Magnetism and Electromagnetism

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

the left hand rule.

The material used for the magnetic circuit of a relay should have:

low retentivity.

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 property of an electric circuit that opposes a change of current in that circuit is known as:

inductance.

When the current decreases rapidly in a highly inductive circuit, the self induced voltage:

is very high.

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.

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

decrease.

The property of an electric circuit that opposes any change in current through that circuit is called:

inductance.

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

reversing the current flow.

To produce one volt in a conductor, the number of lines of force in one second would be:

100 M.

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 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.

Inductance is one factor affecting the amount of self induced voltage in a coil. Another factor is the:

rate of change of current in that circuit.

The unit of measurement for inductance is the:

henry.

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

relative motion.

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.

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

decrease.

When a change of current in one circuit induced a voltage into another circuit, this process is known as:

mutual induction.

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

inductance and resistance.

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

increase in strength.

Current flow in a conductor produces:

a magnetic field.

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.

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

eddy currents.

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

increase.

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

induction.

The symbol used to denote inductance is:

L.

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

increase.

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

• flux density

• velocity

If the current flow and a conductor increases, the magnetic field on the conductor will:

increase.

What is a magnetic material?

Any material that is attracted to a magnet and has magnetic domains.

What are the two classifications of magnets?

a) temporary

b) permanent

What is a magnetic pole?

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

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.

Describes it to behaviors of magnetic poles when they are placed next to each other.

a) like poles repel each other

b) unlike poles attract each other

In what direction do the imaginary magnetic lines of force of a closed loop flow?

From the north pole to the south pole.

Define electromagnet.

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

Define flux.

The total lines of force surrounding a magnetic material. The symbol is Φ.

Define flux density.

The number of magnetic lines of force per-unit area. The symbol is β.

Define reluctance.

The opposition of a material to carry magnetic flux. The symbol is Rm.

Define residual magnetism.

The magnetic effect that remains in a material after the magnetizing force has been removed.

Define retentivity.

The level of ability of a material to retain residual magnetism.

Define

permeability.

The ability of a material to concentrate flux as compared to air.The symbol is μ.

Define saturation.

Saturation is reach when all the domains have been aligned.

What effect does decreased current in a conductor on the magnetic field around the conductor?

The magnetic field decreases.

If the current in a conductor is switched off, what happens to the magnetic field around the conductor?

The magnetic field around the conductor collapses.

Left hand rule for conductors

Thumb - direction of current

Fingers - lines of force

Left hand rule for coils

Thumb - indicates north pole

Fingers - indicates the direction of electron flow

When using the left hand rule for conductors, what does the thumb indicate?

The direction of electron flow in the conductor.

If the electron flow is into the page, will the direction of the lines of force the clockwise or counterclockwise?

Counterclockwise.

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.

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

An electro magnet has a soft iron core. If the core is replaced with one lower permeability, what happens to the strength of the magnetic field?

The strength of the magnetic field decreases.

Why is silicon steel used to make cores for coils?

To reduce hysteresis losses.

Name two types of iron loss that can occur in the core of an electro magnet.

a) eddy currents

b) hysteresis

Why are iron coil core laminated?

To reduce eddy current losses.

Define hysteresis.

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

When are eddy currents produced?

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

When does the core of an electromagnet become saturated?

When an increase of current in the coil does not produce an increase and magnetism in the core.

Define electromagnetic induction.

Induction of voltage from one system to another via magnetic lines of flux.

Define Faraday's Law.

A voltage will be produced whenever there is relative motion between a conductor and a magnetic field.

Flemings left-hand rule.

Thumb - indicates the thrust of conductor

Pointer finger - indicates direction of flow

Middle finger - indicates current flow

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

Relative motion.

Define induced voltage.

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

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

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

Decreases.

Describe self induction.

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

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.

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.

What two factors affect the amount of induced voltage?

a) rate of change

b) inductance of current

Define inductance.

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

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.

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

L, henry, H.

State two applications that use the principle of mutual induction.

a) ignition coils

b) transformers