Electromagnetic Induction

Question 1

what is electromagnetic induction?

Answer 1

the creation of a voltage (and maybe current) in a wire with is experiencing a change in magnetic field.

Question 2

in which 2 situations do you get EM induction?

Answer 2

1. an electrical conductor (a coil of wire is often used) moves through a magnetic field.
2. the magnetic field through an electric conductor changes (get bigger or smaller or reverses)

Question 3

how can you test the 2 situations where you get EM induction?

Answer 3

• by connecting an ammeter to a conductor and moving the conductor through a magnetic field (or moving a magnet through the conductor)
• the ammeter will show the magnitude and direction of the induced current.

Question 4

what happens if the direction of movement is reversed?

Answer 4

the voltage/current will be reversed too.

Question 5

what do you need to do to get a bigger voltage?

Answer 5

INCREASE:

1. the strength of the magnet
2. the number of turns on the coil
3. the speed of movement

Question 6

how does a simple generator work?

Answer 6

• generators rotate a coil in a magnetic field (or a magnet in a coil)
• their construction is pretty much like a motor
• as the coil spins, a current is induced in the coil. this current changes direction every half turn.
• a.c. generators have slip rings and brushes so the contacts don’t swap every half turn.
• this means they produce a.c. voltage, as shown by these CRO displays. note that faster revolutions produce not only more peaks but higher overall voltage.
• power stations use a.c. generators to produce electricity, they just get the energy needed to turn the coil or magnetic field in different ways.

Question 7

what current do transformers work with?

Answer 7

alternating current ONLY.

Question 8

how do transformers work?

Answer 8

• transformers change the size of the voltage of an alternating current.
• they all have two coils, the primary and the secondary, joined with an iron core.
• when an alternating voltage is applied across the primary coil, the magnetically soft (iron) core magnetises and demagnetises quickly. this induces an alternating voltage in the secondary coil.
• the ratio between the primary and secondary voltages is the same as the ratio between the number of turns on the primary and secondary coils.

Question 9

what do step-up transformers do?

Answer 9

• INCREASE the voltage.

- they have more turns on the secondary coil than the primary coil.

Question 10

what do step-down transformers do?

Answer 10

• decrease the voltage.

- they have more turns on the primary coil than the secondary.

Question 11

how can you calculate the output voltage from a transformer?

Answer 11

• you can calculate the output voltage from a transformer from the input voltage and the number of turns on each coil.
• V p / V s = N p/ N s
• this equation can be used either way up, theres less rearranging to do if you put whatever you’re trying to calculate on top.
• the number of turns on the secondary coil divided by the number

Question 12

transformers are…

Answer 12

nearly 100% efficient so…

power in = power out
VpIp = VsIs
primary voltage x primary current = secondary voltage x secondary current

Question 13

how do transformers make transmitting mains electricity more efficient?

Answer 13

• step up and step down transformers are used when transmitting electricity across the country.
• the voltage produced by power stations is too low to be transmitted efficiently.
• power = VI, so the lower the voltage, the higher the current for a given amount of power, and current causes wires to heat up.
• a step-up transformer is used to boost the voltage before it is transmitted.
• step down transformers are used at the end of the journey to reduce the voltage so its more useful and safer to use.