Magnetic Fields IB Physics

Daily Question #5Daily Question #5

a) What is the magnitude of the electric

field at a point which is 2.0m away from

the centre of a sphere carrying a

charge of +7.3×10-5C?

b)What would be the force on a

charge of +0.56µC placed at that point?

MagneticMagnetic FieldField

How are they produced?

By moving electric charge

Electric current Electron in atom

Flow of moving charge Field around a permanent

magnet has its origin in the

movement of electrons in certain atoms. (ferrous)

Magnetic Field LinesMagnetic Field Lines

By convention the direction of a magnetic field is

from magnetic north to a magnetic south

Density of lines represent strength of field

Earth’s Magnetic Field

Straight wire carrying currentStraight wire carrying current

circular field around the wire

field strength is proportional to the current and

inversely proportional to distance from the wire

direction of field lines can be predicted using

‘right-hand grip rule’

depends on the medium around the wire.

‘Right-hand grip rule’ (the thumb points in the

conventional current direction

and fingers indicate direction of

the magnetic field)

Magnetic Field PatternMagnetic Field Pattern

Single turn of wire

Magnetic Field PatternMagnetic Field Pattern

Magnetic Field PatternMagnetic Field Pattern

Solenoid

long coil of wire with turns which don’t overlap (helical)

Strong uniform magnetic field inside the coil (similar to a permanent bar magnet field pattern)

Magnetic Field PatternMagnetic Field Pattern

Solenoid

Magnetic field

Increasing the current will increase

strength of magnetic field

the more turns of wire, the stronger the

magnetic field

Direction can be found using ‘right-hand

grip rule’

Reversing the direction of the current,

reverses the polarity of the magnetic field.

Magnetic Field PatternMagnetic Field Pattern

Solenoid

A copper wire is placed in a magnetic field.

a) Does copper feel a force from the magnet?

b) When a current passes through it, does it

produce its own magnetic field? If yes, what

is its pattern?

.

Daily Question #6Daily Question #6

When a current-carrying wire is placed in a

magnetic field, the magnetic interaction between

the two results in a force.

Force is maximum when current and field are

at right angles

Force is zero if the current is parallel to the field.

The direction of the force, current or magnetic field is given by Fleming’s left-hand rule:

Force (thumb)

Current (second finger)

Field (first finger)

It only applies if current and field are at right angles.

Force on a currentForce on a current--carrying wirecarrying wire

Experiments show that the force is

proportional to:

magnitude of magnetic field, B

magnitude of current, I

length of the wire, L sine of the angle θ, between the

field and current

Magnetic Field StrengthMagnetic Field Strength

BILsinFor sin

IL

FB

B – magnetic field strength

(magnetic flux density)

θ – angle between the

current and the direction of

the magnetic field

I – current

L – length of the conductor 1T (Tesla) = 1NA-1m-1

DB

Daily Question #7Daily Question #7

A measured current is flowing across a small, uniform

magnetic field.

a) In which direction is the force on the wire?

b) In which direction is the force on the balance?

c) When the current is flowing, the balance indicates that

there is an extra mass of 4.20×10-2g on the balance. What

extra downward force is this?

d) If the current is 1.64A and the length of the field 8.13cm,

what is the strength of the magnetic field?

N

MagneticMagnetic Force Force onon a a movingmoving chargecharge

A single charge moving through a magnetic field also feels a force, proportional to:

magnitude of magnetic field, B

magnitude of the charge, q

the velocity of the charge, v sine of the angle θ, between the

velocity of the charge and the field

sinBqvF

θ – angle between the

velocity of the charge and

the magnetic field

v – velocity

q - charge

MagneticMagnetic Force Force onon a a movingmoving chargecharge

t

qI

t

Lv

Since the force on a moving charge is

always at right angles to the velocity of the

charge the resultant motion can be circular.

MagneticMagnetic Force Force onon a a movingmoving chargecharge

Left-hand rule (remember that the conventional direction of

current is that of the movement of positive charges)

MagneticMagnetic Force Force onon a a movingmoving chargecharge

Magnetic force is zero if:

charge moves parallel or

antiparallel to the magnetic field;

charge is not moving;

electrically neutral particles (q=0)

The figure shows a uniform magnetic field. Which of the three paths—1, 2, or 3—does the particle follow?

Daily Question #8Daily Question #8

+

ParallelParallel conductorsconductors

Two wires carrying currents will

produce magnetic fields which

interact and exert mutual forces if

they are brought close together.

By Newton’s 3rd Law, forces are

equal.

Antiparallel wires

Parallel or Antiparallel wires?

Definition of AmpereDefinition of Ampere

If wires are 1 m apart and force is 2×10-7 N, then wires are both

carrying a current of 1A.

CoulombCoulomb

The Coulomb can now be defined as

charge transferred when 1Ampere flows for

1 second.