18.1 – Magnetic Effect of a Current

A compass placed near to a current carrying wore will show a deflection. This is because a current carrying wire generates a magnetic field around it.

As the current flows from A to B, the magnetic field generated around the wire AB causes the compass needles to deflect.

The left compass has been placed above the wire AB. The right compass has been placed below the wire AB. The magnetic field is actually circling the wire AB.

Unlike the magnetic field from a bar magnet, the field around a current carrying wire does not go from/to poles. The field is circular in nature.

Magnetic field due to current in a straight wire

1. Direction of magnetic field when the direction of the current is away from you

Current flowing this way.
Often referred to as “into the page/screen”

Magnetic field lines can be see here circling the wire
in a clockwise direction.

 

2. Direction of magnetic field when the direction of the current is towards you

Current flowing this way.
Often referred to as “out of the page/screen” Magnetic field lines can be see here circling the wire
in an anticlockwise direction.
Caution
Remember, magnetic fields extend to infinity, just like electric fields & gravitational fields. They get weaker the further we are from the wire. This can be seen from the field lines being spaced further and further from each other.
Direction Symbols: Representing INTO and OUT OF a 2-D Surface
As the paper / screen is only 2-dimensional, it his difficult to show things that are 3-dinemsional. We will frequently see the following symbols:

coming OUT OF the screen / page

 

going INTO the screen / page

 

It helps to image them as the ends of an arrow:

If this arrow is coming towards you then all you will just see the tip coming towards you:

and if the arrow is going away from you, you will see the tail feathers moving away from you:

 

The Right Hand Grip Rule
The direction can be determined using the right hand grip rule.

This is a useful way of remembering the direction of the (circular) magnetic field lines B (shown by curled fingers) relative to the current I (shown by thumb).

This is not a law of physics. Just a way to remember the direction of magnetic field produced by a current in a wire.

This rule is defined using the direction of current meaning the direction of conventional current.

 

Magnetic field due to current in a single loop of wire

The magnetic filed around a loop could be investigated by placing a piece of card as shown above and using a plotting compass to show the direction of the field lines.

The field will have the following shape:

Note: For clarity only some direction arrows are shown on the magnetic field here. (You should always draw an arrow on each fired line.

 

Note how the direction of the field can be determined from the right hand grip rule.

Note that there is a  straight field line running through the centre of the loop.

Note hat near the wire the field is almost circular around the wire.

 

Magnetic field due to current in a solenoid

A solenoid is just a coil of wire.

Current flowing through the wire shown above will create magnetic fields around each coil of wire. These add together to create a field like this:

The direction of the above filed is determined by the Right Hand Grip Rule (for solenoids) as we saw back in the chapter on magnetism.

Example
Determine the polarities of each end of the following solenoids.

(a)

 

 

(b)

 

 

 

(c)

 

 

 

(d)

 

 

(e)

 

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