## 6.7 Magnetic force and field - Questions

1 -
How is the direction of a magnetic field defined ?

2 -
Draw the magnetic field around one permanent magnet.

3 -
Draw the magnetic field between two permanent magnets.

4 -
Draw the magnetic field that the earth is creating.

5 -
What happens when electrical charges move, for example electrons in a wire ?

6 -
Draw the magnetic field around a wire with a current. Describe the hand rule to get the field direction.

7 -
Explain what the field looks like around a circular wire.

8 -
Draw the magnetic field around a coil with a current.

9 -
What is the hand rule for a coil ?

10 -
What happens when a wire with a current is placed in an external field ?

11 -
What is the formula for a wire with a current that is placed in an external field ?

12 -
What is the hand rule for a wire with a current that is placed in an external field ?

13 -
What happens when a free charge such as an electron move in an external field ?

14 -
What is the formula for free charge that moves in an external field ?

15 -
What is the hand rule for a free charge that moves in an external field ?

16 -
What is the magnetic flux density ?

17 -
Give a difference between an electric field and a magnetic field.

18 -
What is the definition of the unit Ampere ?

## 6.7 Magnetic force and field

If one put a small compass needle (magnet) in a magnetic field then
north will point in the field direction. When you draw the magnetic field lines this
means that they always goes from a North pole to a South pole.

Moving charges give rise to a magnetic field.
So a wire with a current will produce a magnetic field.

When a wire with a current is placed in an external field then the field from the current
in the wire combines with the external field to create a force on the wire. The force is
perpendicular to both the field direction and the current (wire) direction.

Look at the data booklet:

F = B•I•L

Where B is the magnetic flux density in Tesla, I is the current in Amperes and L is the length of the
wire in meters and F is the force in Newtons.

If the field is not perpendicular to the current but there is an angle θ between the current and
the field then the formula changes to:

F = B•I•L•sin(θ)

Use your right hand. Put your fingers in the field direction and the thumb in the current direction. The force
is the sticking out of the palm of your hand.

When a free charge such as a free electron moves through a magnetic field the electron will experience a force
perpendicular to both the field direction and the direction of the movement.

Look at the data booklet:

F = B•q•v

Where B is the magnetic flux density in Tesla, q is the charge in Coulomb and v is the velocity in m/s
and F is the force in Newtons.

If the field is not perpendicular to the velocity but there is and angle θ between the velocity and
the field then the formula changes to:

F = B•q•v•sin(θ)

Use your right hand. Put your fingers in the field direction and the thumb in the current direction. The force
is the sticking out of the palm of your hand. However, remember that electrons that move create a current
in the opposite direction of their movements. So another way of doing it is to now use your left hand and let
the thumb be in the direction of the electron movement.

The magnetic flux density (B) gives the strength and direction of a magnetic field. It is a vector quantity.
It has the unit Tesla (T).

An electric field can be created by a single charge which sends a field in all directions but a magnet always
have both a north pole and a south pole with field lines between them. You cannot have a monopole magnet only
dipole magnets but you can have both a mono-charge and a di-charge electrical field.

When a current going through two 1 meter long parallel conductors that are 1 meters apart is 2x10^{-7}N
then the current is exactly 1 Ampere.