Electric Fields/Magnetic Fields/Gravitational Fields

What is a force field?

• a region in which a non-contact force is experienced by an corresponding interacting particle

what is an electric field

• a region in which a charged particle will experience a non-contact force

what is electric field strength?

• force per unit charge experienced by a charged particle when placed in field

what is the equation used to calculate the force a charge experiences in an electric field

F=EQ

State Coulomb’s law

• the force between two charges in directly proportional to the product of their charges and inversely proportional to the square of their separation

Describe the electric field pattern around a positive point charge

• a radial field, acting outwards

• if around negative, then points inward

describe the uniform electric field between parallel plates

• points from positive to negative, field lines are equally distant from each other

what equation is used to calculate the electric field between parallel plates

• E = V/d

• what is electric potential

• the amount of energy required to move a positive test charge from infinity to a given point in a n electric field

What are equipotentials? How to calculate work done in moving charge along an equipotential

• lines along which the electric potential remains the same

• no work is done as electric potential doesn’t charge

What is a magnetic field?

• a region which a magnetic material experiences a non-contact force

What is magnetic flux density?

• a measure of strength of a magnetic field

• number of magnetic field lines that pass through a given pass through a given area

What are the factors that determine the force exerted on a charge moving through a magnetic field?

• magnetic flux density

• charge

• velocity of particle (perpendicular to field)

what is the equation used to calculate the force exerted on a charge moving through a magnetic field

F = Bqvsin(theta)

Describe and explain the path taken by a charge in a magnetic field

• the charge will move in a circular path

• because the magnetic force always acts perpendicular to the charge’s motion and so acts as a centripetal force

What occurs when a current-carrying wire is placed in a magnetic field

• wire will experience a force due to the permanent magnetic field interacting with the wire’s magnetic field

What are the factors that affect the force experiences by a current-carrying wire placed in a magnetic field?

• the magnetic flux density of the field

• the current passing through the wire

• the length of the wire

• F=BILsintheta - for a wire

• F = BvQ for a charged particle

What is Lenz’s law?

• the direction of induced current is opposite to the motion causing it

• can use to predict the direction of an induced emf

what does the distance between field lines represent in a uniform field?

• the magnitude of force

• the further away the field lines are, the less the force particle experience

explain electromagnetic induction

• when a conducting rod moves relative to a magnetic field, the electrons in the rod will experience a force ad build up on one side, which causes an emf to be induced in the rod

• also occurs if you move a magnetic relative to a coil wire, as the coil forms a complete circuit, a current is induced

what is Faraday’s law

• the magnitude of induced emf is equal to the rate of change of flux linkage

Explain why magnet takes longer to reach the ground when it moves through coil (using Lenz’s law)

• as magnet approaches the coil, there is a change of flux through the coil so an emf and current is induced

• due to Lenz’s law, the direction of induced current is such as to oppose the motion of magnet, so the same pole as the pole of magnet approaching the coil will be induced to repel the magnet. this causes magnet to slow down, due to electrostatic forces of repulsion

• as magnet passes through centre of coil, there is no change in flux, so no emf is induced

• as magnet leaves coil, there is a change in flux, current induced opposite the motion of magnet, so an opposite pole is induced by magnet causing it to slow down again due to electrostatic force of attraction.

Lenz’s law is a direct consequence of…

conservation of energy

• it ensures that the electrical energy gained by the induction of a current is offset by an equal amount of energy being removed.

What does Newton;s Law of Gravitation state

• the gravitational force between two point masses is proportional to the product of their masses and is inversely proportional to the square of the separation of their centres.

what is always true about the gravitational force between two masses

• it is always an attractive force

what is a gravitational field?

• a region in which any object with mass will experience a non-contact force

State the equation for gravitational potential

State the equation for gravitational potential energy

• V= - GM/r

• E = -GMm/r

what is gravity

• the universal attractive force which acts between all matter

What is gravitational potential?

• the potential energy per unit mass. at any point in field

• zero potential is defined at infinity, hence at a point close to a mass the potential of an object would be negative

how to calculate work done by moving mass in a field

• work done = mass*change in gravitational potential

why is gravitational potential a negative value

• work needs to be done to move an object from the inside the field to outside the field.

• since outside the field’s potential is defined as zero then the potential inside the field must be negative

how is the orbital period related to the radius of a circular orbit

T² is proportional to R³

What type of matter is affected by a gravitational field

any object or matter with mass will experience an attractive force

What is a point mass?

• a theoretical object that has a mass but no dimensions

• all of its mass acts at a single point

compare electric fields and gravitational field

• gravitational fields are regions in which a mass experiences a force due to its mass whereas electric fields are regions in which a charge experiences a force due to its charge

• both types of field have an infinite range

• in each type of field the force varies as an inverse square

• the force between masses is always attractive whereas the force between charges can be attractive or repulsive

What is special about uniform field on parallel plates

• field strength is the same at all points

How to calculate root mean square of voltage/current

• Vrms = V0 /root 2, V0 is the peak voltage, so if see question with peak voltage/peak current, use root mean square values

How does charging an electric toothbrush work?

