5.4 Magnetic effects of electric currents

ALL magnets have two polesNORTH seeking pole

SOUTH seeking pole

Breaking a magnet produces two magnets!

N S

N S

NN SS

Opposites attract!

Opposite poles attract and like poles repel

(Ferro) Magnetic materials

(Ferro) Magnetic materials

Iron (steel), Cobalt and Nickel

Magnetic induction

Magnetic induction

When a magnetic material is close to a magnet, it becomes a magnet itself

We say it has induced magnetism

NSNS

magnet

Hard and Soft Magnetism

Soft Magnetism

Pure iron is a soft magnetic material

It is easy to magnetise but loses its magnetism easily

NS

before after

Iron nail

SN

NS

Not a magnet

N

Hard Magnetism

Steel is a hard magnetic material

It is harder to magnetise, but keeps its magnetism (it is used to make magnets!)

NS

before after

Steel paper clip

NNS

It’s a magnet!

N

S

S N

Magnetic field

Magnets and electric currents produce magnetic fields around them.

In a magnetic field, another magnet, a magnetic material or a moving charge will experience a magnetic force.

www.physchem.co.za

Magnetic field lines

We can represent the magnetic field around a wire or magnet using field lines.

Magnetic field lines

The arrows show the direction a compass needle would point at that point in the field.

Magnetic field lines

The closer the field lines are, the stronger the magnetic force felt

The arrows show the direction a compass needle would point at that point in the field.

Note that magnetic field is a vector quantity

Earth’s Magnetic Field

N

S

Remember the North of a compass needle points to the geographic north pole (i.e. the geographic North pole is a magnetic south pole!)

Moving charges (currents)

Moving charges (electric currents) also produce a magnetic field

http://www.sciencebuddies.org

Conventional current – electrons flow in the opposite direction

Magnetic field around a straight wire

Stronger field closer to wire

Magnetic field around a flat circular coil

http://physicsed.buffalostate.edu

Magnetic field around a solenoid

The Motor Effect

When a current is placed in a magnetic field it will experience a force. This is called the motor effect.

The Motor Effect

The direction of the force on a current in a magnetic field is given by Flemming’s left hand rule.

Centre finger = Conventional Current

First finger = Field direction

Thumb = Motion

Remember, conventional current is opposite the flow of electrons!!

Sample question

In this example, which way will the wire be pushed? (red is north on the magnets)

Another sample question!

An electron approaches a bar magnet as shown. What is the direction of the force on the electron?

NS

Defining Magnetic Field BThe size of the force on a wire in a field

depends on the size of the field (B), the length of wire in the field (L) and the current in the wire (I)

Defining Magnetic Field B

In other words , F α BIL, or F = kBIL

Defining Magnetic Field BF = kBIL

We can make k = 1 by defining the Tesla as the magnetic field when the force on 1 m of wire carrying a current of 1 A is 1 N.

Force on a current in a fieldThus the force on a length L of wire

carrying a current I in a magnetic field B is given by F = BILsinθ where θ is the angle between the current and the magnetic field.

The force on a moving charge in a magnetic field

Since a current experiences a force in a magnetic field, and a current is just made of moving charges, moving charges themselves must experience a force in a magnetic field.

www.nearingzero.net

The force on a moving charge in a magnetic field

Consider a positive charge q moving with speed v.

vq

Magnetic field B out of the slide

The force on a moving charge in a magnetic field

In time Δt the charge will have moved a distance L = vΔt

vq

The force on a moving charge in a magnetic field

The current is given by I = q/Δt

vq

The force on a moving charge in a magnetic field

Given that F = BILsinθ F = B(q/Δt)vΔt = qvBsinθ

vq

The force on a moving charge in a magnetic field

The fact that this force is always at right angles to the velocity means that the charge will move in a circle (if the speed is constant)

vq

Note; If the force is

perpendicular to the motion,

no work is done.

Bubble tracks

5.4 Magnetic force questions