Motors use the effect of forces on current-carrying conductors in magnetic fields

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Students learn to:

1. discuss the effect on the magnitude of the force on a current-carrying conductor of variations in:

the strength of the magnetic field in which it is located

↑B = ↑F

the magnitude of the current in the conductor

↑I = ↑F

the length of the conductor in the external magnetic field

↑l = ↑F

the angle between the direction of the external magnetic field and the direction of the length of the conductor

↑θ = ↑F

Use F = B I l \sin \theta\, to easily find the above.


2. describe qualitatively and quantitatively the force between long parallel current-carrying conductors:

The Right hand grip rule is used to determine the direction of the magnetic field around a current carrying conductor.
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The Right hand grip rule is used to determine the direction of the magnetic field around a current carrying conductor.

{F \over l}=k{I_1 I_2 \over d}

We know that a wire with a current flowing though it will create a magnetic field around it. And we know that if we have two wires with current flowing though them that are parallel to each other then the magnetic fields will interact and create a force on the wires.

If the current of the two wires are flowing in the same direction then they will attract. If the current is flowing in opposite directions they will repel. You can work this out by applying the right hand grip rule (see below) to the wires. If the current is in the same direction then the magnetic field will be in the same direction and the two wires will attract to create one big magnetic field. The opposite can be said when the current in the wires is in the opposite direction. The above formula only works for long wires.

3. define torque as the turning moment of a force using:

\tau\ = F d

Torque is just a turning moment.


4. identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field

The motor effect is the action of a force experienced by a current carrying conductor in an external magnetic field. The direction of the force can be determined by the right hand push rule. Remembering that magnetic field lines go from N to S. The magnitude of the force can be determined by F = B I l \sin \theta\,

The Motor Effect is the action of a force experienced by a current carrying conductor in an external magnetic field.
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The Motor Effect is the action of a force experienced by a current carrying conductor in an external magnetic field.

5. describe the forces experienced by a current-carrying loop in a magnetic field and describe the net result of the forces

If a current is flowing though the coil, and an external magnetic field is present then there will be a force acting on the coil. The direction of the force can be determined by the right hand push rule. This is the basis for a motor, in which two opposite forces on opposite sides, rotate and create a torque, creating a spinning motion used in motors. The net force of the forces on the coil is a force which causes the coil to spin.

DC Electric Motor - Maximum Force

When the coil has rotated to the position shown below, then the two forces for both sides of the motor will be acting in opposite directions along the same line of action and hence they will cancel each other out. Momentum however pushes them past this point, along with the change in direction of the current due to the split ring commutator, the coil will start to experience a turning force again.

DC Electric Motor - Zero Force

6. describe the main features of a DC electric motor and the role of each feature

The DC electric motor uses the motor effect to create a continuous spinning motion.

The DC Electric Motor uses the motor effect to create a continuous spinning motion. Its main features are a magnetic field, commutator, brushes, coil and an armature.
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The DC Electric Motor uses the motor effect to create a continuous spinning motion. Its main features are a magnetic field, commutator, brushes, coil and an armature.


A motor must have a magnetic field to work. This field is provided by either permanent magnets or electromagnets. Remember that magnetic field lines travel from north to south.


If you apply the right hand push rule to the two wires currents perpendicular to the magnetic field lines you will find that the two side push in opposite directions, which act together to move the coil. However the force will act keeping the coil perpendicular to the magnetic field lines. This is where the commutator comes into play. By changing the direction of the current at the right time thus changing the direction of the force, and thus it will spin continuously.


The brushes just keep the current flowing into the commutator, without sparking. They allow a sliding contact with the commutator, allowing them to be stationary while the commutator is spinning.


The armature is the thing that the coil is wrapped around. This is usually iron. It gives the coils mass, resulting in momentum, and also holds the coils in place.


A current must also be present in the coil.

7. identify that the required magnetic fields in DC motors can be produced either by current-carrying coils or permanent magnets

The magnetic field in a DC motor can be produced using either a permanent magnet, or electromagnet (made using a current-carrying coil and an iron core). If an electromagnet is used it is placed parallel to the commutator.

Students:

1. solve problems using:

{F \over l}=k{I_1 I_2 \over d}


Where;

F = Force (N)

l = length of wire (m)

k = Magnetic force constant (on data sheet)

I1 = Current in first wire (Amps)

I2 = Current in second wire (Amps)

d = distance separating wires (m)


2. perform a first-hand investigation to demonstrate the motor effect

The apparatus shown below is set up, where a wire is placed on an electronic balance. The wire is connected to a variable power source. Permanent magnets are placed on either side of the wire as shown. When no current is passed though the wire the electronic balance is zeroed. Now a current is passed though the coil, depending on the direction of the current the electronic balance will measure a positive or negative value. However the value has changed meaning the wire is experiencing a force. This shows the motor effect.

Motor Effect Experiment

3. solve problems and analyse information about the force on current-carrying conductors in magnetic fields using:

A current carrying conductor in a magnetic field creates a force.

F = BIlsinθ


Where;

F = Force (N)

B = Magnetic field strength (T)

I = Current in wire (Amps)

l = Length of wire (m)

θ = Angle between current and magnetic field


The direction of the force can be obtained by applying the right hand push rule.


4. solve problems and analyse information about simple motors using:

τ = nBIAcosθ


Where;

τ = torque (Nm)

n = number of coils

B = magnetic field strength (T)

I = current (A)

A = area of coil (m2)

θ = angle

5. identify data sources, gather and process information to qualitatively describe the application of the motor effect in:

the galvanometer

A galvanometer is used to measure the magnitude and direction of direct current (DC). It uses the motor effect to do this.


(A diagram can be seen in the 2006 HSC Exam Paper on page 18.)


So when a current is applied the pointer and coil will turn. It has a spring in it so that it will return to zero. It can go both ways showing the direction of the current. The magnets used are shaped so that the field is perpendicular to the plane of the coil. This allows a uniform scale and allows the angle to be constant. As the force on the coil = BIlsinθ, and as sinθ, l and B are constant, Force \propto Current.

the loudspeaker

Speakers create sound waves from electrical impulses. The motor effect in the speaker is used for movement in 1 dimension, not a spinning motion like the DC electric motor. When a current is present in the coil, and a magnetic field, the coil will have a force pushing it out. The spring brings it back in to normal position. This movement creates sound waves.

Loudspeaker