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P7 D) Motor Effect
P7 D) Motor Effect
If we place a current-carrying wire is place in between the poles (north and south) of two magnets, the two magnetic field will interact with each other resulting in a force being exerted onto the current-carrying wire. The two magnetic fields are:
We are going to look at all of the forces for the motor effect starting with the force from the two magnets first. The magnetic field lines from magnets always go from north (positive) to south (negative). There will be a uniform magnetic field between the flat parts of the two magnets; a uniform magnetic field is where the field lines are straight, parallel and have an equal distance between the lines. The magnetic field lines from the magnet are shown below.
- The magnetic field from the two magnets
- The current-carrying wire
We are going to look at all of the forces for the motor effect starting with the force from the two magnets first. The magnetic field lines from magnets always go from north (positive) to south (negative). There will be a uniform magnetic field between the flat parts of the two magnets; a uniform magnetic field is where the field lines are straight, parallel and have an equal distance between the lines. The magnetic field lines from the magnet are shown below.
The current-carrying wire also produced a magnetic field, and the magnetic field produced from the current-carrying wire is shown below; this diagram does not include the magnetic field produced by the two magnets.
On the above diagram, there is a dot that is representing the current-carrying wire. The red dot means that the current is travelling towards you and out of the page (if there was an ‘x’ rather than a dot, it would mean that the current in the wire is travelling away from you and into the page). We need to know the direction of the current because the direction of the current determines the direction of the magnetic field produced by the wire. We can work out the direction of the magnetic field produced by the wire by using the ‘right-hand thumb rule’, where the thumb is the direction of the current and the curls of the fingers show us the direction of the magnetic field produced. For the above diagram, the current is coming out of the page, so our thumb will be pointing away from the page. When we look at the curls on our right hand, we see that they are anticlockwise. This means that the magnetic field produced by the current-carrying wire is anticlockwise.
Interacting of Both Magnetic Fields
The two magnetic fields will interact with one another, which produces a force. The diagram below shows the interaction between the two magnetic fields and the direction of the force that is produced.
Interacting of Both Magnetic Fields
The two magnetic fields will interact with one another, which produces a force. The diagram below shows the interaction between the two magnetic fields and the direction of the force that is produced.
The interaction of the two magnetic fields causes the magnetic fields to deviate, which exerts a force on the wire. This is known as the motor effect. This is indicated on the diagram by the orange arrow. The force that is produced is always in the same direction relative to the magnetic field and the direction of the current. The force that is produced is not huge, but it is enough to cause the current-carrying wire to move slightly in the direction of the orange arrow. A change in the current direction or the position of the magnetics will result in a change in the direction of the force.
The greatest force occurs when the current-carrying wire passes through the poles of two magnets at 90°. If the current-carrying wire passes at an angle that is less than 90°, the force produced will be smaller. If the wire travels in the same direction as the magnetic field from the magnets, there will be no force exerted on the current-carrying wire.
The greatest force occurs when the current-carrying wire passes through the poles of two magnets at 90°. If the current-carrying wire passes at an angle that is less than 90°, the force produced will be smaller. If the wire travels in the same direction as the magnetic field from the magnets, there will be no force exerted on the current-carrying wire.
Fleming’s Left-Hand Rule
We are able to use Fleming’s left-hand rule to determine the direction of the force exerted. To use Fleming’s left-hand rule, you place you left hand with your thumb, first finger and second finger at right angles to each other like the diagram shows.
We are able to use Fleming’s left-hand rule to determine the direction of the force exerted. To use Fleming’s left-hand rule, you place you left hand with your thumb, first finger and second finger at right angles to each other like the diagram shows.
You point your first finger in the direction of the magnetic field between the magnetics from north to south (positive to negative). Next, you point your second finger in the direction of the current. After you have the first and second position in their position, the direction of your thumb will show you the direction of the force exerted on the wire. The easy way to remember what finger means what is:
In order to use Fleming’s left-hand rule, we need to have 2 out of the 3 variables.
Let’s now show that Fleming’s left-hand rule works for the original diagram at the start. The original diagram is shown below
- First finger is Field between magnet (FF)
- seCond finger is Current (CC)
- thuMb is the direction of force, which is referred to as Motion (MM)
In order to use Fleming’s left-hand rule, we need to have 2 out of the 3 variables.
Let’s now show that Fleming’s left-hand rule works for the original diagram at the start. The original diagram is shown below
The first finger for Fleming’s left-hand rule is the field between the magnetics, which goes from north to south (positive to negative). For the above diagram, this is from right to left, so the first thing will be pointing rightwards.
The second finger in the current. The wire is represented by a dot, which means that the current is coming out of the screen towards you. This means that our second finger will be pointing towards you.
We now look at the position of the thumb to determine the force that is exerted on the wire. When are fingers are in these positions, the thumb is pointing upwards, which means that the force will be upwards.
The second finger in the current. The wire is represented by a dot, which means that the current is coming out of the screen towards you. This means that our second finger will be pointing towards you.
