Topic 7 - Magnetism And Electromagnetism

Question 1

In what direction do the lines on a magnetic field go?

Answer 1

They go north to south.

Question 2

If the field is stronger, are the lines closer or further apart?

Answer 2

Closer - the force is weaker if the lines are further apart.

Question 3

Is magnetism a contact or non-contact force?

Answer 3

It is a non-contact force felt on magnetic objects within the magnetic field.

Question 4

Will like or unlike poles repel each other?

Answer 4

Like. Unlike poles attract.

Question 5

Where is the magnetic force the strongest on a magnet?

Answer 5

At the poles.

Question 6

Will a magnetic object and a magnet attract at either pole?

Answer 6

Yes - they attract no matter what the pole.

Question 7

Using a compass, how do we know that Earth’s core is magnetic?

Answer 7

Because compasses contain small bar magnets that always point north when they aren’t near another magnet, showing that the Earth has an electromagnetic field.

Question 8

What is the difference between a permanent and an induced magnet?

Answer 8

A permanent magnet produces its own magnetic field. On the other hand, an induced magnet becomes magnetic when in a magnetic field.

Question 9

What is the force between a permanent and induced magnet? (Hint - A)

Answer 9

Attractive

Question 10

How is a magnetic field produced from a wire?

Answer 10

When current flows through a wire, it produces a magnetic field in the form of concentric circles. A compass will show you the magnetic field.

Question 11

What happens if you change the direction of the current in a wire?

Answer 11

The direction that the magnetic field is changes.

Question 12

What is a solenoid?

Answer 12

A solenoid is a wire wrapped into a coil to make the magnetic field stronger.

Question 13

Why do solenoids make the magnetic field stronger?

Answer 13

Because the fields of the wire line up together and go in the same direction, producing a stronger field.

Question 14

What is the magnetic field inside a solenoid?

Answer 14

Strong and uniform.

Question 15

What is the magnetic field outside the solenoid the same as?

Answer 15

A bar magnet.

Question 16

How can you increase the strength of a solenoid?

Answer 16

By placing an iron core inside. This is an induced magnet.

Question 17

What happens when current stops flowing through a solenoid?

Answer 17

The magnetic field disappears.

Question 18

What is the name for a solenoid with an iron core?

Answer 18

An electromagnet.

Question 19

How can electromagnets be used?

Answer 19

1) In scrapyards they can be turned on and off to attract magnetic metals.
2) Within circuits to act as switches - see the book for more information.

Question 20

What is the motor effect?

Answer 20

The motor effect occurs when two magnetic fields interact when a wire with current flowing through it is placed between two poles. This causes a force to be exerted and can cause the wire to move.

Question 21

At what angle will magnetic fields interact in the motor effect?

Answer 21

90 degrees. They won’t interact if they’re parallel.

Question 22

What two methods can be used to increase the force in the motor effect?

Answer 22

1) Increasing the strength of the magnetic field.

2) Increasing the current

Question 23

What 3 factors does the force acting on a conductor in a magnetic field depend on?

Answer 23

1) The magnetic flux density (the strength of the field)
2) The current
3) The length of the conductor.

Question 24

What is the equation for the size of the force in the motor effect?

Answer 24

F=BIl ( B I L)
B is magnetic flux density in Tesla
I is current
L is length

Question 25

What is Fleming’s left hand rule and how do you find the direction of the current?

Answer 25

See the pages in the textbook, okay?

Question 26

In detail, how does a dc motor work?

Answer 26

A conductor is held between two poles with a current flowing through. It has two side-arms. The forces act on these, causing the motor to spin. A split-ring commutator swaps the contacts every half turn to keep the motor spinning. You can change the direction by changing the poles and you can use Fleming’s left hand rule to predict its direction.

Question 27

Explain in detail how loudspeakers and headphones work.

Answer 27

An ac current is passed through a wire attached to the base of a paper cone. The coil surrounds a pole of a permanent magnet which is surrounded by the other pole, causing the cone to move because of force in the wire. As current changes, the direction of the force changes, changing the movement of the cone. This creates variations in pressure, creating sound waves. By controlling the frequency of the ac you can alter the sound wave.

Question 28

What is the definition of the generator effect?

Answer 28

The generator effect is the induction of a potential difference (and current in a complete circuit) in a wire which is moving relative to a magnetic field or is experiencing a change in magnetic field.

Question 29

How does the generator effect create an ac?

Answer 29

In a complete circuit, a magnet moved in a coil of wire will create a ‘blip’ of current. If it is moved in the other direction the current is reversed. By moving the magnet back and forth or by swapping the polarities, this creates the ac. This also works if you move a conductor in a magnetic field.

Question 30

Why does induced current always oppose the change that made it?

Answer 30

Because the magnetic field created by an induced current acts against the change, whether that’s movement or change of the field it’s in.

Question 31

Name two ways you can increase the size of an induced p.d.

Answer 31

1) Increasing the speed of the movement (cutting through more magnetic field lines in a shorter time)
2) Increasing the magnetic field strength (thus increasing the number of lines that can be cut).

Question 32

How do alternators use the generator effect to create an alternating current?

Answer 32

It is arranged in the same way as a motor, but instead of a split-ring commutator, they have slip rings and brushes to avoid the contacts changing every half turn, creating an alternating current.

Question 33

How do dynamos generate direct current?

Answer 33

They work in the same way as a motor with a split-ring commutator.

Question 34

What does an oscilloscope show?

Answer 34

The p.d generated in coils over time.

Question 35

Describe how ac and dc appear on an oscilloscope.

Answer 35

Ac goes up and down over the x axis. Dc goes up and down but never crosses the x axis, instead staying positive.

Question 36

What does the peak of the line show and how will increasing the frequency of revolutions change the display on an oscilloscope?

Answer 36

The peaks show the generated p.d at that time. The peaks will increase.

Question 37

Describe in detail how a microphone works.

Answer 37

Sound waves hit a diaphragm attached to coils of wire around a magnet (solenoid), moving the coil in the magnetic field and generating a current. The louder the sound, the more the diaphragm moves and the more current is generated.

Question 38

What do transformers do and how do they work?

Answer 38

A transformer changes the p.d of ac ONLY. They have two coils of wire known as the primary and secondary, which are joined by an iron core. As current passes through the primary, the core magnetises and demagnetises quickly, inducing current in the secondary coil and creating a current in a complete circuit. The ratio between the pd at the primary and secondary is the same as the ratio of their coiled wires.

Question 39

What does a step up transformer do?

What does a step down transformer do?

Answer 39

A step up transformer increases pd and has more secondary coils.
A step down transformer decreases pd and has more primary coils.

Question 40

What is the transformer equation?

Answer 40

Vp / Vs = np / ns
V is input (p) or output (s) pd
np is number of turns on the primary
ns is number of turns on the secondary
These values can be reversed (ie Vs / Vp = ns / np)

Question 41

If the efficiency of the transformer is 100%, what equation can you use?

Answer 41

Vp x Ip = Vs x Is
V is p.d or voltage 
I is current
p is primary coil
s is secondary coil