Magnetic Fields and Electromagnetism Notes

Magnetic Field Around Wires & Solenoids

• When a current flows through a conducting wire, a magnetic field is produced around the wire.
• A conducting wire is any wire that has current flowing through it.
• The shape and direction of the magnetic field can be investigated using plotting compasses.
  • The compasses would produce a magnetic field lines pattern.
• The magnetic field is made up of concentric circles.
• A circular field pattern indicates that the magnetic field around a current-carrying wire has no poles.
• As the distance from the wire increases, the circles get further apart.
  • This shows that the magnetic field is strongest closest to the wire and gets weaker as the distance from the wire increases.

Right-Hand Thumb Rule

• The right-hand thumb rule can be used to work out the direction of the magnetic field.
• Thumb points along the direction of the current.
• Other fingers give the direction of the field.
• Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field.
• Side View:
  • A circle with a dot in the center shows that current is flowing out of the plane.
• Top View:
  • A circle with a cross in the center shows that current is flowing into the plane.

Magnetic Field and Current

• If there is no current flowing through the conductor, there will be no magnetic field.
• Increasing the amount of current flowing through the wire will increase the strength of the magnetic field.
  • This means the field lines will become closer together.

Magnetic Field Around a Solenoid

• When a wire is looped into a coil, the magnetic field lines circle around each part of the coil, passing through the center of it.
• To increase the strength of the magnetic field around the wire, it should be coiled to form a solenoid.
• The magnetic field around the solenoid is similar to that of a bar magnet.
• The magnetic field inside the solenoid is strong and uniform.
• One end of the solenoid behaves like the north pole of a magnet; the other side behaves like the south pole.
• To work out the polarity of each end of the solenoid, it needs to be viewed from the end.
  • If the current is traveling around in a clockwise direction, then it is the south pole.
  • If the current is traveling around in an anticlockwise direction, then it is the north pole.
• If the current changes direction, then the north and south poles will be reversed.
• If there is no current flowing through the wire, then there will be no magnetic field produced around or through the solenoid.

Poles of a Solenoid

• N Pole: End view of current traveling anticlockwise.
• S Pole: End view of current traveling clockwise.

Electromagnets

• A solenoid can be used as an electromagnet by adding a soft iron core.
• The iron core will become an induced magnet when current is flowing through the coils.
• The magnetic field produced from the solenoid and the iron core will create a much stronger magnet overall.
• The magnetic field produced by the electromagnet can be switched on and off.
  • When the current is flowing, there will be a magnetic field produced around the electromagnet.
  • When the current is switched off, there will be no magnetic field produced around the electromagnet.
• Changing the direction of the current also changes the direction of the magnetic field produced by the iron core.

Factors Affecting Magnetic Field Strength

• The strength of the magnetic field produced around a solenoid can be increased by:
  • Increasing the size of the current which is flowing through the wire.
  • Increasing the number of coils.
  • Adding an iron core through the center of the coils.
• The strength of an electromagnet can be changed by:
  • Increasing the current will increase the magnetic field produced around the electromagnet.
  • Decreasing the current will decrease the magnetic field produced around the electromagnet.

Applications of the Magnetic Effect

• Electromagnets are used in a wide variety of applications, including:
  • Relay circuits (utilized in electric bells, electronic locks, scrapyard cranes etc.)
  • Loudspeakers & headphones

