Electromagnetism

Question 0

What happens to the magnetism of a current carrying coil

Answer 0

The magnetic flux lines combine into a large magnetic field that enters the coil at one end passes through, then spreads out and works its way around the outside of the coil back to the other end. The magnetic field is much stronger inside the coil because the flux lines are much closer together (higher flux density).

The polarity of the electromagnetic coil is determined by the left hand rule for coils. If the fingers are curled around a coil in the direction of current flow the thumb is pointing in the direction of magnetic flux (the south pole)

Question 1

What is the left hand rule

Answer 1

Magnetic flux lines rotate around a conductor carrying electric current. If the left hand is gripping a conductor with the thumb pointing in the direction of DC current flow the fingers will be wrapped around in the direction of magnetic flux rotation.

Question 2

What are the three ferromagnetic metals

Answer 2

Iron, cobalt, nicket

Question 3

What are magnetic domains

Answer 3

Ferromagnetic metals always have magnetic properties but usually the magnetic domains (small clusters of aligned force) are pointing in different directions and so cancel each other out. If they are inserted into an electromagnetic coil the external magnetic force will align all of the magnetic domains so they are no longer cancelling each other out. This creates a magnetic many times more powerful than the electromagnetic coil alone. This will be lost when the current is cut.

Question 4

Why can steel but not iron be permanently magnetised

Answer 4

The carbon atoms in steel make it harder for the magnetic domains to return to random alignment after the external magnetic field is lost.

Question 5

What can remove the permanent magnetism on iron

Answer 5

Heat or repeatedly striking the steel will allow the magnetic domains to return to a random alignment thus removing the permanent magnetism .

Question 6

What will happen to two parallel conductors with current flowing in:

the same direction
opposite directions

Answer 6

In the same direction the flux lines will combine and the conductors will attract each other.

In opposite directions the conductors will repel each other.

Question 7

What is the left hand rule for motors

Answer 7

If a current carrying conductor is placed in a magnetic field it will experience forces on it. The direction of movement is determined by:

If the left palm is touching the conductor with the thumb pointing in the direction of current flow and the fingers are pointing the the direction of the magnetic field (south). The force on the conductor will be perpendicular to both the external magnetic field and the direction of current flow. It will push straight up through the palm.

Question 8

Describe a basic single phase generator

Answer 8

An iron armature with conductors wrapped around it rotating in an external magnetic field. The movement of the coil in the field creates a current in the coil which in turn induces a magnetic field in the iron armature.

Do not understand fully. Seek more information and add visual aids to this card.

Question 9

What is coulomb’s law

Answer 9

Colombs law dictates the strength of attraction or repulsion between two charged objects.
(8.99 x 10⁹) x (q₁q₂)/r²

q1 and q2 represent the strength of the two charges in coulombs. If both charges are alike (both negative of positive) the product will be positive and the objects will repel. If one negative and one positive the product will be negative and the objects will attract.

The product is divided by square of the distance in meters (r^2).
Because this is a force equation the result will be in units of newtons (N).

Question 20

What is the direction of magnetic flux

Answer 20

North to south

Question 21

What is electric field

Answer 21

Every charged object creates an electric field that interacts with every other charged object in the universe. However the effect of this charged is proportional to the square of the distance between the two objects so in practice charges far apart exert vey little force on each other.

Electric field lines extend away from positive charges and towards negative charges (add visual aids).

Question 22

What happens to the electric field lines of two fixed

Alike charges
Opposite charges

Answer 22

The field lines of two alike charges will bend away from each other
The field lines of two opposite charges will bend towards each other and combine.

Question 23

What will positive and negative charges do in an electric field

Answer 23

Positive charges will go in the direction of the field lines (south)
Negative charges will go against the field lines (north)

Question 24

What is the formula for the force on a charged particle in an electric field

Answer 24

The force is in the same direction
F = Q E

force (vector) = charge x electric field (vector)

Force is measured in newtons
charge is coulombs

Question 25

What is the symbol and unit for magnetic flux

Answer 25

φ

Unit: webber (wb)

Question 26

What is the symbol and unit for flux density

Answer 26

Variable: B
Unit: Webbers per metre squared wb/m²

Question 27

What factors determine the strength of the force on a current carrying conductor in a magnetic field.

Answer 27

F = BIL

F is force measured in newtons.
B is the flux density of the external field measured in Webbers per metre.
I is the strength of the current measured in amps.
L is the effective length of the conductor (only the component that is at right angles to the field) thus the current is reduced using the sine rule.

Question 28

What is the relationship between Webbers and teslas

Answer 28

Webber is the unit of magnet flux
Tesla is the unit of magnetic field
One Tesla is defined as one webber per square meter.

Question 29

Explain faradays law (for generators)

Answer 29

A constant magnetic field will not induce current in a stationary conductor. Current will only be generated if the conductor is moving relative to the magnetic field like a generator or the two are stationary but field is coming and going like a transformer. The stronger the field and the faster the change the greater the induced voltage will be.
The strength of the voltage is determined by
∆φ/∆t

Question 30

What happens if you push a magnetic through an open loop of wire

Answer 30

There will be no resistance because an open loop of wire cannot conduct a current. The resistance to movement in a generator is created by the interaction between the external magnetic field and the field created by the generated current. Thus the current used in the circuit the greater the force needed to generate it.

Question 31

What is magnetic saturation in an electromagnet

Answer 31

When he external magnetic field is strong enough to push all of the magnetic domains in the same direction the metal is magnetically saturated

Question 32

What is lenzes law for induced emf

Answer 32

Current that is induced by an changing magnetic field will always oppose the magnetic field inducing it

Question 33

Explain self induction in a current carrying conductor (back emf)

Answer 33

When a switch is closed (in a DC circuit) an induced emf will be created that opposes the current flow. After a second the emf will disappear because static magnetic fields do not induce current. When the circuit is opened the magnetic field will collapse producing an emf in the same direction of current flow.

Question 34

What is the unit of inductance

Answer 34

Unit: Henry (H)
Variable: L

A circuit has an inductance of one henry if it induces one volt of emf when the rate of change in the circuit is one amp per second.
This is an impracticably large unit for most purposes.
E = -L ∆i/∆t
The minus sign is because the induced voltage will always be in the opposite direction to the current.

Question 35

Explain mutual inductance

Answer 35

Mutual inductance is how transformers operate. Magnetic flux induced by the AC current in one circuit induces a current in another circuit that is linked only by magnetic flux.

Question 36

What is the formula for voltage induced in the secondary coil of a transformer (relating to field strength)

Answer 36

E₂ = −N₂ ∆φ₂/∆t

N is number of turns
φ is the flux density
t is time
- sign is because the induced voltage will always be in opposition to the flux.
The voltage in a secondary coil of transformer is num

Question 37

Explain energy stored in an inductive circuit

Answer 37

An inductive circuit creates magnetic fields which are a form of stored energy. When the current disappears the field collapses which creates a voltage in the direction of the current flow. This causes arching as the increased voltage pushes through the air.
The energy released is proportional to the energy required to create the magnetic field against the back emf.
The energy stored is given by the formula 1/2 LI²

Question 38

Explain lenzes law and current direction with respect to a changing magnetic field

Answer 38

Both an increasing and a decreasing magnetic field will induce a current in a conductor. As a field grows (positive ∆φ/∆t value) it will induce a current that opposes it. Thus the current will be negative with respect to the field. The minus sign in the equation corrects for this.

As a field decreases (negative ∆φ/∆t value) it will induce a current that supports the magnetic field. Thus the current will be positive with respect to the field. The minus sign corrects for this.