1 . Motors use the effect of forces on current-carrying conductors in magnetic fields 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 The magnitude of the force is proportional to the magnetic field strength. Thus, an increase in magnetic field strength will cause an increase in the force on the wire and a decrease in magnetic field strength will cause a decrease in force on the wire.

The magnitude of the current in the conductor The magnitude of the force is also proportional to the current (l). Thus, an increase n current will result in an increase in force whilst a decrease in current will lead to a decrease in force. The length of the conductor in the external magnetic field The magnitude of the force is also proportional to the length (L).

Thus, an increase in the length of the wire within the field will result in an increase in force on the wire, whilst a decrease in the length of the wire in the magnetic field will lead too The angle between the direction of the external magnetic field and the direction of the length of the conductor. The force is at its maximum when the current carrying conductor is at right angles o the field (sinks=1), and is zero when the conductor is parallel to the field The magnitude of the force is proportional to the components of the magnetic field that is at right angles to the current carrying conductor (sinks =1).

The above points can be mathematically expressed as: F = noblest(0) Where: magnitude of the force (N) N = number of turns of wire magnetic field strength (T) current (amps) Sins= angle between the magnetic field and current. Describe qualitatively and quantitatively the force between long parallel current- carrying conductors : F/l =kill/d The force (F) per unit length (l) between two wires carrying currents IL and 12 separated by a distance (d) in a vacuum is given by: [F/l= K IL ii/d] Two parallel wires each carrying a current, will exert a force on the other.

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This happens because each current produces a magnetic field (as shown in Rooster’s experiment). Therefore each wire will find itself carrying a current across the magnetic field produced by the other wire and hence experiences a force the magnetic field strength at a distance, d, from a long straight conductor carrying a current, l, can be found using the formula: kid]

Where AXES-Ana-2 the magnitude of the force experienced by a length ,L, of a conductor due to an extended magnetic field is: [F/l= K IL ii/d] or rearranged to make F the subject: 121 (kill The size of the force is proportional to the two currents and the common length of the wires ; The size of the force is inversely proportional to the perpendicular distance between the two wires, meaning that if they are further away from each other, the force is less. If both currents are in the same direction, then the conductors will attract ; both currents are in opposite directions then the conductors will repel

Define torque as the turning effect of a force using: t = FDA Torque is the turning effect of a force acting on an object which causes it to rotate. [T=FDA] ; If torque is applied at an angle less then 90 or 90 degrees to the point of application then the formula T = FDA sine is used. Where T= torque (N. M) d= distance from rotational axis (m) O = angle to the point of application ; From the formula above torque depends on both the size of the force and how far from the turning point it is applied. The further a force is from the turning point, the greater the effect it has. T=nabobs] = number of coils A = area of coil (AMA) Cobs= angle between the plane of the coil to the magnetic field. Notice the force is always perpendicular to the magnetic field meaning that the force is the same throughout and the torque will change. Force only stays the same on the two sides of the coil but not the front and back. The torque on a coil will vary and look something like this: Identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic field The motor effect is due to the force acting on a current – carrying conductor in a magnetic field.

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 through a coil where an external magnetic field is present then there will be forces acting on the coil. That is, each side of the coil that is perpendicular to the magnetic field will experience a force due to the motor effect. By the right hand palm rule, the left side (in the diagram below) will experience a force moment. The coil will experience maximum torque at this point.

When the coil has rotated to the position shown below, then the two forces for OTOH sides of the motor will be acting in opposite directions along the same line of action and hence they will cancel each other out. However momentum pushes the coil past this point, along with the change in direction of the current due to the split ring commutate, the coil will start to experience a turning force again. Description of a coil rotating 360 degrees At this point the torque is maximum as the plane of the coil is parallel to the magnetic field, or coos(O) = 1 .

