electromagnetism

ElectromagnetismElectromagnetism

Unit 21

Unit 21

Electric motors are machines that use magnetism and electricity to make things move. We use d.c. motors to drive electric trains. The

electric current is supplied to the train from overhead wires or from the rails below.

Electric motors are machines that use magnetism and electricity to make things move. We use d.c. motors to drive electric trains. The

electric current is supplied to the train from overhead wires or from the rails below.

contentscontents

Force on a Current-carrying Conductor

D.C. Motors

http://www.youtube.com/watch?v=HQdLFEiVeCA

Force on a Current-carrying Conductor

D.C. Motors



Unit 21.1: Magnetic Effect of a Current

• When the circuit is closed, a compass A placed above the wire XY would point to the East. Another compass B is placed below the wire would point to the West.

• A current-carrying conductor produces a magnetic field around it.

Demonstrating the magnetic effect of a current—Oersted’s Experiment

Fig. 21.4 Oersted’s experiment

A Straight Wire

1. A straight wire carrying a current produces circular

lines of force.

What happens when the direction of current is reversed?

The direction of the magnetic field will also be reversed!

A Straight Wire

The strength of the magnetic field in astraight wire is stronger when

•

a larger current flows through it.

•

(the circular lines of force are closer) i.e. nearer the wire

Direction of arrow

Direction of current or magnetic field

Current-carrying wire is perpendicular

to the plane of board.

current is directedout

of paper

(point of arrow)

current is directedinto

paper

(tail of arrow)

A Flat Circular Coil

A flat coil carrying a current produces

circular lines of force around the wires

and

almost parallel lines of force in the middle.

Unit 21.1: Magnetic Effect of a CurrentTest Yourself 21.11. A current flows in a long straight wire in the direction shown in

Figure 21.17. Draw, in the diagram, the pattern and direction of the magnetic field produced.

Answer:

Unit 21.1: Magnetic Effect of a CurrentTest Yourself 21.12. (a) Draw the magnetic field lines around a current-carrying solenoid.

(b) Name three ways to increase the magnetic field strength of a solenoid.Answer:(a)

(b) 3 ways to increase magnetic field of solenoid:• Increase the no. of turns per unit length of the solenoid,• Increase the magnitude of the current• Place a soft iron core in the solenoid.

Fig 21.9 pp 418

force on a current-carrying conductor2. Current-carrying conductor 2. Current-carrying conductor

The setup investigates the interaction between a current and a magnetic field. The setup investigates the interaction between a current and a magnetic field.

soft iron c-core

thick bare wire

powerful magnadur magnet

2V power pack or

lead-acid accumulator

The direction of the force can be deduced by using this rule.The direction of the force can be deduced by using this rule.

3. Fleming’s left-hand rule 3. Fleming’s left-hand rule

force on a current-carrying conductor

Motion(thumb)Motion(thumb)

Field(first finger)

Field(first finger)

the fingers are at right angles to one another

Current(second finger)

Current(second finger)

To explain the force exerted on the wire, consider the combined magnetic fields due to the current flowing through the straight wire and the magnets.

To explain the force exerted on the wire, consider the combined magnetic fields due to the current flowing through the straight wire and the magnets.

Fleming’s left-hand ruleFleming’s left-hand rule


magnetic field between two magnadur magnets

magnetic field due to the current in the wire

N

S

Fleming’s left-hand rule Fleming’s left-hand rule


The two fields acting in the same direction combine to give a stronger field, but the two fields opposing each other combine to give a weaker field.

The unbalanced fields on both sides exert produce a force that exerts on the wire.

The two fields acting in the same direction combine to give a stronger field, but the two fields opposing each other combine to give a weaker field.

The unbalanced fields on both sides exert produce a force that exerts on the wire.

combined magnetic field

Further explanation Unit 21.2: Force on Current-carrying Conductors

Worked Example 21.1Figure 21.20(a) shows a wire placed between two magnetic poles.

(a) If the current in the wire flows from A to B, in which direction does a force act on the wire?

(b) What will happen if the current flows from B to A instead?

Fig. 21.20(a)

Unit 21.2: Force on Current-carrying ConductorsWorked Example 21.1 – Solution(a) By using Fleming’s Left-Hand Rule, we find that the force acts

vertically downward on the wire AB (Figure 21.20(b)).

(b) If the current flows from B to A, the force reverses in direction and acts vertically upward.

