Chapter 9: Magnets and Currents

Hard

________ Magnetic Materials- such as steel, and alloys called Alcomox and Magnadur, are difficult to magnetize but do not readily lose magnetism.

electromagnets

They are used in ________; because their magnetic effect can be switched ‘ ‘ on and "off.

strength of field

Increasing the current increases the ________.

magnet poles

The field is similar to that from a bar, and there are ________ at the end of coil.

weak magnetic field

If an electric current is passed through a wire, as shown below, a(n) ________ is produced.

North seeking pole

N pole

South seeking pole

S pole

Magnetic Materials

is one which can be magnetized and is attracted to magnets

Hard Magnetic Materials

such as steel, and alloys called Alcomox and Magnadur, are difficult to magnetize but do not readily lose magnetism

Soft Magnetic Materials

such as iron or mumetal are relatively easy to magnetize, but their magnet is temporary

Non-magnetic Materials

include metals such as brass, copper, zinc, tin, and aluminum as well as non-metals

https

/ spark.iop.org/electromagnets-everyday-use

https

/ www.youtube.com/watch?v=j_F4limaHYI

https

/ www.youtube.com/watch?v=fiQ38p6vb8o

In this case, it is the magnetic field that is moving rather than the wire, but the result is the same

field lines are being cut

https

/ youtu.be/3HyORmBip-w

What is a magnetic field?

A region where magnetic objects experience a force
-strongest at the poles
-field strength decreases as distance from magnetic increases

What are magnetic field lines?

lines that map out the invisible magnetic field around a magnet
-always go from north to south
-never touch or cross other field lines
-the closer the field lines the stronger the magnet

Waht do arrows on field lines represent?

The direction of the force that would be experienced by the north pole of a magnet placed in the field .

What causes magnetic forces?

They are caused by the interaction of magnetic fields the attraction or repulsion between magnetic poles can be increased if

• the current is increased

• a stronger magnet is used

• the length of wire in the field is increased

What are permanent magnet (Hard magnets)?

a magnet made from a material that keeps its magnetism for a long time and is hard to magnetize (eg steel)

What are temporary magnets (soft magnets)?

magnet that loses its magnetism after a short time and is easy to magnetize(eg Iron)

What are Induced magnets?

magnetic material that becomes a temporary magnet because it is in the magnetic field of another magnet.

Give examples of magnetic materials.

material that is attracted to a magnet and can be magnetized
eg. Iron,steel,nickle,cobalt

What are methods of magnetization?

-stroking with a magnet
-hammering it in a magnetic field
- using a dc in a coil

What are methods of demagnetisation?

-hammering
-heating
-placing in a coil with an AC wire

What are Electromagnets?

a soft metal core made into a magnet by the passage of electric current through a coil surrounding it. Can be turned on and off
The strength of field is increased by
- increasing the current
-increasing the number of turns in the coil

Magnetic field around a wire

-the magnetic field lines form concentric circles
-the field is strongest close to the wire
-increasing the current increases the strength of the field.

Magnetic field around a coil

-the field is similar to that from a bar magnet,and there are magnetic poles at the end of the coil
-increasing the current increases the strength of the field
-increasing the number of turns on the coil increases the strength of the field

State Fleming's left hand rule

a rule that gives the direction of the force on a current-carrying wire in a magnetic field according to the directions of the current and the field

• the field direction is from the N pole of a magnet to the S pole

• the current direction is from the + terminal of a battery round to the - terminal. The rule only applies if the current and the field direction are at right angles

What is the Turning effect of curent?

When a force causes an object to turn
-increase the current
-using stronger magnet
increasing number of turns on the coil
-increasing the area of the coil

What is Electromagnetic Induction?

the process of generating a current by moving an electrical conductor relative to a magnetic field

What is Induced emf and current in a moving wire?

When a wire is moved across a magnetic field a small emf is generated in the wire
the induced emf can be increased by
-moving the wire faster
-using a stronger magnet
-increasing the length of wire in the magnetic field - looping the wire several times through the field

What is Induced emf and current in a coil?

When a bar magnet is pushed into a coil an emf is induced in the coil
the induced emf can be increased by
-moving the magnet faster
-using a stronger magnet
-increasing the number of turns on the coil

Reverse direction

-if the magnet is pulled out of the coil the direction of the emf is reversed
-If S pole instead of N ole is pushed into the coil, this reverses the current direction
-if the magnet is held still,no field lines are cut, so no induced emf or current.
-moving the wire in the opposite direction
-turning the magnet round so that the field direction is reversed
-if the wire is not moving,or is moving parallel to the field lines ,there is no induced emf or current

State Faraday's Law.

The emf induced in a conductor is proportional to the rate at which magnetic field lines are cut by the conductor

State Lenz's Law.

An induced current always flows in a direction such that it opposes the change which produced it

What is Fleming's right hand rule?

A rule that gives the direction of an induced current on a moving wire in a magnetic field depending on the direction of the movement/force and the field.

What is an Ac Generator?

A device that changes mechanical energy into electrical energy.
When the coil is rotated it cuts magnetic field lines so an emf is generated.
The coil rotates upwards, downwards and so the current flows backward,forwards.
The current is maximum when the coil is horizontal
It is zero when the coil is vertical
-increasing the number of turns of turns on the coil
-increasing the area of the coil
-using a strong magnet
-rotating the coil faster

What is a transformer?

A device that increases or decreases the voltage of alternating current
when ac flows in the primary coil it sets up an alternating magnetic field in the core and in secondary coil.

What is a step-down transformer?

a transformer that decreases voltage

What is a step-up transformer?

a transformer that increases voltage