At this point the torque is less then the frame before because an angle exists between the plane of the coil and the magnetic field. ‘e. Cooks < cosO ) At this point the torque is minimum because each force is in opposite position to one another is directed outwards and cancel each other out. Also cos(90) = O. coil past this point. Also the spilt ring commutator reverses the direction of the current so the coil can maintain the same direction of rotation and torque. Now the current has changed direction the coil can maintain the same direction of rotation and torque.

We have reached the first frame again, and the process repeats. Describe the main feature of a DC electric motor and role of each feature The DC electric motor uses the motor effect to create a continuous spinning motion. The main features of a DC electric motor are shown below. Permanent magnets: provide an external magnetic field in which the coil rotates. As Rotating coil: carries a direct current that interacts with the magnetic field, producing torque. Armature: is made of ferromagnetic material and allows the coil to rotate freely on an axle.

The armature and coil together are known as the rotor (moving parts of the motor). The armature protrudes from the motor casing, enabling the movement of the coil to be used to do work. Split ring Commutate: Reverses the current of the coil every half revolution to maintain consistent direction and torque. It is a mechanical switch that automatically changes the direction of the current flowing through the coil when the torque falls to zero. Brushes: Supply continuous current to the commutate as it is rotating.

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 an electromagnet (made using a current-carrying coil and an iron core). Students: 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; therefore a current is passing through the wire. Permanent magnets of opposite polarity are placed on either side of the wire as shown. When no current is passed through the wire the electronic balance is zeroed. ; When a current is passed though the wire, 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. ; To show a stronger reading place the magnets closer, the wire should move more, creating a greater reading on the electronic balance.

Identify data sources, gather and process information to qualitatively describe the application of the motor effect in: – the galvanometer Galvanometer: Structure: A galvanometer is made up of two curved magnets (of different polarity), a spring and a moving coil attached with a needle wrapped around a soft iron core. ; The soft Ron core increases the strength of the radial magnetic field. ; The spring produces an equal and opposite torque that counterbalances the torque produced by the motor effect.

Operation: A galvanometer is a device that measures the magnitude and direction of small DC currents. ; When a current flows through the coil and the radial magnetic field, the coil will experience a force due to the motor effect which then produces a torque. This torque causes the coil and the needle to move. ; The radial magnetic field ensures that the torque is always constant as the plane of the coil is always parallel o the magnetic field, this means that the angle is always O and the amount of movement of the needle is only dependent from the strength of the current. T = NUBIAN, given other variables remain constant T -? l) ; This also means that the scale is linear and uniform since only the current can affect the needle’s movement. ; The needle will stop rotating once the spring produces an equal and opposite torque to counterbalance the torque produced by the motor effect to move the needle. Loud Speaker: A loud speaker consists of a circular magnet that has north poles on the outside ND a south pole in the middle. The coil is wrapped around the South Pole which is in the middle and the coil is connected to the cone which produces sound waves which is connected at the back of the coil. Loudspeakers are devices that transform electrical energy into sound energy. ; When a current is present in the coil, and the magnetic field, the coil will have a force pushing it in or out due to the motor effect. The motor effect in the speaker is used for movement in 1 dimension (in or out). ; The direction of movement of the coil can be determined using the right hand push rule.

When the current is clockwise, the force on the coil is into the page and when the current is anti-clockwise the force on the coil is out of the page. ; The coil is connected to the cone which creates sound waves in the air as the coil moves in or out, compressing or expanding the air waves. ; When the magnitude of the current increases and therefore force increases causing the coil to move in and out more which increases the loudness of the sound. 2. The relative motion between a conductor and magnetic field is used to generate an electrical voltage.