Relay Circuits

• Electromagnets are commonly used in relay circuits.
• Relays are switches that open and close via the action of an electromagnet.
• A relay circuit consists of:
  • An electrical circuit containing an electromagnet.
  • A second circuit with a switch which is near to the electromagnet in the first circuit.
• When a current passes through the coil in Circuit 1, it attracts the switch in Circuit 2, closing it enables a current to flow in Circuit 2.
• When a current flows through Circuit 1, a magnetic field is induced around the coil.
• The magnetic field attracts the switch, causing it to pivot and close the contacts in Circuit 2.
• This allows a current to flow in Circuit 2.
• When no current flows through Circuit 1, the magnetic force stops.
  • The electromagnet stops attracting the switch.
  • The current in Circuit 2 stops flowing.
• Scrapyard cranes utilize relay circuits to function:
  • When the electromagnet is switched on it will attract magnetic materials.
  • When the electromagnet is switched off it will drop the magnetic materials.
• Electric bells also utilize relay circuits to function.
• When the button K is pressed:
  • A current passes through the electromagnet E creating a magnetic field.
  • This attracted the iron armature A, causing the hammer to strike the bell B.
  • The movement of the armature breaks the circuit at T.
  • This stops the current, destroying the magnetic field and so the armature returns to its previous position.
  • This re-establishes the circuit, and the whole process starts again.

Loudspeakers & Headphones

• Loudspeakers and headphones convert electrical signals into sound.
  • They work due to the motor effect.
• A loudspeaker consists of a coil of wire which is wrapped around one pole of a permanent magnet.
• An alternating current passes through the coil of the loudspeaker.
• This creates a changing magnetic field around the coil.
• As the current is constantly changing direction, the direction of the magnetic field will be constantly changing.
• The magnetic field produced around the coil interacts with the field from the permanent magnet.
• The interacting magnetic fields will exert a force on the coil.
• The direction of the force at any instant can be determined using Fleming's left-hand rule.
• As the magnetic field is constantly changing direction, the force exerted on the coil will constantly change direction.
• This makes the coil oscillate.
• The oscillating coil causes the speaker cone to oscillate.
• This makes the air oscillate, creating sound waves.

Investigating the Field Around a Wire

• The magnetic field patterns due to currents in straight wires and in solenoids can be investigated using:
  • A thick wire
  • A solenoid (a wire wrapped into a coil) – for example, a metal slinky
  • Cell, ammeter, variable resistor and connecting wires
  • Cardboard with holes (the holes must be large enough for the wire to fit through)
  • Clamp stand
  • Iron filings or a compass
• Spread the iron filings uniformly on the cardboard and place the magnetic needle on the board.
• Tap the cardboard slightly and observe the orientation of iron filings.
• When the current direction is reversed, the compasses point in the opposite direction showing that the direction of the field reverses when the current reverses.

Experiment 1: Plotting the magnetic field around a wire

1. Attach the thick wire through a hole in the middle of the cardboard and secure it to the clamp stand

• Secure the wire vertically so it sits perpendicularly to the cardboard

1. Attach the ends of the wire to a series circuit containing the variable resistor and ammeter on either side of the cell

Using plotting compasses:

1. Place plotting compasses on the card and draw dots at each end of the needle once it settles

• Make sure to draw an arrow to show the direction of the field at different points

1. Move the compass so that it points away from the new dot, and repeat the process above
2. Keep repeating the previous process until there is a chain of dots on the card
3. Then remove the compass, or compasses, and link the dots using a smooth curve - this will be the magnetic field line
4. Repeat the whole process several times to create several other magnetic field lines

Using iron filings:

1. If using iron filings, simply pour the filings onto the cards and gently shake the card until the filings settle in the pattern of the magnetic field around the wire

Experiment 2: Plotting the magnetic field around a solenoid

1. Attach the thick wire through a hole on one side of the cardboard and loop it through a hole on the other side of the cardboard and secure it to the clamp stand

• Secure the wire so it forms a circular loop around the cardboard

1. Attach the ends of the wire to a series circuit containing the variable resistor and ammeter on either side of the cell

Using plotting compasses:

1. Follow the procedure outlined in Experiment 1

• Note: this can be carried out using a solenoid, but since a solenoid is essentially many circular loops, the pattern around a circular loop can be extended to give the pattern around a solenoid

Using iron filings and a solenoid:

1. Take a solenoid (a metal slinky works well for this) and thread it through pre-made holes in a piece of card
2. Pour the filings onto the card and gently shake the card until the filings settle in the pattern of the magnetic field around the solenoid