Fig. 21.20(b)

Unit 21.2: Force on Current-carrying ConductorsWhy does a current-carrying conductor experience a force when placed in a magnetic field?

Fig. 21.21(a) & (b) Separate magnetic fields of a current flowing through a wire and of two magnetic poles


Fig. 21.21(c) Superimposed field patterns of (a) and (b)


Fig. 21.21(d) Combined magnetic field when the wire is placed between the poles of the magnet

Unit 21.2: Force on Current-carrying ConductorsWhy does a current-carrying conductor experience a force when

placed in a magnetic field?

From Fig. 21.21(d), we can see that there is a stronger field on one side of the wire at A, since all the magnetic field lines are in the same direction. At B, the combined field is weaker due to opposing magnetic field lines.

A force then acts on the wire from the stronger field to the weaker field.

Fig 21.21(d) Combined magnetic field when the wire is placed between the poles of the magnet.

4. Force on a beam of charged particles 4. Force on a beam of charged particles

Fleming’s left hand rule can be applied to all moving charges. Fleming’s left hand rule can be applied to all moving charges.


motion field (magnetic)

Conventional current flow

Electron current flow

The conventional current travels in an opposite direction to that of the electron flow.

The conventional current travels in an opposite direction to that of the electron flow.

force on a beam of charged particles force on a beam of charged particles


xx xx xx xx xx

xx xx xx xx xx

xx xx xx xx xx

xx xx xx xx xx

magnetic field into paper

path of positively charged particle(part of a circle)

positively charged particle

direction of positively charged particle before

entering the magnetic fieldxx

motion (force)

current

force on a beam of charged particles force on a beam of charged particles


xx xx xx xx xx

xx xx xx xx xx

xx xx xx xx xx

xx xx xx xx xx

magnetic field into paper

path of electron or negatively charged particle

(part of a circle)

electron or negatively charged

particle

direction of electron or negatively charged particle

before entering the magnetic field

xx

motion (force)

current

Unit 21.2: Force on Current-carrying ConductorsForce on a moving charge in a magnetic field

Fig. 21.22(a) A positively charged particle in a magnetic field is deflected upwards in a circular path.

• When a beam of positive charges enter the magnetic field region, it is deflected upwards in a circular path as the moving charges experience a force perpendicular to its direction of motion.• The direction of the force can be predicted by Fleming’s Left-hand rule.

Unit 21.2: Force on Current-carrying ConductorsForces between two parallel current-carrying wires• Currents in opposing directions cause repulsion.

Fig. 21.24 Combined magnetic field due to currents in the opposite direction

Unit 21.2: Force on Current-carrying ConductorsForces between two parallel current-carrying wires• Currents in similar directions cause attraction.

Fig. 21.25 Combined magnetic field due to currents in the same direction.

d.c. motors

5. Turning effect on a current carrying coil5. Turning effect on a current carrying coil

A current- carrying coil placed in a magnetic field of a horseshoe magnet experiences a turning effect.

A current- carrying coil placed in a magnetic field of a horseshoe magnet experiences a turning effect.

d.c. motors

turning effect on a current carrying coilturning effect on a current carrying coil

A catapult field is produced when the field produced by the coil superimposes on the field of the horseshoe magnet. A catapult field is produced when the field produced by the coil superimposes on the field of the horseshoe magnet.

increasing the number of turns on the coil

increasing the magnitude of the current

inserting a soft iron core within the coil to concentrate the magnetic lines of force

increasing the number of turns on the coil

increasing the magnitude of the current

inserting a soft iron core within the coil to concentrate the magnetic lines of force

d.c. motors

turning effect on a current carrying coilturning effect on a current carrying coil

The turning effect can be increased byThe turning effect can be increased by

principles of a d.c. motorprinciples of a d.c. motor

make use of the turning effect of a current-carrying coil in magnetic field to convert electrical energy to mechanical (kinetic) energy

works on direct current

are the basic components in electric fans, hair dryers and many other electrical appliances

make use of the turning effect of a current-carrying coil in magnetic field to convert electrical energy to mechanical (kinetic) energy

works on direct current

are the basic components in electric fans, hair dryers and many other electrical appliances

d.c. motors

an opened up d.c. motor


a. when the circuit is closed, current flows from the battery through P and X, through the coil and back to the battery through Y and Q

a. when the circuit is closed, current flows from the battery through P and X, through the coil and back to the battery through Y and Q

