P15 Electromagnetism

Question 1

What is a magnet?

Answer 1

An object capable of exerting a magnetic force.

Question 2

What is a non contact force?

Answer 2

Force exerted between two objects, even when they are not touching, such as the force of gravity.

Question 3

Where does the magnetic field flow to?

Answer 3

From North to South

Question 4

What is a permanent magnet?

Answer 4

Magnet made from a magnetic material. Its magnetism cannot be turned on or off, unlike an electromagnet.

Question 5

What is an induced magnet?

Answer 5

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

Question 6

What is a magnetic field?

Answer 6

magnetic field
Area surrounding a magnet that can exert a force on magnetic materials.

Question 7

What is a solenoid?

Answer 7

A solenoid consists of a wire coiled up into a spiral shape. When an electric current flows, the shape of the magnetic field is very similar to the field of a bar magnet. 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 is an electromagnet?

Answer 8

A magnet made by wrapping a coil of wire around an iron bar and passing an electric current through the coil.

Question 9

What is the motor effect?

Answer 9

The effect where a force is exerted on a wire carrying a current in a magnetic field.

Question 10

What is the equation for the motor effect force?

Answer 10

To calculate the force on a wire carrying a current at right angles to a magnetic field, use the equation:

force = magnetic flux density × current × length

Question 11

What is flemings left hand rule?

Answer 11

the forefinger is lined up with magnetic field lines pointing from north to south
the second finger is lined up with the current pointing from positive to negative
the thumb shows the direction of the motor effect force on the conductor carrying the current

Question 12

How does a dc motor work?

Answer 12

current in the left hand part of the coil causes a downward force, and current in the right hand part of the coil causes an upward force
the coil rotates anticlockwise because of the forces described above
When the coil is vertical, it moves parallel to the magnetic field, producing no force. This would tend to make the motor come to a stop, but two features allow the coil to continue rotating:

the momentum of the motor carries it on round a little
a split ring commutator changes the current direction every half turn
Once the conducting brushes reconnect with the commutator after a half turn:

current flows in the opposite direction through the wire in the coil
each side of the coil is now near the opposite magnetic pole
This means that the motor effect forces continue to cause anticlockwise rotation of the coil.

Question 13

What does magnetic flux density do?

Answer 13

Measure the strength of the magnetic field.

Question 14

What happens to the magnetic field in a solenoid if the current is increased or decreased?

Answer 14

When current is reversed the field is reversed.
When current is increased the field gets stronger.

Question 15

What is the right hand solenoid rule?

Answer 15

An electric current passes through a solenoid, resulting in a magnetic field. When wrapping the right hand around the solenoid with the fingers in the direction of the current, the thumb points in the direction of the magnetic north pole

Question 16

What is electromagnetic induction?

Answer 16

The production of a potential difference (voltage) when a conductor, such as a wire, is moved through a magnetic field or exposed to a varying magnetic field. If the conductor is part of an electric circuit, an induced current will flow.

Question 17

What is the generator effect?

Answer 17

When motion between a conductor and a magnetic field creates electricity, ie a magnet is moved into a coil of wire.

Question 18

An induced potential difference or induced current will increase if?

Answer 18

the speed of movement is increased
the magnetic field strength is increased
the number of turns on the coil is increased

Question 19

How does an ac generator(Alternator) work?

Answer 19

As one side of the coil moves up through the magnetic field, a potential difference is induced in one direction. As the rotation continues and that side of the coil moves down, the induced potential difference reverses direction. This means that the alternator produces a current that is constantly changing. This is alternating current or ac.

Question 20

How to increase the maximum voltage on a alternator?

Answer 20

increasing the rate of rotation
increasing the strength of the magnetic field
increasing the number of turns on the coil

Question 21

How does a dynamo dc generator work?

Answer 21

n a dynamo, a split ring commutator changes the coil connections every half turn. As the induced potential difference is about to change direction, the connections are reversed. This means that the current to the external circuit always flows in the same direction.

Question 22

How to increase the maximum voltage on a dynamo?

Answer 22

increasing the rate of rotation
increasing the strength of the magnetic field
increasing the number of turns on the coil

Question 23

What is a transformer?

Answer 23

An electrical device that increases, or decreases, the potential difference (voltage) of an alternating current.

Question 24

What are the 2 different transformers and what do they do?

Answer 24

a step-up transformer increases the voltage
a step-down transformer reduces the voltage

Question 25

How does a transformer work?

Answer 25

a primary voltage drives an alternating current through the primary coil the primary coil current produces a magnetic field, which changes as the current changes the iron core increases the strength of the magnetic field the changing magnetic field induces a changing potential difference in the secondary coil the induced potential difference produces an alternating current in the external circuit

Question 26

What is the equation for a transformer?

Answer 26

Vp is the potential difference in the primary (input) coil in volts (V) Vs is the potential difference in the secondary (output) coil in volts (V) np is the number of turns on the primary coil ns is the number of turn on the secondary coil vp/vn =np/ns In a step down transformer vsvp

Question 27

Power supplied to the primary coil = Power used in the secondary coil equation

Answer 27

Vp= voltage primary coil Ip=current primary coil Vs=Voltage secondary Is= Current secondary VpIp=VsIs

Question 28

When a wire( connected to an ammeter) is moved through the poles of a ushaped magnet why does the ammeter detect current?

Answer 28

p.d. induced in wire when it cuts magnetic field lines, induced p.d. causes current to pass through wire and ammeter while wire is cutting field lines

Question 29

Why is alternating pd produced when the alternators coil spin between the poles of an u shaped magnet?

Answer 29

p.d. induced in coil when sides of coil cut across field lines, p.d. reverses direction each time coil is at position where its sides are moving parallel to the field lines, this happens every half-turn of coil so one full turn of coil corresponds to one full cycle of a.c.

Question 30

What is the function of a split ring commutator in a dynamo generator?

Answer 30

split-ring commutator reconnects coil opposite way round in circuit every half-turn, each time coil is perpendicular to magnetic field lines, so induced p.d. does not reverse its polarity

Question 31

Explain why the alternator produces peak voltage when the coil is parallel to magnetic field lines:

Answer 31

when coil is spinning, sides of coil cut across magnetic field lines directly when plane of coil is parallel to field lines, induced voltage is peak value here because sides cross directly through field lines

Question 32

Why is D.c not used in transformers

Answer 32

d.c. in primary coil does not produce alternating magnetic field so no p.d. induced in secondary coil

Question 33

Why is steel not used as an iron core for transformers?

Answer 33

steel core not so easily magnetised and demagnetised, when a.c. passes through primary coil, steel core would not produce as strong a magnetic field as iron core, so induced p.d. in secondary coil much smaller or transformer with steel core less efficient

Question 34

Explain how an electric bell works?

Answer 34

when current passes through coil of electromagnet, core becomes magnetised and attracts iron armature which is pulled onto electromagnet and this causes gong to hits bell and ring, movement also opens ‘make and break’ switch stops current through electromagnet, electromagnet loses magnetism so no longer attracts armature which springs back causing switch to close so current passes through the coil again, process repeated so bell rings continuously

Question 35

How to make the coil spin in an anticlockwise direction

Answer 35

reverse the poles of the magnet reverse the direction of current flow

Question 36

Lenz's Law

Answer 36

The direction of an induced potential difference always opposes the change that produces it

Question 37

Flemings left hand rule

Answer 37

With forefinger (magnetic field) pointing left to right, and second finger (current) pointing down, your left thumb (force) will point towards you. This is the direction in which the force acts.