Electromagnetism

Question 1

What happens when an electric current flows through a conductor?

Answer 1

A magnetic field is produced

Question 2

How can you use the right hand thumb rule to determine the direction of magnetic field lines in the magnetic field produced in a conductor with current running through it?

Answer 2

• Point your thumb in the direction of the flow of current (on paper, a dot denotes the current flowing towards you and an x denotes the current flowing away from you)
• The direction your fingers are wrapping around is the direction of the magnetic field lines

• The magnetic field lines will be concentric circles with the distance between them getting larger as distance increases from the wire
• This is an indicator that the wire has no poles

Question 3

What are the factors which influence the strength of a magnetic field in a conducting wire that has current running through it?

Answer 3

• The size of the current (so also resistance of the wire)
• The distance away from the wire (as distance increases, the strength of the field decreases)

Question 4

How are electromagnets made and why are they made this way?

Answer 4

• A conducting wire with current running through it is coiled to concentrate the strength of the magnetic field
• A soft magnetic material is placed inside the coil
• This is usually iron, and is known as an iron core
• When current runs through the wire, the iron core will become magnetic
• This is advantageous as it will make a much stronger magnet compared to if you simply used a wire, and can be switched on and off by simply switching on and off the current

The direction of the magnetic field lines can be reversed by reversing the direction of current flow (as is made clear by the right hand thumb rule)

Question 5

How can the strength of an electromagnet’s magnetic field be increased?

Answer 5

• Increasing the size of the current running through the wire
• Adding more turns to the coil of wire
• Increasing the strength of the soft magnet (iron core) or simply adding one

Adding more turns increases the current because each point of the wire has the same current so you are increasing the size and concentration of current

Question 6

What does the magnetic field pattern for a flat circular coil look like?

Answer 6

The magnetic field lines circle around each part of the wire and pass directly through the centre of it as seen below

Question 7

What does the magnetic field pattern for a solenoid look like?

A solenoid is the coiled wire in an electromagnet

Answer 7

• Around the solenoid, it is similar to that of a bar magnet as one end acts as the north pole and one end acts as the south pole
• Inside the solenoid, the magnetic field is uniform and strong

• You can work out which end is the north pole and which end is the south pole by viewing it from the end and observing the direction current is travelling in
• If the current changes direction the poles will be reversed

Question 8

What happens when a charged particle moves in a magnetic field in a way that is not parallel to the direction of the field?

Answer 8

• A force is exerted on the charged particle, causing it to deflect
• This means if a wire carrying a charge cuts through magnetic field lines, a force will be exerted on the wire as a force is exerted on each individual electron
• This is known as the motor effect

This effect is a result of two interacting magnetic fields (because the moving electrons will produce a magnetic field which is constantly moving with the movement of the electrons)

Question 9

How can the motor effect be used to make simple d.c. electric motors?

Answer 9

• A freely rotating coil of wire is placed in the middle of a uniform magnetic field (between two bar magnets)
• When current flows through this wire (at 90 degrees to the direction of the magnetic field), a force will be exerted on the wire due to the interacting magnetic fields
• As current flows in opposite directions on each side of the coil, one side will be pushed down and the other will be pushed up (the direction of the force can be explained by Fleming’s left hand rule), allowing it to spin
• When in the vertical position, the coil will no longer spin due to it having a balanced force up and down (so no resultant force), so here the split ring comunicator switches the contacts so the current begins travelling in the reverse direction which reverses the direction of the forces allowing it to continue spinning
• The split ring communicator will continue to swap the direction of current every half turn to ensure it continues to spin in the same direction

Alternatively an A.C power supply can be used

Question 10

How can you increase the speed of a d.c. motor by increasing the size of the force applied to the coil?

Answer 10

• Increase the size of the current
• Increase the strength of the magnetic field of the permanent magnets
• Increase the number of turns in the coil
• Make sure the wire is 90 degrees to the magnetic field lines of the permanent magnet for maximum effect - if they are parallel there will be no effect

The direction the coil spins can be changed by not only reversing the direction of the current but also by reversing the poles of the permanent magnets

Question 11

How can the motor effect be used to make loudspeakers?

Answer 11

• A coil of wire is wrapped around the pole of a permanent magnet
• An alternating current is passed through this coil, creating a changing magnetic field
• This will interact with the field from the permanent magnet, exerting a force on the coil
• The direction of the force can be determined using Fleming’s left hand rule, and the force exerted on the coil will be constantly different as the magnetic field is constantly changing
• This causes the coil to vibrate in a different way depending on the size or speed of the current running through it, making the coil vibrate and therefore making the speaker cone vibrate, creating sound waves

Question 12

How can Fleming’s be used to predict the direction of the force when a wire
carries a current perpendicular to a magnetic field?

Answer 12

• Point your index finger in the direction of the magnetic field
• Point your middle finger in the direction of the current
• Your thumb will be pointing in the direction of the force

Remember that current should flow from the positive to the negative terminal (conventional flow) and magnetic field lines are always from north to south