Electromagnetism

Question 1

Describe the construction of electromagnets:

Answer 1

A solenoid is wrapped around a magnetically soft material. When there is a current in the cool of wire, a magnetic field is induced in the metal.

Same field as bar magnet - ends of a solenoid act like the North Pole and South Pole of a bar magnet

Question 2

Sketch and recognise magnetic field patterns for a straight wire, a flat circular coil and a solenoid when each is carrying a current:

Answer 2

a field around a straight, current-carrying wire is made up of concentric circles with the wire in the middle (series of circles around the wire)

A field around a current-carrying solenoid is strong, uniform, and outside the coil - the field is just like the one around a bar magnet! The ends of a solenoid act like North and South Pole.

A field around a current-carrying flat circular coil is similar to that of a bar magnet. Magnetic field likes are concentric ellipses around the coil.

Question 3

Why is there a force on a charged particle when it moves in a magnetic field, as long as its motion is not parallel to the field lines

Answer 3

if something with a charge is moving across a magnetic field, it (the electrons in a current/charged particles) will experience a force from the field. Unless of course it is parallel in which case it wont need to as it is already in place with the field.

Question 4

What happens to a current-carrying wire in a magnetic field?

Answer 4

A force is exerted on it.

In simple d.c. , coils on a spindle and forces act one up and one down, it rotates
The split ring commutator is a clever way of swapping the contacts every half turn to keep the mirror rotating in the same direction
The direction of the motor can be reversed by swapping polarity of the dc supply or swapping over magnetic poles

Loud speaker- a.c. electrical signals from an amplifier are fed to coil of where in the speaker which is wrapped around base of cone. Coil surrounded by permanent magnet, so ac signals cause a force on coil and make it move back and forth. Movements make cone vibrate and create sounds.

Question 5

What speeds up a simple DC electric motor?

Answer 5

More current
More turns in coil
Stronger magnetic field

Question 6

When is voltage induced in a conductor or a coil?

Answer 6

Voltage is induced in a conductor or coil when it moves through a magnetic field or when a magnetic field changes through it (when wire is experiencing a change in magnetic field)

Question 7

What factors affect the size of the induced voltage? (electromagnetic induction)

Answer 7

Increase strength of magnet
More turns in coil
Increase speed of movement

Question 8

Describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field:

Answer 8

Generators rotate a coil in a magnetic field (or a magnet in a coil)- construction much like a motor
As coil spins, current induced on coil. The current changes direction every half turn. Instead of split ring commutators, ac generators have slip rings and brushes so the contacts don’t swap every half turn.

(Produce ac voltage. power stations use ac generators to produce electricity)

Question 9

Transformers with ac or dc?

Answer 9

Transformers only work with alternating current!

Question 10

What does a transformer do?

Answer 10

Change the size of an alternating voltage (voltage of an alternating current) by having different numbers of turns on the input and output sides.

Question 11

Describe the structure of a transformer.

Answer 11

They have two coils; primary and secondary, joined with an iron core.

The ratio between the primary and secondary voltages is the same as the ratio between the number of turn on the primary and secondary coil.

Question 12

What’s a step up transformer?

Answer 12

Increase the voltage. (Step it up) they have more turns on the secondary coil than the primary coil.

Question 13

What’s a step down transformer?

Answer 13

Decreases the voltage. (Steps it down) they have more turns on the primary coil that secondary coil.

Question 14

How does a transformer work?

Answer 14

When an alternating voltage is applied to the primary coil, an alternating magnetic field is produced. The alternating magnetic field flows through soft iron core, which magnetises and demagnetises quickly.

The secondary coil interacts with the varying magnetic flux and induces an alternating voltage.

the ac output is equal to ac input.

Question 15

What’s the benefit of the soft iron core being laminated?

Answer 15

Reduces heat loss

Question 16

Explain the use of step-up and step-down transformers in the largescale generation and transmission of electrical energy;

Answer 16

In the national grid (the system which transmits electricity across the country) transformers are used

Voltage produced by power stations are too low to be transmitted efficiently. (Too much current flowing causes wire to heat up losing energy as thermal)
Power = V x I, so lower the voltage, higher the current for a given amount of power.

A step up transformer increases voltage before transmitted. Step down transformer used at the end of journey (when reaches home/business) to reduce voltage so it’s useful (in appliances that we use) and safer to use.

Question 17

How does one induce a current (electromagnetic induction)

Answer 17

Move a conductor through a magnetic field to cut the field lines, inducing a current.

Question 18

What’s the relationship between input and output voltages and the turns ratio for a transformer?

Answer 18

input voltage/output voltage = number of turns on primary/number of turns on secondary

Vp Np
— = —
Vs Ns

(Can swap both sides round together so Vs and Ns are on the top- Put what you’re trying to find on top)

Question 19

Transformers are nearly 100% efficient so…

Answer 19

Input power = output power

VpIp = VsIs

Primary voltage x primary current = secondary voltage x secondary current

(P=IV)

Question 20

An electric current in a conductor produces what?

Answer 20

If you run an electric current through anything that conducts (e.g. a wire) a magnetic field will be produced around it!

Question 21

What is magnetic induction?

Answer 21

The creation of a voltage (and possibly current) in a wire which is experiencing a change in magnetic field.