Physics IGCSE Edexcel Magnetism and Electromagnetism

Give an example of a magnetically soft metal?

iron

Give an example of a magnetically hard metal?

steel

If the current running through a coil is changed from d.c. to a.c. what will happen to the magnetic field.

The polarity of the magnetic field is constantly changing, due to the current which is changing direction at 50 Hz.

Describe the pattern of a uniform magnetic field?

Equally spaced, parallel magnetic field lines.

How do you produce a uniform magnetic field?

You get two permanent bar magnets you put the North pole of a magnet facing the South pole of a magnet, at a certain distance apart.

How can you investigate the size and direction of a magnetic field around a magnet?

Place a plotting compass by one side of a magnet, then draw a dot in the direction the needle is pointing. Then put another plotting compass in the that direction and repeat.

How do you rearrange the equation I1 xV1 = I2 x V2 to compare input and output the currents?

I1/I2 = V1/v2

How do you calculate efficiency?

useful energy output/total energy input

The transformer is heating up, what is the effect on the output of the transformer?

The output will decrease, as useful electrical energy is being converted into waste thermal energy, meaning less energy reaches the secondary coil and is transmitted further. It is less efficient as it has a lower amount of useful output energy over the same input energy.

Explain the meaning of 50 Hz alternating current?

The current changes direction 100 times per second.

Explain why d.c. from a d.c. generator must be changed to a.c. before transmission?

Transformers lower the amount of current and raise the amount of voltage before transmission and they rely of a.c.

How does a transformer work?

Transformers step up or step down the voltage, the current flowing through the primary coil produces a magnetic field, the current is constantly changing as it is a.c., this means the magnetic field around the primary coil is also constantly changing. The iron core strengthens this magnetic field. The coil of the second coil cuts the magnetic field lines inducing a voltage across the secondary coil.

A student investigates how to produce a voltage.

He hangs a magnet from a spring, above a coil that is connected to a data logger. The student pulls the magnet through the coil to X and then releases it. The magnet moves up and down through the coil.

The data logger produces this graph of voltage against time.Explain why the data logger records a varying voltage.

Voltage is induced when field lines are cut, the direction of the voltage produce changes when the direction of magnets movement changes, when the magnet slows down the voltage produced decrease, so the amplitude of the wave decreases

The head of an iron nail is placed close to the north pole of a bar magnet. The head of the nail is attracted towards the bar magnet until they touch and it sticks to the magnet. Explain what causes this to happen?

The north pole of the magnet induces a south pole in the head of te nail, and the opposite poles attract each other.

A laptop battery charger contains a step-down transformer.

A student notices that the charger becomes warm when it is working. Suggest how this will affect the output of the transformer. (2)

The output of the transformer will be reduced, as useful electrical energy will be converted to heat energy and wasted, meaning less energy is available for the laptop. So the transformer will not be 100% efficient. Also the resistance will increase with temperature meaning that less current will flow.

The useful energy transfer in the wind turbine is

A chemical energy to electrical energy

B gravitational potential energy to electrical energy

C kinetic energy to electrical energy

D sound energy to electrical energy

(1)

C

The generator output voltage is then increased to 132 kV for transmission. Explain why electrical energy is transmitted using very high voltages. (4)

If there is a higher voltage there will be a lower current, as the same amount of power is being transmitted and Power = Current x Voltage