We now look at the position of the thumb to determine the force that is exerted on the wire. When are fingers are in these positions, the thumb is pointing upwards, which means that the force will be upwards.
Question
A typical exam question will ask you to find the direction of the force exerted. Here is an example.
A current is passed through a wire that is between opposite poles of two magnets. In which direction does the wire experience a force.
A typical exam question will ask you to find the direction of the force exerted. Here is an example.
A current is passed through a wire that is between opposite poles of two magnets. In which direction does the wire experience a force.
We are able to the direction of the force by using Fleming’s left-hand rule.
The first finger in Fleming’s left-hand rule is the direction of the magnetic field, which is from north to south (positive to negative). For the magnets in the diagram above, the magnetic field is from right to left; it is leftwards.
The second finger is the current for the wire. The wire is represented by an “x”, which means that the current is travelling into the screen. Therefore, our second finger will be pointing towards the screen.
We now have 2 fingers in position, which means that we can look at the direction that our thumb is pointing to determine the direction of force exerted on the wire. Our thumb is pointing up the screen, which means that the force exerted on the wire is upwards on the screen. I have added the direction of the force on the diagram below.
The first finger in Fleming’s left-hand rule is the direction of the magnetic field, which is from north to south (positive to negative). For the magnets in the diagram above, the magnetic field is from right to left; it is leftwards.
The second finger is the current for the wire. The wire is represented by an “x”, which means that the current is travelling into the screen. Therefore, our second finger will be pointing towards the screen.
We now have 2 fingers in position, which means that we can look at the direction that our thumb is pointing to determine the direction of force exerted on the wire. Our thumb is pointing up the screen, which means that the force exerted on the wire is upwards on the screen. I have added the direction of the force on the diagram below.
There are two different ways that we can change the direction of the force produced.
- One way is to change the direction of the current. For this question, the current was going into the page, which resulted in the force being upwards. If we change the direction of the current so that the current was coming out of the page, the force produced would be downwards. If we wanted to show the change in the direction of the current on the above diagram, we would change the x/ cross to a dot.
- Another way that we can change the direction of the force produced is to swap the poles of the magnets around. When we swap the poles of the magnets around, the north will be on the left and the south will be on the right.
Size of the Force
The magnitude of the force on the wire in a magnetic field is dependent on three factors:
Here is the formula for working out the force that exerted on the wire:
The magnitude of the force on the wire in a magnetic field is dependent on three factors:
- The magnetic flux density (B), which is the strength of the magnetic field produced by the magnets. The stronger the magnetic field is/ the greater magnetic flux density, the greater the force exerted on the wire. The weaker the magnetic field is/ the weaker magnetic flux density, the weaker the force exerted on the wire. We are able to work out the strength of the magnetic field/ magnetic flux density by looking at how many field lines there are on the diagram. Field lines that are closer together mean that there is a greater magnetic field strength and therefore there will be a greater force that acts on the wire. We measure the magnetic flux density in teslas (T).
- The size of the current through the wire (I). A stronger current through the wire will produce a stronger magnetic field around the wire, which will result in a greater force being exerted on the wire. A weaker current means a weaker force exerted on the wire. We measure current in amps (A).
- The length of the wire that is in the magnetic field of the magnets (L). The greater the length of the wire in the magnetic field of the magnets, the greater the force exerted on the wire. The length of the wire is measured in metres (m).
Here is the formula for working out the force that exerted on the wire:
The force, like all force, is measured in Newtons (N).
This formula only works if the wire is at 90° to the magnetic field produced by the magnets. If the wire passes through the magnetic field produced by the magnets at an angle that is less than 90°, the force exerted on the wire would be less than this formula.
Example
A current of 8 A passes through a wire that is in a magnetic field. The length of the wire that is in the magnetic field from the magnets is 5 cm. The value for the magnetic flux density is 0.65 teslas. Find the force that is exerted on the wire.
We find the force exerted on the wire by using the formula below.
This formula only works if the wire is at 90° to the magnetic field produced by the magnets. If the wire passes through the magnetic field produced by the magnets at an angle that is less than 90°, the force exerted on the wire would be less than this formula.
Example
A current of 8 A passes through a wire that is in a magnetic field. The length of the wire that is in the magnetic field from the magnets is 5 cm. The value for the magnetic flux density is 0.65 teslas. Find the force that is exerted on the wire.
We find the force exerted on the wire by using the formula below.
B in the formula above is the magnetic flux density and it is 0.65 teslas. I is the current, which is 8 A. And, l is the length of the wire in the magnetic field, which is 5 cm. B and I are the question are in the correct units, but the length of the wire is not. The length of wire should be measured in metres, but currently it is measured in centimetres. We can convert centimetres into metres by divide by 100. This gives us a value for l of 0.05 m (5 ÷ 100 = 0.05). We can now sub the values into the formula; we sub B in as 0.65, A in as 8 and l in as 0.05.
The force exerted on the wire is 0.26 N.