Electromagnetism

Question 1

what is an induced magnet?

Answer 1

A temporary magnet, made when a magnetic material is placed in a magnetic field. The induced magnetism is lost when moved out of the magnetic field.

Question 2

what is a permanent magnet?

Answer 2

Magnet made from a magnetic material. Its magnetism cannot be turned on or off, unlike an electromagnet and always causes a force on other magnets, or on magnetic materials.

Question 3

features of a magnetic field diagram

Answer 3

• the magnetic field lines never cross each other
• the closer the lines, the stronger the magnetic field
• the lines have arrowheads to show the direction of the force exerted by a magnetic north pole
• the arrowheads point from the north pole of the magnet to its south pole
• the field lines are semi circles that start more centred on the magnet but then are further apart

Question 4

what evidence shows that the core of the earth must be magnetic?

Answer 4

The needle of a compass points in the direction of Earth’s magnetic field (the Earth’s core, which is made from iron and nickel, produces this magnetic field)

Question 5

how do you plot the magnetic field pattern of a magnet using a compass?

Answer 5

1. place the plotting compass near the magnet on a piece of paper
2. mark the direction the compass needle points
3. move the plotting compass to many different positions in the magnetic field, marking the needle direction each time
4. join the points to show the field lines
5. the needle of a plotting compass points to the south pole of the magnet.

Question 6

define solenoid

Answer 6

A straight coil of wire which can carry an electric current to create a magnetic field.

Question 7

how can solenoid arrangement increase the magnetic effect of the current?

Answer 7

The field inside a solenoid is strong and uniform. The small magnetic fields caused by the current in each coil add together to make a stronger overall magnetic field.

Question 8

what happens when a current flows along a wire?

Answer 8

• a magnetic field is produced around the wire
• the field lines are in circles
• the strength of the magnetic field is greater:
• closer to the wire
• if the current is increased

Question 9

define electromagnet

Answer 9

A magnet made by wrapping a coil of wire around an iron bar and passing an electric current through the coil (solenoid with an iron core)

Question 10

what is the motor effect?

Answer 10

when a current flows through a wire which is in a magnetic field, the wire experiences a force and moves

Question 11

rough relationship between direction of force, magnetic field and current in the left hand rule

Answer 11

the wire experiences a force perpendicular to the external magnetic field and to the direction of the current flow

Question 12

what do the fingers represent in the left hand rule and what position should the fingers be in?

Answer 12

• make thumb and first finger into a gun shape (right angle)
• point the second finger 90 degrees forward from the first finger
• thuMb= force/ Motion
• First Finger= magnetic Field
• seCond finger= Current

Question 13

what does the size of the induced force depend on in the motor effect? give units

Answer 13

the size of the current in the wire,
the strength of the magnetic field and the strength of the magnet
* the magnetic flux density (T)
* the current (A)
* the length of the wire (m)
(all multiplied together)

Question 14

how is rotation caused in an electric motor?

Answer 14

• A wire carrying a current creates a magnetic field. This can interact with another magnetic field, causing a force that pushes the wire at right angles.
• A coil of wire carrying a current in a
  magnetic field experiences a force that tends to make it rotate. This effect can be used to make an electric motor.

Question 15

what happens if an electrical conductor moves relative to a magnetic field or if there’s a change in the magnetic field around a conductor?

Answer 15

a potential difference is induced across the ends of the conductor

Question 16

how does a moving-coil loudspeaker and headphones work?

Answer 16

The motor effect is used inside headphones, which contain small loudspeakers. In these devices, variations in an electric current cause variations in the magnetic field produced by an electromagnet. This causes a cone to move, which creates pressure variations in the air and forms sound waves.
1. an alternating current passes through the coil of wire
2. this creates a changing magnetic field around the wire
3. the coils magnetic field interracts with the magnetic field of the permanent magnet
4. this makes the coil vibrate (due to motor effect)
5. the speaker cone vibrates causing pressure variations in the air, which are sound waves
- To make a loudspeaker cone vibrate correctly, the electric current must vary in the same way as the desired sound.