Force on a Current-Carrying Conductor

• A current-carrying conductor produces its own magnetic field
  • When interacting with an external magnetic field, it therefore will experience a force
• A current-carrying conductor will only experience a force if the current through it is perpendicular to the direction of the magnetic field lines
  • A simple situation would be a copper rod placed within a uniform magnetic field
• When current is passed through the copper rod, it experiences a force which makes it move
• Two ways to reverse the direction of the force (and therefore, the copper rod) are by reversing:
  • The direction of the current
  • The direction of the magnetic field
• The direction of this force depends on:
  • The direction of the field.
  • The direction of the current.
• Reversing either of the above will reverse the direction of the force.
• Changing either the direction of the current or the direction of the magnetic field will change the direction of the force on the wire.
• By changing the direction of both the force on the wire remains upwards

Left Hand Rule

• The direction of the force (aka the thrust) on a current carrying wire depends on the direction of the current and the direction of the magnetic field
• All three will be perpendicular to each other
  • This means that sometimes the force could appear to be acting either into or out of the page
• The direction of the force (or thrust) can be worked out by using Fleming's left-hand rule:
  • Thumb: Thrust
  • First Finger: Field
  • Second Finger: Current

DC Motor

• The motor effect can be used to create a simple d.c electric motor
• The simple d.c. motor consists of a coil of wire (which is free to rotate) positioned in a uniform magnetic field:
• This causes the coil to rotate since it experiences a turning effect
• The turning effect is increased by increasing:
  • The number of turns on the coil
  • The current
  • The strength of the magnetic field
• The force supplied by a motor can be increased by:
  • Increasing the current in the coil.
  • Increasing the strength of the magnetic field.
  • Adding more turns to the coil.

Operation of a DC Motor

• When the current is flowing in the coil at 90° to the direction of the magnetic field:
  • The current creates a magnetic field around the coil
  • The magnetic field produced around the coil interacts with the field produced by the magnets
  • This results in a force being exerted on the coil
• The direction of the force can be determined using Fleming's left-hand rule
  • As current will flow in opposite directions on each side of the coil, the force produced from the magnetic field will push one side of the coil up and the other side of the coil down
  • This will cause the coil to rotate, and it will continue to rotate until it is in the vertical position
  • In the vertical position momentum keeps the coil turning until the magnetic force takes over again
• The split ring commutator swaps the contacts of the coil
  • This reverses the direction in which the current is flowing every half turn
  • This keeps the current leaving the motor in the same direction (d.c)
• Reversing the direction of the current will also reverse the direction in which the forces are acting
  • As a result, the coil will continue to rotate
• The split-ring commutator reverses the direction of the current in the coil every half turn
  • This will keep the coil rotating continuously as long as the current is flowing

Factors Affecting the D.C Motor

• The speed at which the coil rotates can be increased by:
  • Increasing the current
  • Use a stronger magnet
• The direction of rotation of coil in the d.c motor can be changed by:
  • Reversing the direction of the current
  • Reversing the direction of the magnetic field by reversing the poles of the magnet
• The force supplied by the motor can be increased by:
  • Increasing the current in the coil
  • Increasing the strength of the magnetic field
  • Adding more turns to the coil

Charged Particles in a Magnetic Field

• When a current-carrying wire is placed in a magnetic field, it will experience a force if the wire is perpendicular
  • This is because the magnetic field exerts a force on each individual electron flowing through the wire
• Therefore, when a charged particle passes through a magnetic field, the field can exert a force on the particle, causing it to deflect
  • The force is always at 90 degrees to both the direction of travel and the magnetic field lines
• The direction can be worked out by using Fleming's left-hand rule
• In the case of a electron in a magnetic field the second finger points in the opposite direction to the direction of motion
  • Conventional current is said to flow opposite to the direction of flow of electrons
  • The finger represents current
  • An alternative is to use the right hand to work out directions for charged particles
• If the particle is travelling perpendicular to the field lines:
  • It will experience the maximum force
• If the particle is travelling parallel to the field lines:
  • It will experience no force
• If the particle is travelling at an angle to the field lines:
  • It will experience a small force