Jasmine

Question 1

Recall the force law between parallel current-carrying wires, in words and symbols.

Answer 1

Force is equal to (length x magnetic force constant x current x current) divided by distance.

Question 2

Recall the effect on parallel current-carrying conductors of currents travelling:

a. in the same direction
b. in opposite directions

Answer 2

a. they are attracted
b. they are repelled

Question 3

Describe the essential principle behind all electric motors.

Answer 3

The motor effect, which states that a current-carrying conductor in a magnetic field experiences a force.

Question 4

Define the following terms (in an electric motor):

a. rotor
b. armature
c. field structure
d. commutator
e. brushes

Answer 4

a. rotating part consisting of the armature and the coil
b. the laminated soft iron core
c. also know as the stator, it is the magnetic field which is set up by electromagnets or permanent magnets.
d. allows for a change in direction of the current in the coil or rotor
e. transfer electricity from the power source to the rotor

Question 5

Describe an experiment you did to investigate the generation of an electric current.

Answer 5

• using a coil connected to a galvometer
• move a magnet into out of the coil
• or move the coil over the magnet
• when there is relative motion between them, a current is generated

Question 6

Define electromagnetic induction.

Answer 6

Electromagnetic induction involves the generation of electricity (the flow of electrons) in a coil with the use of a moving magnetic field. There must be relative motion between them.

Question 7

Describe Faraday’s experiment with a magnet and a coil to demonstrate electromagnetic induction.

Answer 7

Faraday set up an experiment using a two seperated coils (think transformer) around a soft iron core.

He observed that when the switch was closed (or opened) that a brief spike in current was observed in the second coil.

Explanation: As the magetic field built up (switch closed) or collapsed (switch open) there was relative motion between the 2nd coil and the magnet field. A current was spike was generated, however when there was no relative motion (switch staying closed), the current dropped off.

Question 8

a. Describe an experiment you did to investigate the factors affecting the generation of an electric current.
b. Explain the way in which you controlled any variables in the experiment

Answer 8

a. - using relative motion between a coil and a magnet - t_he number of turns in the coil_, the speed of the motion between them and t_he strength of the magnetic field were varied_
b. - keep the number of coils and height the same, then vary the strength of the magnet (bar magnet -> neodymium magnet) - keep height and strength of magnet the same and vary the number of coils - keep strength of magnet and number of coils the same and vary the height

Question 9

Describe a rule that enables us to find the direction of the induced current.

Answer 9

Lenz’s law enables us to find the direction of the induced current. It states that if a magnet is moved relative to a coil wrapped around a soft iron core, then the magnetic field in the oft iron core will be set up to oppose the force of the magnet, which sets up a current in the coil.

Question 10

Explain how Lenz’s law leads to the concept of a back emf.

Answer 10

Lenz’s law states that when a magnet is moved toward a coil, a magnetic field will be set up in the soft iron core to oppose the motion of the magnet. This concept is shown in back emf, when an emf is set up to oppose the supply emf in a motor.

Question 11

Explain how back emf relates to the law of conservation of energy.

Answer 11

If there was no back emf set up in a motor, the speed of the motor would continue to increase when the voltage did not increase. the greater speed would mean you are getting energy form nothing, breaking the law of conservation of energy. Back emf ensures this does not happen.

Question 12

It is said that ‘an electric motor acts as a motor and a generator at the same time’. Explain this statement.

Answer 12

When an electric motor is running, back emf (the generator effect) is produced to oppose the supply emf (the motor effect).

Although the net force still causes the motor to turn it is still generating power in the opposite direction.

Question 13

Account for the need for a ‘starting resistance’ in a large electric motor.

Answer 13

A starting resistance is needed because when a motor starts;

• back emf is zero (motor isn’t moving)
• a high starting current is produced (V= IR)
• as the motor speeds up the back emf increases causeing the current to decrease.

. A starting resistance prevents the circuit from burning out when it is first turned on, and drops off as the back emf increases.

Question 14

Compare AC and DC generators

Answer 14

Ac and DC generators differ in their commutators. All other componets are the same.

AC generators have slip-ring commutators,

but DC generators have a split-ring commutator to change the direction of current flow.

It is true to say all generators are AC but with DC the negative component of current is flipped at the commutator.

Question 15

Predict whether AC or DC generators predominate in large scale generation. Justify your answer.

Answer 15

AC is used in large-scale generation because our main power supply is in AC, so no conversion is needed.

AC can be steppped up/down to reduce lossess in transmission lines.

By steepping down the end consummer can use a safer voltage

Slip-rings also take longer to wear down than a split-ring.

Question 16

Compare the views of Westinghouse and Edison on electricity generation and distribution in the late 19th century.

Answer 16

• Edison began providing the people of the US with DC current, strongly believing this was the way of the future. - He was willing to build the required power stations to provide more people with electricity, not wanting to lose his customers to AC current. - Westinghouse believed AC current was the way of the future, purchasing the patent rights from Tesla. - He believed that AC was far more efficient and that he could take over as the main power distributor.

Question 17

Explain why Westinghouse triumphed over Edison.

Answer 17

• Edison used public displays of killing animals, leading to the invention of the electric chair, to attempt to show AC current was unsafe. -

In 1889, DC power was switched on at Willamette Falls Station. It was destroyed by a flood, leading to the commission of AC power for attempted number two. - The International Electro-Technical Exhibition in Germany 1891 was the first display of AC three-phase power, generated 175 km away. -

In 1893, the contract to produce power from Niagara Falls to Buffalo was awarded to Westinghouse, with the generators bearing Tesla’s name. - Westinghouse triumphed because he could prove that AC was more efficient and could be transmitted over longer distances with minimal energy loss.

Question 18

Describe how high voltage transmission lines are insulated from their supports.

Answer 18

• “Chain insulators constructed either of ceramic segments joined together with metal links or of rubber discs with a fibre-glass core.”
• They are designed to prevent build up of dust and shed rainwater, to prevent conduction.

Question 19

Describe how high voltage transmission lines are protected from lightning strikes

Answer 19

• The top wire is a shield conductor, which safely carries the current from lightning to the earth if it is struck. - Metal towers also do this, as they are separated from the main power supply to prevent a power surge. - The towers are well-spaced to ensure only one tower is affected.

Question 20

Describe how energy conservation determines the current transformations in a transformer.

Answer 20

P=VI.

Energy conservation determines the current in a transformer because the power does not change. (In an Ideal transformer the power on across the primary and secondary coil has to be the same)

If the voltage is higher then the current will be lower, and vice versa.

Question 21

Identify the type of transformer which results in the current being greater in the secondary coil.

Answer 21

Step-down transformer, because if the current is greater in the secondary coil, the voltage must be stepped down, as the power does not change, P=VI.

Question 22

Outline the impact of the development of transformers on society.

Answer 22

Positives

• Allows for remote power stations (cleaner air), power stations can be situated near coal mines
• Lower energy losses (less greenhouse gases, cheaper electricity)
• power transmission across long distances - step voltage up or down is needed for household and transmission use
• varying voltages can be used in the household (low for electronics)
• Negavities
• Cheap electricity leads to green house gasses
• don’t want to swap to renewables impacting future generations
• out of sight out of mind mentalitiy.

Question 23

Explain why eddy currents in transformers are a problem.

Answer 23

inefficientcies

Eddy curents are set up in the sof iron core

Power is lost through heat generation

Question 24

Describe how eddy currents are reduced in transformers.

Answer 24

• laminated soft iron core
• teeth cut into the metal which reduce the area of eddy currents loops -
• oil baths and fins which cool transformers
• Fans to promote air flow