• The supply creates a changing magnetic field in the iron core

• Rate of change of flux in toothbrush coil is equal to rate of change of flux in charger coil (for an ideal transformer)

• The changing flux linkage in the coil of the toothbrush induces an e.m.f. according to Faraday’s law

• E = – N dphi/dt so to step down the e.m.f. there must be fewer turns in the toothbrush coil

• The e.m.f. in the toothbrush coil must be larger than the toothbrush battery

• Diode is included so battery is not discharged by the alternating e.m.f.

What is the neutral point?

• when you have two magnetic put close together, the result shape of field has neutral points where two magnetic fields cancel out

Why in a electron beam deflection, electron travel in a circular path?

• magnetic force always acts in the perpendicular direction to the field and velocity

• Magnetic force always acts towards the center of the circle

• so provides a centripetal force on the electron

Why do electrons have constant kinetic energy and speed as they move in a circle in the electron beam setup?

• the magnetic force is always perpendicular to the path of the electrons, so this force does not affect their speed.

For an electron experiencing both electric and magnetic fields, what is the condition to travel in a straight line with out deflection?

Electric force = magnetic force

What do transformer do?

• They increase or decrease the voltage of alternating current electrical power supplies. They do not work with direct current power supplies

• In all transformers, the primary coil produces a magnetic field

• this magnetic field links with the secondary coil

• with alternating currents, the magnetic field of primary coil varies continuously

• so magnetic flux linkage through the secondary coil varies continuously

• the change of this magnetic flux linkage induces a varying emf in the secondary coil

• Vs/Vp = Ns/Np = Ip/Is

Why is laminated iron sheets used in iron core? Why iron core used in transformer?

• to reduce energy loss- by preventing eddy currents to form which disturb flux linkage and have heating effects

• to increase magnetic flux

When object position parallel to magnetic field, what is the induced emf?

• 0V

• because the object does not cur magnetic field, so there is no change in flux

When you have an alternating current in a coil, how is the current of the object induced

• there is a change in magnetic field

• field cuts across conductor

• magnetic flux linkage changes

• emf induced which causes current to induce

Explain the jumping metal, and why immediately falls back

• B field is Hoproduced by solenoid

• flux lines cut washer

• induced emf and the current in the washer

• B field of solenoid opposite to B field of the washer

• there’s a repulsive force that lift the washer

• after, there’s a steady current, so no changing of flux

How does a loudspeaker convert sound waves into electrical signals

• sound waves make the diaphragm vibrate

• there is a change in flux linkage as coil cuts field lines

• this induces voltage across coil

• frequency of sound wave is frequency of induced voltage

Explain how magnetic slows down in copper tube, and why not plastic

• the magnetic flux cuts copper tube

• induction occurs, induces current in copper tube

• which creates a magnetic field

• that is the opposite direction to the magnetic’s magnetic field

• which repels and slows down the magnet

• there’s no induction effect for a plastic tube

Explain why the magnitude of the current varies as magnet moves upward

• flux density at coil changes with distance

• so there’s change in magnetic flux

why is there an alternating current in a coil when magnet oscillates

• there is a change in magnetic field around the ring due to the oscillating magnet

• induces emf

• changing direction of magnet changes the direction of emf

• magnitude of emf depends on the rate of change in magnetic flux linkage/ speed of magnet

why dc current does not give emf reading

• there must be a change in magnetic flux

• but current is constant, there isn’t a change in magnetic field

three ways of inducing emf

• move coil in and out of field

• use ac instead of dc

• switch current on and off/variable resistor

remember to mention when see iron core

• iron core becomes magnetised/ increases magnetic field

• there is a magnetic field in the core

Explain the shape of the graph when magnet drops to a coil

• emf increases as magnet gets closer

• coil cuts the field

• emf = - dNphi/dt

• negative emf because of Lez’s law, opposite direction as the change in magnetic flux, as magnetic flux direction changed, induced emf changes

• the speed of magnet increases as magnet falls, emf increases with speed

• negative maximum > positive graph maximum because of magnet leaving at faster speed

• hence less time for magnet to leave than enter, so the negative part is less wide

• but the area of both part is the same because area is proportional to the change in flux

Does frequency affect emf

yes

both frequency and current affect emf

Explain how using the motor as generator slow the car down

suggest why regenerative braking will not fully stop a car

• energy is required to turn generator

• this energy is transferred form kinetic energy

• greater KE transfer at higher speed, so when the speed is slow, the braking effect becomes negligible