# 25.4.1 Magnetic Fields in Wires, Coils & Solenoids

### Magnetic Fields in Wires, Coils & Solenoids

• Magnetic field patterns are not only observed around bar magnets, magnetic fields are formed wherever current is flowing, such as in:
• Long straight wires
• Long solenoids
• Flat circular coils

#### Field Lines in a Current-Carrying Wire

• Magnetic field lines in a current carrying wire are circular rings, centered on the wire
• The field lines are strongest near the wire and become further part away from the wire
• Reversing the current reverses the direction of the field
• The field lines are clockwise or anticlockwise around the wire, depending on the direction of the current
• The direction of the magnetic field is determined by Maxwell’s right hand screw rule
• This is determined by pointing the right-hand thumb in the direction of the current in the wire and curling the fingers onto the palm
• The direction of the curled fingers represents the direction of the magnetic field around the wire
• For example, if the current is travelling vertically upwards, the magnetic field lines will be directed anticlockwise, as seen from directly above the wire
• Note: the direction of the current is taken to be the conventional current ie. from positive to negative, not the direction of electron flow

#### Field Lines in a Solenoid

• As seen from a current carrying wire, an electric current produces a magnetic field
• An electromagnetic makes use of this by using a coil of wire called a solenoid which concentrates the magnetic field
• One ends becomes a north pole and the other the south pole
• Therefore, the magnetic field lines around a solenoid are very similar to a bar magnet
• The field lines emerge from the north pole
• Which is the north or south pole depends on the direction of the current
• This is found by the right hand grip rule
• This involves gripping the electromagnet so the fingers represent the direction of the current flow of the wire
• The thumb points in the direction of the field lines inside the coil, or in other words, point towards the electromagnet’s north pole

#### Field Lines in a Flat Circular Coil

• A flat circular coil is equal to one of the coils of a solenoid
• The field lines will emerge through one side of the circle (north pole) and leave the other (south pole)
• As before, the direction of the north and south pole depend on the direction of the current
• This can be determined by using the right hand thumb rule
• It easier to find the direction of the magnetic field on the straight part of the circular coil to determine which direction the field lines are passing through

#### Worked example

✓ Concentric circles

✓ Increasing separation between each circle

✓ Arrows drawn in anticlockwise direction

#### Exam Tip

Remember to draw the arrows showing the direction of the field lines on every single field line you draw. Also, ensure that in a uniform magnetic field, the field lines are equally spaced.

### Factors Affecting Magnetic Field Strength

• The strength of the magnetic field of a solenoid can be increased by:
• Adding a core made from a ferrous (iron-rich) material eg. an iron rod
• Adding more turns in the coil
• When current flows through the solenoid with an iron core, it becomes magnetised, creating an even stronger field
• The addition of an iron core can strengthen the magnetic field up to a several hundred times more
• When more turns are added in the coil, this concentrates the magnetic field lines, causing the magnetic field strength to increase

### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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