Outline Michael Faraday discovery of the generation of an electric current by moving a magnet Electromagnetic induction is the opposite of the motor effect, I. E. The conversion of mechanical energy into electrical energy: an induced Neff is produced in a conductor when it is left in a changing magnetic field. ;The effect can be shown in the following experiments by Faraday: Relative movement between magnet and coil experiment: connected too galvanometer. ; When the magnet (which produces a magnetic field around it) and the coil are stationary, no current is observed in the coil and no deflection by the galvanometer. When the magnet or coil is moving (which causes a change in the magnetic field around the coil), a current (or an induced Neff) is observed in the coil. ; Thus Michael Faraday concluded that a current will be induced in a coil when there is a change in the magnetic field around it, which is achieved when there is relative movement between the magnet and the coil. The size of the induced current can be increased by: ; Increasing the speed of the relative motion between the coil and the magnet ; Increasing the strength of the magnetic field Increasing the number of turns in the coil

Iron ring experiment: Michael Faraday conducted another experiment to illustrate electromagnetic induction. ;He had an iron ring, on one side of the ring he wound a primary circuit and connected it to a battery and a switch, on the other side he wound a secondary circuit with a galvanometer attached. ;There was no current induced in the secondary circuit when the switch was open or when there is a steady current flowing in the primary circuit as there is no magnetic field change around the primary circuit. During the process of closing or opening the switch S, a change in he magnitude of the current in the primary circuit occurred causes a change in the magnetic field around the primary circuit, this change in magnetic field passes through the secondary circuit inducing a current into the secondary circuit and according to Lens’s law creates its own magnetic field to oppose the original changing magnetic field from the primary circuit. ; Thus to put it simply as long as a conductor is left in a changing magnetic field (caused by the primary circuit), a current is induced in the conductor. Secondary circuit) Define magnetic field strength B as magnetic flux density The strength of the magnetic field, B, is also known as the magnetic flux density. It is the amount of magnetic flux passing through a unit area. Describe the concept of magnetic flux in terms of magnetic flux density and surface area Magnetic flux is the number of magnetic field lines passing through a surface area A at right angle to the field. It is represented by phi = BAA. If the magnetic flux is not flowing at right angles to area then phi = Backs(O). Magnetic flux density (or the magnetic field B) is the number of magnetic flux or magnetic field lines passing Note Areas don’t have to be the same,. The two diagrams to the right have different surface areas. Notice that both diagrams have the same amount of flux I. E. That is they have 2 magnetic field lines passing through a given area, however they have different magnetic flux densities. ; The bigger rectangle is mamma and has 2 magnetic field lines so it has 2/mamma, that is 0. 2 magnetic flux density. The smaller rectangle on the other hand has an area of mama and has 2 magnetic field lines so it has 2/mama, that is 1 magnetic flux density which is 5 times larger then the magnetic flux density of the bigger rectangle. Describe generated potential difference as the rate of change of magnetic flux through a circuit ; Faraday Law of Induction states: “The induced Neff in a circuit is equal in magnitude to the rate at which the magnetic flux through the circuit is changing with time. ; Mathematically this is expressed as Thus the Neff visually can be represented as the derivative of the magnetic flux (note be careful about the negative sign). Account for Lens’s law in terms of conservation of energy and relate it to the production of back Neff in motors The law of conservation of energy states that energy can not be destroyed or reared but merely transformed into one form to another. Lens’s law is a direct consequence to this and states that “The direction of the induced Neff is such that the current it produces creates a magnetic field that opposes the change that causes it”.

Suppose that the coil and the magnet does not obey Lens’s law and instead the induced current in the coil creates a magnetic field that attracts the magnet. This would cause the magnet to accelerate and therefore produce a current in the coil. This violates the law of conservation of energy because energy has been created; Hereford Lens’s law must always be obeyed. ; Lens’s law and the conservation of energy also apply too rotating motor.

As the coil rotates due to a supply Neff the magnetic flux passing through the coil is constantly changing. (magnetic flux is Max when the plane of the coil is perpendicular to the magnetic field and is minimum when the plane of the coil is parallel to the magnetic field. ) This changing magnetic flux induces an Neff known as the back Neff that opposes the rotation caused by the supply Neff. Suppose that this was not the case and the back Neff does not oppose the supply Neff , this would mean

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