Magnets and Magnetic field and force

Facts about magnets Not all materials are attracted to a magnet. Only ferromagnetic materials are attracted to a magnet. Examples of ferromagnetic materials are iron, nickel, cobalt, neodymium.

Facts about magnets Different magnets have different strengths. The strength of the magnet does not necessarily depend on the size of the magnet. The strength of the magnetic depends on the alignment of the magnetic domains in the magnet.

Facts about magnets The magnetic domains act as tiny magnets within a magnet. The more the magnetic domains point in the same direction, the stronger the magnetic field of the magnet.

Facts about magnets Magnets have 2 poles (north and south) Like poles repel Unlike poles attract Magnets create a MAGNETIC FIELD around them

Facts about magnets When a bar magnet is broken into smaller pieces, each small piece will have its own north and south pole.

Facts about magnets When suspended, a bar magnet will align itself with the Earth’s magnetic field. The north pole of the magnet points toward the north geographic pole of the Earth.

Directional compass The needle of a compass is a bar magnet. The tip of the compass needle that points to the north direction is the north pole of the compass needle.

Magnetic field of a bar magnet

Magnetic Field A bar magnet has a magnetic field around it. This field is 3D in nature and often represented by lines LEAVING north and ENTERING south To define a magnetic field you need to understand the MAGNITUDE and DIRECTION We sometimes call the magnetic field a B-Field as the letter “B” is the SYMBOL for a magnetic field with the TESLA (T) as the unit.

Magnetic field The magnetic field of a bar magnet is strongest at the poles.

Magnetic field between like poles

Magnetic field between unlike poles

Draw the magnetic field lines between the two south poles N S S N

Earth’s geographic and magnetic poles

Permanent magnets vs Electromagnets retains a magnetic force Can not be turned on and off Electromagnets Use electricity to create a magnetic field They can be controlled (turned on and off) Their force or strength of field can be controlled

Comparison between gravitational, electric and magnetic forces Field The force is applied on a: Direction of the force Gravitational field mass same direction as the field Electrical force electric charge opposite or in the same direction as the field Magnetic field perpendicular to the plane created by v and B

Direction of the magnetic force? Right Hand Rule To determine the DIRECTION of the force on a POSITIVE charge we use a special technique that helps us understand the 3D/perpendicular nature of magnetic fields. Basically you hold your right hand flat with your thumb perpendicular to the rest of your fingers The Fingers = Direction B-Field The Thumb = Direction of velocity The Palm = Direction of the Force For a NEGATIVE charge the force is in the opposite direction (where the back of your hand is facing)

Magnetic force on a moving charge: Another version of right-hand rule

Our direction guides up +z in:west in:-x south north -y +y out:east out:+x up down in:in -z left right out:out down

Magnetic force on a moving charge 𝑭=𝒒𝒗𝑩𝒔𝒊𝒏𝜽 F = magnetic force in N q = charge in C [if q is negative, do not plug in the negative sign in the equation] v = the velocity of the charge in m/s B = magnetic field in Tesla (T)  = the angle between B and v

Charges moving in a wire Up to this point we have focused our attention on PARTICLES or CHARGES only. The charges could be moving together in a wire. Thus, if the wire had a CURRENT (moving charges), it too will experience a force when placed in a magnetic field. You simply used the RIGHT HAND ONLY and the thumb will represent the direction of the CURRENT instead of the velocity.

Magnetic force on current-carrying wire 𝐹=𝐵𝐼𝐿𝑠𝑖𝑛𝜃 F = force in N B = magnetic field in Tesla I = current in Ampere (A) L = length of wire in the field in m  = angle between B and I

Example A 36-m length wire carries a current of 22A running from right to left. Calculate the magnitude and direction of the magnetic force acting on the wire if it is placed in a magnetic field with a magnitude of 0.50 x10-4 T and directed up the page. 0.0396 N into the page

Direction of magnetic force on current-carrying wire