Magnetic Fields Flashcards
Why are magnets called dipoles?
Magnets have a North Pole and a South Pole
What is a magnetic field?
A region where a magnet exerts a force on objects made from magnetic materials or on other magnets
How do you represent magnetic fields?
Using arrows or flux lines to indicate the strength and direction fo the field in the region surrounding the magnet
Describe the field lines for magnets
- Flux lines represent the direction of the force experienced by the both pole of. magnet at any point in a magnetic field (N->S)
- A magnetic field is strongest where its flux density is highest and this is shown as flux lines closest together
- A magnetic field with twice the strength is drawn with twice the number of flux lines per unit area in the same region
- The magnetic field of more than one magnet is the combined field of the individual magnets
- Flux lines never cross
- If there is more than one magnets, the magnetic fields cancel out in some places and there is a neutral point
Which direction do magnetic field lines go?
- North to South
- Closer the lines are together the stronger the field
How do you mark a neutral point?
With an x
What will a magnet freely suspend in a magnetic field do?
Align itself with the field
What is magnetic flux density?
- The strength or intensity of a magnetic field is its magnetic flux density B also known as B field
- Magnetic flux density is the number of magnetic flux lines that pass through an area of 1m^2
How is magnetic flux density measured?
Magnetic flux density is measured in Teslas (T)
What is Tesla?
Tesla is the flux density that causes a force of 1N on a 1m wire carrying a current of 1A at right angles to the flux
What do moving charges cause?
- Moving charges cause a magnetic field, which we describe using flux lines
- When current flows in a wire or any other long straight conductor, a magnetic field is induced around the wire
How is the magnetic flux around a current carrying wire shown?
- As concentric circles indicating the magnitude and direction of the flux
1. Moving away from the wire, flux lines are further apart because the field gets weaker
2. If you look at the wire with the conventional current flowing away from you the flux lines circle the wire in a clockwise direction
What does the x indicate?
Current flowing away from you
What does the dot indicate?
Current flowing towards you
How do you work out the direction of a magnetic field around a current-carrying wire?
- Use the right hand rule
1. Thumb points in direction of conventional current
2. Curled up fingers point in the direction of the field
What is the direction of the magnetic field if you loop the wire into a coil?
If you loop the wire into a coil, the filed is doughnut shaped while a coil with a length (a solenoid) form a field like a bar magnet
What is a solenoid?
A solenoid is a current carrying coil of wire that produces magnetic flux (this is also described as an electromagnet formed form a coil of wire)
What is the magnetic flux around a solenoid like?
The magnetic flux outside a solenoid is similar to the magnetic flux for a bar magnet, with the North Pole at one end of the coil and the South Pole at the other end, depending on the current direction
Why does the current-carrying wire in a magnetic field move?
Because a force acts on it, the magnetic field making the wire move is called the catapult field
What is the catapult field due to?
The catapult field is due to the combined effect of the current carrying wire’s flux and the static flux
Why will a wire carrying a current in a magnetic field experience a force?
- If you put a current-carrying wire into an external magnetic file (e.g. between two magnets) the field around the wire and the field from the magnets are added together
- This causes a resultant feel (lines closer together show where the magnetic field is stronger) and this bunched up lines cause a ‘pushing’ force on the wire
What is the direction of the force on a current carrying wire in a magnetic field always?
- The direction of the force is always perpendicular to both he current direction and the magnetic field (given by FLHR)
- If the current is parallel to the field lines the size of the force is 0N as there is no component of the magnetic field perpendicular to the current
What is Fleming’s Left-Hand Rule (FLHR)?
- Thumb point in direction of force
- First finger is in direction fo the external uniform magnetic field
- Second finger points in direction of conventional current