onto the other. The capacitor remains neutral.

What factors determine the capacitance of the capacitor?

Surface area of the plates, Distance between the plates, Permittivity

The higher the permittivity of the dielectric, the greater...

its ability to transmit an electric field between the plates and the higher the capacitance.

What does an RC circuit do?

It charges the capacitor.

Why does the capacitor store only 1/2QV?

The rest of the energy goes to the resistor.

Define farad

One farad is equal to one coulomb per volt.

τ=

RC

What does time constant tell you?

Time constant gives you an indication of how long the capacitor takes to discharge.

Define time constant

The time constant is the time taken for V0 , I0 or Q0 to reduce to 37% of its original value when discharging a capacitor.

What are charge, current and voltage when looking at capacitors?

Exponential

What components only work with AC current?

Motors, Transformers, Mains

What components only work with DC current?

Capacitors, LEDs/Diodes, Batteries, Solar cells, Electronic

What is turning AC to DC current called?

Rectification

The root mean square value of an alternating current is...

the value of the direct current which would give the same heating effect (power) as the alternating current in the same resistor

What is an equipotential surface?

A region where the potential is equal. There is no potential difference when moving from one place on an equipotential surface to another.

Where is zero Ep set to in electric fields around a point charge?

Infinity

The electric potential at a certain position in an electric field is the...

work done per unit positive charge when a positive test charge is moved from infinity to that position

What is an electric field line?

It is the path that a single point charge would follow if placed in the field.

What is an equipotential line?

It is a line joining points of equal potential. Equipotential lines are always perpendicular to field lines. In a uniform field they are evenly spaced.

How can you tell the strength of an electric field from a field line diagram?

Stronger fields have more field lines per unit area - the field lines are closer together.

How can you find electric potential energy from an electric field strength against distance graph?

The area under the graph represents the energy transformed when we moved a charge from a one point to another in an electric field. We would have to do work against the force of the field so there is an energy change.

What is a uniform field?

A uniform field is one in which the electric field strength is constant. This means that the field lines must be parallel to each other and equally spaced.

How would you approximate a uniform electric field?

It can be approximated by placing two conducting plates parallel to each other and maintaining a voltage between them. It is only an approximation because of 'edge effects', therefore any experiments must avoid the edge area.

What happens to the field lines if two electric plates move together?

The field lines will get closer together. E=V/d so if d gets smaller, E gets bigger so the electric strength will increase.

What happens to a charged particle if it is placed between charged plates?

It accelerates towards the oppositely charged plate. In the diagram it has no initial horizontal velocity.

What happens to a charged particle if it enters the region between charged plates?

It continues to move horizontally at a steady rate while accelerating in the vertical plane. It therefore follows a parabolic path.

Give the equation relating to the electrostatic force between point charges in a vacuum

F= 1/4piε0 . Q1Q2/r^2

F= electrostatic force

Q1 and Q2 = charges

r = distance between their centres

ε0 = permittivity of free space (a vacuum)

If the electrostatic force F is negative what does that mean?

It means the force is attractive - the charges will be made to accelerate towards each other. One charge is positive and one is negative.

If the electrostatic force F is positive what does that mean?

It means the force is repulsive - the charges will be made to accelerate away from each other. Both charges are the same sign.

State Coulomb's law

The magnitude of the electrostatic force of interaction between two point charges is directly proportional to the scalar multiplication of the magnitudes of charges and inversely proportional to the square of the distances between them. F=ke q1q2/r^2 where q is charge, r is the seperation distance and ke is the proportionality constant.

What is a test charge?

A 'test charge' is used to determine the strength of a magnetic field. A 'test charge' should be positive. The direction it moves in then shows the direction of the electric field.

How is a test charge used?

It is placed in the electric field at a particular point and the force exerted on it is measured. The size of its charge and that force can be used to measure the electric field strength at that point.

Define electric field strength

The electric field strength is defined as force per unit charge. It is the force a test charge would experience if it was placed at that point in the electric field.

Is the electric field strength a scalar or a vector?

Vector so you have to add electric field strengths by resolving them into horizontal and vertical components and adding those.

What is a radial field?

A field where the field lines radiate outwards from a point charge. The radial electric field strength will therefore vary - getting weaker as you move away from the charge that is the source of the field.

Define a volt in an electric field

There is a potential difference of one volt between two points within an electric field, if one joule of energy is required to move a charge of one coulomb between those two points.

What will the voltage be if you bring a positive charge towards another positive charge?

Positive at any point in the positive charge's field as you will have to do work against repulsion

What will the voltage be if you bring a negative charge towards a positive charge?

Negative because you would not have to do work - it would be attracted - the energy you supplies would therefore be negative and the potential around a negative charge would be negative too.

Is electric potential vector or scalar?

Scalar as it is a point in space and points have no direction. This means that you can simply add potentials to get a net result.

What are the similarities between electric and gravitational fields?

Both inverse square law fields, both non-contact fields and both are infinite range forces

What are the differences between electric and gravitational fields?

Coulomb force relates to the interaction of charges whereas gravitational force relates to the interaction of masses

Masses always attract but charges may attract or repel

The Coulomb force is much stronger than the gravitational force - the constants of proportionality differ

What is the equation that compares work done in gravitational and electric fields

ΔW=mΔV

What are simple capacitors made from?

Big plates of metal close to each other but not touching

How do capacitors work?

The battery pushes electrons through the wire away from its negative terminal towards one of the plates. The plate gains more electrons and negatively charged. The electrons in the other plate are repelled by the build up of electrons in the first plate. So they move off the second plate. The electrons leaving the second plate complete the circuit.