Magnetic Fields Flashcards
What is a magnetic field?
A magnetic field is a region where a force is exerted on magnetic materials
What can magnetic fields be represented by?
Magnetic fields can be represented by field lines (also called flux lines)
In what direction do magnetic field lines go in?
Field lines go from the north to the south pole of a magnet
The closer together the magnetic field lines…
the stronger the field
What is there around a wire carrying electric current?
There is a magnetic field around a wire carrying electric current
What happens when current flows through a wire?
When current flows in a wire or any other long straight conductor, a magnetic field is induced around the wire
Describe the field lines of the magnetic field around a a current carrying wire
The field lines are concentric circles centred on the wire
How can you work out the direction of the magnetic field around a current carrying wire?
You can work out the direction of the magnetic field around a current carrying wire using the right hand rule
Explain how to use the right-hand rule
1- Stick your right thumb up like you’re hitching a lift
2- Your thumb points in the direction of conventional current
3- Your curled fingers point in the direction of the field
What will a wire carrying a current in a magnetic field experience?
A wire carrying a current in a magnetic field will experience a force
Explain what happens if you put a current carrying wire into an external magnetic field
If you put a current-carrying wire into an external magnetic field (eg. between two magnets) the field around the wire and the field from the magnets are added together. This causes a resultant field - lines closer together show where the magnetic field is stronger. These bunched lines cause a ‘pushing’ force on the wire
What is the direction of the force on a current carrying wire in an external magnetic field?
The direction of the force is always perpendicular to both the current direction and the magnetic field, its given by Fleming’s left hand rule
What is the size of the force on a current carrying wire in an external magnetic field if the current is parallel to the field lines?
If the current is is parallel to the field lines the size of the force is 0N - there is no component of the magnetic field perpendicular to the current
Explain how to use Flemings left hand rule
- The first finger points in the direction of the external uniform magnetic field (N to S)
- The second finger points in the direction of the conventional current (+ to -)
- The thumb points in the direction of the force in which motion takes place
What is the force on a wire proportional to?
The force on a current-carrying wire at a right angle to an external magnetic field is proportional to the magnetic flux density (B)
Define magnetic flux density (B)
Magnetic flux density is defined as the force on one metre of wire carrying a current of one amp at right angles to the magnetic field
Where does the equation F=BIl come from?
When current is at 90 degrees to the magnetic field, the size of the force (F) is proportional to the current (I), the length of the wire in the field (l), as well as the flux density (B). This gives the equation F=BIl
Is flux density a scalar or vector quantity?
Flux density is a vector quantity with both a direction and a magnitude. Its measured in teslas (T)
What are each of the variables in the equation F=BIl?
- F is the force
- I is the current
- l is the length of the wire in the field
- B is the flux density
What is 1 tesla equal to in terms of other units?
1 tesla = Wb/m^2
What is Wb (weber) the unit of?
Wb is the unit of the number of flux lines
Explain the method of the practical for investigating flux density (F=BIl)
1- A square hoop of metal wire is positioned so that the top of the hoop, length l, passes through the magnetic field, perpendicular to it. When a current flows, the length of wire in the magnetic field will experience a downwards force (Fleming’s left hand rule)
2- The power supply should be connected to a variable resistor so that you can alter the current. Zero the digital balance when there is no current through the wire so that the mass reading is due to the electromagnetic force only. Turn on the power supply
3- Note the mass and the current. Use the variable resistor to change the current and record the new mass - do this for a large range of currents. Repeat this until you have 3 mass readings for each current. Calculate the mean for each mass reading
4- Convert your mass readings into force using F=mg. Plot the data on a graph of force (F) against current (I). Draw a line of best fit
5- Because F=BIl, the gradient of your graph is equal to B*l. Measure the gradient, then divide by length l to get a value for B
6- Alternatively, you could vary the length of wire perpendicular to the magnetic field by using different sized hoops. You could also keep current and wire length the same and instead vary the magnetic field by changing the strength of the magnets
What acts on charged particle in a magnetic field?
A force acts on a charged particle moving in a magnetic field. This is why a current-carrying wire experiences a force in a magnetic field - electric current in a wire is the flow of negatively charged electrons
What is the formula used to calculate the force on a current-carrying wire in a magnetic field that is perpendicular to the current?
F=BIl