using Fleming’s left-hand rule, the left side of the coil experiences a downward force and the right-hand side experiences an equal upward force

using Fleming’s left-hand rule, the left side of the coil experiences a downward force and the right-hand side experiences an equal upward force

d.c. motors


b. this pair of forces causes the coil to rotate anticlockwise until it reaches a vertical position

b. this pair of forces causes the coil to rotate anticlockwise until it reaches a vertical position

at this point, current is cut off because neither X nor Y is in contact with P or Q

at this point, current is cut off because neither X nor Y is in contact with P or Q

d.c. motors


(c) momentum of the coil carries it slightly beyond this vertical position

(c) momentum of the coil carries it slightly beyond this vertical position

half-ring Y will then touch P while X comes into contact with Q

turning forces act again and coil continues to rotate in the same direction

half-ring Y will then touch P while X comes into contact with Q

turning forces act again and coil continues to rotate in the same direction

d.c. motors

If a soft iron cylinder is placed between the curved poles of the magnet in a motor: If a soft iron cylinder is placed between the curved poles of the magnet in a motor:

d.c. motors


this arrangement increases the magnetic field strength and thus increases the turning effect for a given current in the coil

this arrangement increases the magnetic field strength and thus increases the turning effect for a given current in the coil

a radial field will be created

radial field keeps the pair of forces acting on the coil almost constant as it turns

a radial field will be created

radial field keeps the pair of forces acting on the coil almost constant as it turns

Unit 21.3: Force on a Current-carrying Rectangular Coil in a Magnetic Field

How does a d.c motor work?– When current flows through the coil ABCD, using Fleming’s left-hand

rule, a downward force will act on side AB, and an upward force on side CD.

– The coil thus rotates anticlockwise about axis PQ until it reaches a vertical position.

– Here, the current is cut off because X and Y are both not in contact with the carbon brushes

– The turning effect of the coil, however, carries it past the vertical position.– This reverses the current direction in the wire arm CD and now a

downward force acts on it.– Similarly, an upward force acts on wire arm AB.– Hence, the coil continues to rotate in the anticlockwise direction.


How does a d.c motor work?• The purpose of the split-ring commutator is to reverse

the direction of the current in the coil every half a revolution to ensure that the coil will always turn in one direction.

• To increase the turning effect of the coil, we can:1. Insert a soft iron core or cylinder into the coil to

concentrate the magnetic field lines.2. Increase the number of turns in the coil3. Increase the current


Key Ideas1. The d.c. motor works on the principle that a current-carrying coil

in a magnetic field experiences a turning effect.2. The function of a split-ring commutator is to reverse the direction

of current in the coil when the coil passes the vertical position so that it continues to turn in the same direction.

3. The turning effect on the coil can be increased by(a) increasing the current in the coil(b) having more turns on the coil, or(c) inserting a soft iron core or cylinder into the coil.


Test Yourself 21.31. In the d.c. motor, what change(s) must be made so that

the coil rotates clockwise instead of anti-clockwise?

Answer:To change the direction of rotation to turn clockwise, we

can do one of the following:• reverse the poles of the magnets, or• reverse the direction of the current, by switching the

terminals of the battery


Test Yourself 21.32. Explain the purpose of the rheostat in the d.c. motor.

Answer:The resistance of the rheostat is varied so that the current flowing in the coil can be controlled. By lowering the resistance, the current will increase and the turning force on the coil will increase. This results in an increased speed of rotation.


Test Yourself 21.33. State the energy conversion that takes place in the d.c. motor.

Answer:

Electrical energy to mechanical energy.

Purpose of the split ring commutator

• To reverse the direction of the current in the coil every half a revolution whenever the commutator changes contact from one brush to another. This is to ensure that the current continue to flow in the same direction in the coil.

Q: How will the p.d. against time graph look like?

Turning effect on Turning effect on a currenta current--carrying carrying

coilcoil

Turning effect is Turning effect is increased by increased by increasingincreasing(a)(a) number of turnsnumber of turns(b)(b) currentcurrent

Force on a beam Force on a beam of charged of charged

particles in a particles in a magnetic fieldmagnetic field

Force on a currentForce on a current--carrying conductor carrying conductor in a magnetic fieldin a magnetic field

Fleming’sFleming’sLeft hand Left hand

rulerule

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is shown byis shown by

results inresults in

helps to helps to determine the determine the direction ofdirection of

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electromagnetism

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