H216 Electromagnetism

Question 1

Define magnetic field.

Answer 1

A magnetic field is a region of space in which a permanent magnet, a current carrying conductor or a moving charge may experience a force.

Question 2

Describe the characteristics of magnetic field lines.

Answer 2

1. Magnetic field lines come out of north poles and go into south poles.
2. The tangent of the magnetic field lines shows the direction of the magneitc force that a ‘free’ magnetic north pole will experience at that point.
3. Are closer together where the field is stronger
4. They do not intersect. When two or more fields interact, the resultant is a vector addition

Question 3

State the formula for magnetic flux density due to current in a long straight wire.

Answer 3

B = [permeability of free space(Hm-1)][Current in the conductor(A)] / 2pi[Distance from the conductor(m)]

Question 4

State the formula for magnetic flux density due to a flat circular coil.

Answer 4

B = (permeability of free space)(Number of turns)(current in the conductor)/2(radius of coil)

Question 5

State the formula for the magnetic flux density due to a long solenoid.

Answer 5

B = (permeability of free space) (number of turns per unit length) (current in the conductor)

This equation is NOT valid if a ferrous (soft iron) coe is inside the solenoid.

Question 6

Explain the effects of inserting an iron core into a current carrying solenoid.

Answer 6

A soft iron core is one that is easily magnetised and de-magnetised. When a soft iron core is inserted in a solenoid, the core is magnetised in the same direction as the magnetic flux density set up by the solenoid, it adds to the magnetic flux density and strengthens the net magnetic field.

Do NOT say concentrate magnetic field lines.

Question 7

What is the formula of the magnetic force acting on a straight current carrying conductor in a magentic field?

Answer 7

F = BILsinx

Question 8

What is the formula of the magnetic force acting on a moving charged particle in a magnetic field?

Answer 8

F = BQv sinx

Question 9

Define magnetic flux density B.

Answer 9

Magnetic flux density is the force acting per unit current per unit length on a wire carrying a current that is normal to the magnetic field.

Question 10

When is magnetic force maximum and when is it zero?

Answer 10

Magnetic force is maximum when I and B are normal to each other (sin90) and is zero when the conductor is parallel to the field (sin0 or sin180).

Question 11

Explain why for wires carrying current in the same direction they attract.

Answer 11

Individual B-fields cancel out in region between wires and reinforce in region outside wire pair, hence there is greater density of flux lines outside of wire pair hnce it resembles a single wire when very far away.

Question 12

Explain why for wires carrying current in the opposite direction they attract.

Answer 12

Individual B-fields reinforce in region between wires and cancel out in region outside ire pair, hence there is greater density of flux lines in between wire pair.

Question 13

For a moving charge in a magnetic field in circular motion, find an expression for radius r.

Answer 13

Magnetic force provides centripetal force.
Fb = Fc
Bqv(sin90) = mv^2/r
r = mv/Bq

Question 14

For a moving charge in a magnetic field in circular motion, find an expression for period T.

Answer 14

Magnetic force provides centripedal force
Fb = Fc
Bq(v)(sin90) = mrw^2
Bq(rw) = mw^2
Bq = mw = m(2pi/T)
T = 2pim/Bq

Question 15

Describe the motion of a moving charge in a magnetic field undergoing circular motion.

Answer 15

Charged particle has velocity normal to magnetic field and experiences a magntic force perpendicular to both velovity and magnetic field given by Fleming’s left-hand rule. Magnetic field provides centripetal force inside magnetic field and no resultant force acts on the paticle outside field, hence by Newton’s First Law, particle coontinues in its state of constant velocity in a straight line.

Question 16

Why doesn’t magnetic force do work when a moving charge in a magnetic field undergoes circular motion?

Answer 16

Magneitc force is always directed perpendicular to velocity of charged particle hence there is no displacement in the direction of magnetic force and hence no work is done.

Question 17

Explain the differences between charges in a magnetic force and electric field force.

Answer 17

Magentic force only acts on moving charged particle while electric field force acts on both stationary and moving charged particles.
Magnetic force is perpendicular to both magnetic field and direction of motion of charged particle, while electric field force is parallel to the direction of the electric field.
Magnetic force depends on speed and direction of motion of charged particle while electric field force is indepedent of both.
Magnetic force results in circular motion while electric field force results in parabolic motion.