Magnetic fields

In what two ways can you make a magnetic field?

1) By using a permanent magnet.

2) By using an electric current.

In what direction do magnetic field lines travel?

From north to south.

In a magnetic field

where is it strongest?

What is a solenoid?

An electromagnet that uses a coil to concentrate the electric field.

What does a solenoid do?

Makes use of the magnetic field created by an electric current.

What are a solenoid's field lines like?

Like that of a bar magnets'.

What are magnetic field lines like for a long

current-carrying wire?

What happens to the direction of the field if you reverse the current?

It reverses.

What are all magnetic fields created by?

Moving charges.

What are the moving charges in a wire?

The free electrons.

What are the moving charges in a permanent magnet?

The movement of electrons within the atoms of the magnet. Each electron represents a tiny current as it circulates within its atom

Why are non-magnetic materials non-magnetic if they have moving charges?

The fields produced by the electrons cancel each other out.

What does Θ indicate?

Current out of the plane of the paper.

What does ⊗indicate?

Current into the plane of the paper.

What does doing the right hand grip rule tell you?

The direction of the magnetic field lines inside an electromagnet.

How do you demonstrate the right hand grip rule?

• Imagine gripping the coil so that your fingers go around it

• Your thumb points in the direction of the field lines inside the coil - towards the north pole.

What does doing the right hand corkscrew rule tell you?

Gives the direction of field lines around a long

How do you demonstrate the right hand corkscrew rule?

• Imagine pushing a corkscrew into a cork

• The direction you push is the conventional current.

• The field lines go around the direction you're turning the corkscrew.

What does Fleming's left hand (motor) rule tell us?

The direction of force on a current-carrying conductor:

What does the thumb

first finger

First finger - magnetic flux density

B

Second finger - direction of conventional current

I.

What is the strength of a magnetic field?

Its magnetic flux density.

What can a field's magnetic flux density be imagined as representing?

It can be imagined as representing the number of magnetic field lines passing through a region per unit area.

What is the symbol for and units of magnetic flux density?

Symbol = B.

Units = Tesla (T).

If you want to figure out the magnetic flux density and the magnetic field is at right angles to the current

which equation do you use?

where F = force experienced by a conductor

I = current in a conductor

Define B.

B is 1T when a wire carrying current of 1A is placed at right angles to the magnetic field

What happens when an electric current cuts across magnetic field lines?

A force is exerted on the current-carrying conductor.

What is the force when the current of a conductor is parallel to the magnetic field?

0N.

Define torque.

One of the forces * perpendicular distance between the forces.

How do you calculate the force F when the current is passing through the magnetic field at an angle other than 90°?

We need to find the component of B at right angles to the current

In which direction in relation to the conventional current do electrons flow?

Oppositely.

What happens to the force electrons in a wire experience when they flow across a magnetic field?

They transfer this force to the wire itself.

How do we create a beam of electrons?

Beams of electrons are created by having an electron gun with a heated cathode attached to it. A positively charged anode attracts electrons from the negatively charged cathode

How can we change a beam of electron's direction?

By using an electric field between two plates or a magnetic field created by electromagnetic coils.

What three things does the size of a force on a moving charge in a uniform magnetic field depend on?

1) Magnetic flux density

2) Charge Q on the particle.

3) Speed v of the particle.

If magnetic force F on a particle moving at right angles to a magnetic field depends on B

Q

What is the path described by a charged particle moving at right angles to a uniform magnetic field?

The arc of a circle because F is always at 90° to the velocity of the particle.

What is the equation linking F

B

How do we link F=BIL and F=BQv?

1) I is the rate of flow of charge: I=Q/t.

2) Substituting this gives us F=BQL/t.

3) L/t = speed v of the moving particle.

4) Substituting gives us F=BQv.

What is the equation

for an electron

For a charged moving particle

what is F?

In a fine-beam tube

what happens to the beam of electrons that enters the spherical tube (that has a uniform magnetic field)?

What is the centripetal force mv^2/r equal to?

The magnetic force F = Bev.

What equation links the mv^2/r and Bev?

r = mv/Be.

What three things does the equation r=mv/Be show?

1) Faster-moving particles move in bigger circles (r α v)

2) Heavier particles also move in bigger circles

as they have more inertia (r α m)

3) A stronger field makes particles move in smaller circles (r α 1/B)

What is the momentum of a particle in a magnetic field?

Momentum ρ of a particle = Ber.

What happens if the beam of electrons in a fine-beam tube is tilted?

The electrons have a component of velocity along the magnetic field

What does a deflection tube show?

A beam of electrons passing through a combination of electric and magnetic fields.

When will the beam in a deflection tube remain horizontal?

When the electric force and the magnetic force are equal and act in opposite directions.

When electrons travel from a cathode (-ve) to an anode (+ve)

what is the voltage?

What is work done on an electron when it accelerates from a cathode to an anode and why?

Work done is eVca

What is the work done on an electron when it accelerates from a cathode to an anode equal to?

Its kinetic energy: eVca = 1/2mv^2.

What is the equation meaning electric force upward = magnetic force downward?

eE = Bev.

What is speed v in a deflection tube related to and what is the equation encompassing this?

The strengths of the two fields. v = E/B.

As electric field strength E = V/d

what does the velocity v in a deflection tube equal?

How do you calculate the charge/mass ratio of an electron?

Substitute v into eVca = ½mv^2 to get e/m = v^2/2VcaB^2d^2.

What do mass spectrometers do?

Separate ions of different masses and charges by passing them through a uniform magnetic field.

How do mass spectrometers work?

Molecules in a mass spectrometer are given a single positive charge +e in the ion source

What is the deflection in a mass spectrometer of heavier ions?

Less than that of lighter ions.

Why are heavier ions deflected less?

Because r = mv/BQ