Electromagnetic Induction

Question 1

What is electromagnetic induction?

Answer 1

Electromagnetic induction occurs whem a changing magnetic field induces an emf, which in turn, produces a current.

Question 2

What is magnetic flux?

Answer 2

Magnetic flux is based on the amount of magnetic flux density multiplied by the area of the coil. Denoted by phi and measured in weber (Wb).

Question 3

State Faraday’s law of electromagnetic induction?

Answer 3

The magnitude of induced emf directly proportional to the rate of change of flux.

Question 4

How would you demonstrate Faraday’s law using a coil and magnet?

Answer 4

Connect the coil to a galvanometer and move the magnet in and out of the coil, experiment with the speed.

Question 5

How would you demonstrate Faraday’s law using two coils, a switch and galvanometer?

Answer 5

Connect one coil to a switchand battery, the other to a galvanometer. When the circuit is opened and closed, an emf will be induced in the other coil.

Question 6

How would you demonstrate Faraday’s law using two coils, an a.c. source and galvanometer?

Answer 6

Connect one coil to a switch and a.c. source and the other to the galvanometer. Close the switch, the a.c. will induce a current in the second coil.

Question 7

What happens when a north pole of a magnet approaches a coil?

Answer 7

• A current is induced in the coil (Faraday’s law)
• This induced current is such that it creates a north pole facing the magnet
• This results in the magnet being repelled as its north pole approaches the coil’s norht pole. The magnet will oppose the magnetic field change and thus the induced emf.

Question 8

If the pole of a magnet was to create the opposite pole what would theoretically happen?

Answer 8

• The law of conservation of energy would be broken
• The magnet would be attracted to the coil
• The magnet would be accelerated towards the coil
• The rate of change of flux in the coil would be increased
• A larger current would be induced, which would create a larger magnetic pole
• This would attract the magnet even more and so on which would tend towards infinite velocity

Question 9

State Lenz’s law.

Answer 9

The direction of an induced current is always such as to oppose the change producing it.

Question 10

How would you demonstrate Lenz’s law?

Answer 10

By taking two identical copper tubes and two metal cylinders, one strongly magnetic and the other non-magnetic. Drop both from the same position at the same time. The magnet induces an emf, creating a current which forms a magnetic field and opposes the falling magnet.

Question 11

What does rms stand for?

Answer 11

Root mean square

Question 12

What is an oscilloscope?

Answer 12

A type of simplified TV screen that shows varying waveforms based on signal inputs, such as voltage. One of the most commonly used oscilloscopes is the ECG (electrocardiogram) which is used in hospitals to show heartbeat signals.

Question 13

How would you compare rms and peak values of a.c.?

Answer 13

Create a circuit with an oscilloscope, bulb, d.c. voltage supply and rheostat and another circuit with an oscilloscope, bulb and a.c. power source. Adjust the a.c. to peak voltage and d.c. voltage to the same value. Compare the brightness of each bulb. The a.c. circuit should be dimmer. Reducing the d.c. voltage by a factor of root 2 should result in the bulbs being the same brightness, verifying rms to peak ratios.

Question 14

What is mutual induction?

Answer 14

The process by which a changing magnetic field in one coil induces an emf in an adjacent coil.

Question 15

What does the amount of mutual induction depend on?

Answer 15

The rate of change of magnetic flux.

Question 16

The size of induced emf through mutual induction can be increased by:

Answer 16

• Coils being closer together
• Winding the coils on a common soft iron core
• Increasing the number of turns on either or both coils

Question 17

What is self-induction?

Answer 17

Self-induction occurs when a changing magnetic field in a coil induces an emf in the coil itself.

Question 18

What is back emf?

Answer 18

When the changing magnetic field in a coil induces an emf in itself, which opposes the changing current. eg a flash/spark when a hairdryeris turned off.

Question 19

How would you demonstrate self-induction?

Answer 19

Connect a bulb, solenoid, soft iron core and switch in series with a d.c. power supply. Close the switch, the magnetic field increases in the coil. This induces an emf in the coil such as to oppose the change producing it. The back emf delays the build-up of the current in the circuit but does not stop it. The same applies when turning off the circuit. All of this results in the bulb not lighting immediately but taking time to operate.

Question 20

What are some uses of back emfs?

Answer 20

• Smoothing out fluctuations in current as sharp increases and decreases (spikes) are opposed by the self-inductance and are slower to take effect
• Building dimmer circuits

Question 21

What are transformers?

Answer 21

A device used to change the value of an alternating voltage.

Question 22

How does a transformer work?

Answer 22

• The input voltage causes a current to flow in the primary coil
• Current causes an alternating magnetic flux in the iron coil
• The alternating flux induces an emf in the secondary coil. This is the output voltage
• The size of the emf in the secondary coil depends on the number of turns in the secondary coil
• The ratio of turns on the primary coil to turns on the secondary coil determines whether voltage increases, stays the same or decreases.

Question 23

What are some uses of transformers?

Answer 23

• Distributing electricity from generating stations to domestic supplies
• Appliences that require higher/lower voltages than domestic supply

Question 24

Explain distributing electricity from generating stations to domestic supplies.

Answer 24

Current has a heating effect, which translates to massive energy loss when electricity is distributed across the country. To reduce this energy loss, mains electricity is stepped up to EHT (extra high tension) voltages. The EHT is then stepped down before entering premises where 230V is required.

Question 25

Explain appliances that require higher/lower voltages than domestic supply.

Answer 25

There are many appliances which require voltage changes in order to work. One of the most common is the large black plug used for charging phones or for computer speakers. These are sealed transformer units.

Question 26

How would you demonstrate the operation of a transformer?

Answer 26

Set up a transformer with an a.c. input source and a voltmeter in parallel to the input and output. Apply various voltages and note how the ratio of input to output voltages adheres to the ratio of primary and secondary turns.