Topic 12 Magnetism and Electromagnetism

When magnetic circuits are made of laminations this is done in order to decrease the losses due to:

eddy currents.

When flux lines of a moving magnet cut across a stationary conductor, an emf is induced by:

relative motion.

The properties of a coil that will affect the amount of current in that coil are:

resistance.

The magnetic polarity of the coil connected to a dc source can be changed by:

reversing the current flow.

Decreasing the speed of a conductor through a magnetic field would cause the induced voltage to:

decrease.

A coil has a iron core. If the core is removed, the strength of the magnetic field around the coil would tend to:

decrease.

What is a magnetic pole?

A place on the magnet where the force of attraction is greatest.

Solenoid

A long current-carrying coil used to generate a uniform magnetic field within its core.

right-hand grip rule

A rule for finding the direction of the magnetic field inside a solenoid. If the right hand grips the solenoid with the fingers following the direction of the conventional current around the solenoid, then the thumb points in the direction of the magnetic field.

Lenz's Law

Any induced current or induced e.m.f. will be established in a direction so as to produce eff ects which oppose the change that is producing it.

State lenz's law of induction.

Lenz's law of induction states that any induced emf will cause a current to flow in such a direction that it's magnetic effect will oppose the change that produce it.

If the flux density of the magnetic field is decreased, what happens to the amount of induced voltage?

Decreases.

Define electromagnet.

Device that uses current flow through conductors around a core to create magnetism.

The induced voltage set up in a circuit is always in such a direction as to assist the force that produced it. True or false?

False.

Define hysteresis.

It is the heat produced by the flip-flopping of the magnetic domains in a iron core caused by an alternating current.

Inductance is symbolized by __ and is measured in __, symbol __.

L, henry, H.

Describe mutual induction.

Mutual induction takes place when a change in current and one circuit causes a voltage to be induced into another separate circuit.

Describe self induction.

Occurs when a change of current in a circuit causes a voltage to be induced back into that circuit.

What must exist between a magnetic field and a conductor in order to produce an induced voltage?

Relative motion.

When the current in a coil has reached its ohms law value, what is the value of the self induced voltage?

Since current does not change, then there will be no self induced voltage.

What happens to the direction of the lines of force when the direction of the current flow is reversed?

The direction of the lines of force also reverses.

Define magnetic flux density.

The magnetic flux density at a point in space is the force experienced per unit length by a long straight conductor carrying unit current and placed at right angles to the field at that point.

1 Tesla equals ____________.

The magnetic flux density is 1 T when a wire carrying a current of 1 A placed at right angles to the magnetic field experiences a force of 1 N per metre of its length.

Define Magnetic flux Φ.

The magnetic flux Φ through area A is defined as: Φ = BA where B is the component of the magnetic flux density perpendicular to the area.

Define Faraday's Law.

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

magnetic flux linkage

The product of magnetic flux and the number of turns. Unit: weber (Wb).

Define inductance.

The property and electric current that opposes a change of current in that circuit.

Define induced voltage.

The voltage produced in a conductor whenever if there is relative motion between the conductor and magnetic field.

When are eddy currents produced?

They are produced when a magnetic core is subjected to a changing magnetic field.

Flemings left-hand rule.

Thumb - indicates the thrust of conductor Pointer finger - indicates direction of flow Middle finger - indicates current flow

Why are iron coil core laminated?

To reduce eddy current losses.

Current flow in a conductor produces:

a magnetic field.

When the flux lines of a changing magnetic field of one conductor of a coil cut across an adjacent conductor, we have:

a voltage induced in the adjacent conductor.

Whenever there is motion between a conductor and a magnetic field:

a voltage is generated in that conductor.

When and open circuited conductor across a magnetic field:

a voltage is induced in that conductor.

What three factors affect the strength of the magnetic field around the coil?

a) The amount of current through the coil b) The number of turns on the coil c) The core material

State the three factors that determine them amount of induced emf.

a) flux density b) the number of turns in a conductor c) The rate at which lines of force are cut by the conductor which is determined by the velocity and the angle of cut

State two applications that use the principle of mutual induction.

a) ignition coils b) transformers

What two factors affect the amount of induced voltage?

a) rate of change b) inductance of current

Describe the two behaviors of the lines of force of a magnet.

a) they take the path that best support lines of force b) they do not cross each other.

Increasing the current through a conductor will cause the magnetic field around the conductor to:

increase in strength.

If the flux density of a magnetic field is increased and the amount of induced voltage would:

increase.

If the length of conductor is increased which moves through a magnetic field, then the amount of induced voltage will:

increase.

Producing an emf by relative motion between a conductor and a magnetic field is called:

induction.

One weber (1 Wb)

is the flux that passes through an area of 1 m2 when the magnetic flux density is 1 T. 1 Wb = 1 T m2.

The basic principle upon which trans formers operate is:

mutual induction.

When the polarity of the magnetic field is changed, this will cause:

the direction of the emf to change.

The direction of the magnetic field around a current carrying conductor can be determined by using:

the left hand rule.

In order to induce an emf in a conductor, we must have:

• a conductor moving across a magnetic field • a conductor being swept by a magnetic field • relative motion between a conductor and the magnetic field

When conductors, which are carrying current in the same direction, are brought close together, their magnetic fields will:

• aid one another • combine to make a stronger field

The amount of induced emf in a coil will increase with:

• an increase in the number of turns of wire • a stronger magnetic field

The magnitude of an induced emf is determined by the length of a conductor within the flux field and:

• flux density • velocity