EDEXCEL Physics Unit 6 Classified past papers

 Classified past paper of Unit 6: Magnetism and electromagnets 

EDEXCEL IGCSE past papers for code 4PH1/4PH0.

The paper has question from paper 1 and 2 from higher tier.

UNIT 6: Magnetism and electromagnets classified past papers link for download is given below:

Download past paper here.

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Notes can be found on this link in PDF format. Visit notes page Here

Notes Excerpt for unit 6:

Split-ring commutator: continuously rotates with coil between the carbon brush contacts placed between the poles of magnet. This happens because he direction of coil in current is reversed, so there are continuous forces acting on coil to keep it turning.The steps of commutator working are:

a)  Current flowing through the commutator causes side A to move upward and side B to move downwards.

b)  The rotation of coil is clockwise direction.

c)   When the coil is at vertical position, no current passes through the coil. It continues to rotate past the vertical because of its momentum.

d)  Side A is on Rt-hand side, current direction reverses.

e)  Side B is pushed upwards and Side A is pushed downwards. The coil continuous in clockwise direction.

 Loudspeaker with moving coil:

The current in the upper side of the coil is into the paper, the lower side will have out of the paper. Then apply the left-hand rule to both side of the coil, tells that the force is towards the left. When the current reverses, the force experienced by coil is on the right. An A.C. current causes the coil to vibrate in and out and that’s how loudspeaker produces sound waves. 

Electromagnetic induction:

Voltage induction:

Voltage difference is produced when a conducting wire is moved through a magnetic field at the two ends of wire. This type of voltage production is called as induced voltage. Voltage will also be induced if:

Changing magnetic field around stationary conducting wire. In case wire is part of complete circuit, the voltage will cause a current in the circuit. This is called induced current. This way of current induction is known as generator effect, as this is the method of generating electricity.

Induced voltages practical:

Investigating the direction of movement of wire to induce current.

Figure shows different directions which the wire can be moved: XX’, YY’ or ZZ’,

But the voltage and current can only be induced if you move it down along XX’. As the wire must cut the magnetic field lines.

Reversing the direction of current will also reverse the voltage. For example: Moving wire up, makes meter move to right then moving wire down will make meter move left.

Reversing magnetic field direction, reverses the voltage.

Factors affecting size of voltage:

•       Moving wire more quickly induces a greater voltage. When there is no movement there is no voltage.
•         Using stronger/bigger magnets to induce greater voltage at same speed of movement of wire.

Coils and magnets:

Voltage can also be induced in a coil by changing the magnetic field.

When magnet is inserted, current direction makes the X side of solenoid to behave as a north pole. When the magnet is pulled out, the induced current direction is now reversed. And X end of solenoid behaves as the south pole. The current induced in conducting wire generates a magnetic field. This magnetic field opposes the original change caused, which may be movement of conducting wire or magnetic field change.

In picture given above, every time the magnet is moved and a current is induced, a force opposes the movement of magnet. And so, when a magnet is moved work is done. When a magnet is stationary near the coil, there is no induction of voltage.

Electromagnetic flow meter:

Figure shows an instrument to measure the flow rate of oil through oil pipeline.

In a pipe, small turbine is placed, so that he oils flow turns the blades. Magnets are placed at rim of turbine, to move them past the solenoid. The movement of magnets induce a voltage in solenoid which is measured on oscilloscope.

The faster the turbine moves, the greater the voltage is produced.

Generators

The a.c. generator (alternator):

A coil of wire can be moved by turning the axle through the magnetic field, which causes a voltage to be induced between the ends of the coil. The voltage waveform is seen at the screen of the oscilloscope.

From the diagram below: 

(i)                  Coil is vertical position with CD lower than AB. In this position the sides AD and BC are moving parallel to the magnetic field.

(ii)                Coil rotated through 1/4^th, creating the greatest voltage. The sides AD and BC are cutting through the magnetic field at maximum rate.

(iii)              The position of coil is vertical and no voltage is produced.

(iv)               In this position, again maximum voltage is produced but in the opposite direction, i.e., CD is moving down and AB is moving up.

Doubling the speed of rotation of generator, doubles the frequency and maximum value of induced voltage.

Induced voltage could be increased by:

•   Faster rotation of coil.
•   Use of stronger magnets.
•   More number of turns to wire.
•   Wrapping the wire around soft iron core.

The voltage waveform on the right shows that when the generator is rotated twice as quickly, there are 2 effects:

The maximum voltage is twice as large

The frequency is doubled, means interval between peak is halved.

Large scale power production:

Generators used for generating electricity are different than those discussed above. These large generators have stationary coils and rotating electromagnet instead of stationary magnet and rotating coil seen above.

Pros:

No moving part needed to collect electricity being generated.

Steps in coal-fired power-station are:

a)  Water is boiled by burning coal.

b)  Water is converted to high pressure steam to turn the turbine.

c)   The turbine is connected to the generator magnets, which rotate near the stationary coils. The output voltage is 25000 V.

d)  This turbine also powers the exciter (device which produces d.c. current and produces current for rotating magnets, which are electromagnet)

Transformers

Electricity is carried at very high voltages of 400,000 V.

Changing fields and changing currents

When the circuit is closed, current flows and is detected to the ammeter in the second circuit. When the switch is opened, current flow is stopped in circuit1 and no current is detected on ammeter in circuit 2. Closing the switch causes current through coil 1 to grow quickly. So, the magnetic field of coil 1 is also induced. A current is induced in the 2^nd coil by the pushing of the north pole of the 1^st coil. When the switch is open, the magnetic field in coil 1 falls, so it pulls away its north pole from coil.

This causes a change in direction in induced current. Pole away from coil 2. Now the induced current changes direction. The ammeter

reads zero when there is a constant current through coil 1.

Transformers

A device made by using two coils of wire onto soft iron core. The primary coil is connected to a 2V alternating current supply. The current induces a magnetic field which increases and decreases repeatedly. The iron core which carries magnetic field to the secondary coil, the energy is carried in this way from primary to the secondary coil. (Transformers only work with a.c. current as it needs continuously changing magnetic fields).

Transformers allow change of voltage of a supply. Example: model railways have transformers that decrease the mains supply from 230 V to 12V. These are step-down transformers.

To make step-down transformer, the primary coil should have a greater number of turns of coil than the secondary coil.

Step-up transformer: which increases the voltage from 2 V to 12 V.

To make a step-up transformer: The secondary coil must have more turns of wire than primary coil.

Calculation of voltages:

Vp=primary voltage

Np=Number of turns in primary coil

Vs=Secondary Voltage

Ns=number of turns in secondary coil.

Power in transformers:

For 100 % power efficient transformer.

The National Grid

Shows how electricity generated at the power station is distributed around the country. Electricity is transported at very high voltages and low current to reduce the loss.