Magnetic fields

What does the field pattern around a straight, current-carrying wire look like?

What does the field pattern around a flat coil look like?

What does the field pattern around a solenoid look like? How do you tell which end is the north and south pole?

What does the field pattern around a bar magnet look like?

What does the field pattern around 2 bar magnets attracting look like?

What does the field pattern around 2 bar magnets repelling look like?

How do you draw a magnetic field line going into a page?

How do you draw a magnetic field line coming out of the page?

How do you draw a wire with current going into a page?

How do you draw a wire with current coming out of the page?

What are the units of Magnetic Field Strength?

tesla (T)

Describe how you can predict the direction a wire carrying a current in a magnetic field will move

Flemings left hand rule

What is Fleming’s Left Hand rule?

What is Fleming’s right hand grip rule and when is it used?

Used to determine the direction of the magnetic field arround a current-carrying wire

What factors affect the size of the Force on a current carrying wire in a magnetic field?

• Strength of magnetic field

• current in the wire, I

• length L of the wire in the magnetic field

• the angle between the magnetic field and the current direction, θ

What equation links the factors that affect the size of the Force on a current carrying wire in a magnetic field, and what is the meaning and unit of each term?

F = BIL sin θ

• F is the force experienced by the wire in newtons

• B is the magnetic flux density in telsa

• I is the current in the wire in amperes

• L is the length of the wire in the magnetic field in meters

• θ is the angle between the magnetic field and the current direction in degrees

Define magnetic flux density

The strength of a magnetic field

Is Magnetic Flux density a scalar or a vector?

Vector

Define the tesla

1T is the magnetic flux density when a wire carrying a current of 1 A

placed perpendicular to the magnetic field experiences a force of 1N

per metre of its length.

Describe a practical to determine magnetic flux density

• The magnets are placed on a top-pan balance and a stiff copper wire is held perpendicular to the magnetic field between the two poles (as shown in diagram)

• The length L of the wire in the magnetic field is measured with a ruler.

• Using crocodile clips, a section of the wire is connected in series with an ammeter and a variable power supply. The balance is zeroed when there is no current in the wire.

• With a current I, the wire experiences a vertical upward force (predicted by Fleming's left-hand rule). According to Newton's third law of motion, the magnets experience an equal downward force, F.

• This force F can be calculated from the change in the mass reading, m, using F = mg, where g is the acceleration of free fall

• The magnetic flux density B between the magnets can then be determined from the equation B = F/IL

Describe the direction of flow of conventional current compared to the direction of electron flow?

Opposite to each other:

• Convetional is ‘the flow of positive charge’ so goes from positive to negative

• Electron flow goes from negative to positive

If electrons are moving as particles, not in a conductor, then which way should you point your second finger when using Fleming’s Left hand rule?

In the direction the electrons are moving

What direction will the force on the electrons be relative to their motion in a uniform magnetic field?

Tp be completed

Derive F = Bev from F = BIL

F = BIL

v = L / t so L = vt

I = Q / t

∴ F = B (Q / t) (vt) = BQv

The charge, Q, of electrons and protons is e

∴ F = Bev

If there is a centripetal force provided by the magnetic field show how the radius of the path will change if the particle:

1. Moves faster

2. Is more massive

3. There is a stronger magnetic field.

r = mV / BQ, so:

1. faster-moving particles travel in bigger circles (r ∝ v)

2. more massive particles move in bigger circles (r ∝ m)

3. stronger magnetic fields make the particles move in smaller circles (r ∝ 1/B)

What is the difference between magnetic flux density B, magnetic flux Φ and magnetic flux linkage NΦ?

To be completed

Define magnetic flux

The product of the component of the magnetic flux density perpendicular to a given area and that cross-sectional area:

What is the equation for magnetic flux?

ϕ = BA cos θ

What is the unit of magnetic flux?

Wb (weber)

Define magnetic flux linkage

The product of the number of turns in a coil and the magnetic flux

What is electromagnetic induction?

The creation of an electric current in a wire when it is moved through a magnetic field

What is Faraday’s Law?

The magnitude of the induced e.m.f. is directly proportional to the rate of change of magnetic flux linkage

(so ε ∝ ΔNϕ / Δt)

What is Lenz’s Law?

The direction of the induced e.m.f. or current is always such as to oppose the change producing it