Topic 5 - Generation and transmission of electricity

Question 1

What is current and voltage?

Answer 1

Current - The flow of electrical charge (e.g. electrons) measured in Amps or Coulombs/s

Voltage - voltage is an electrical pressure giving a measure of the energy transferred measured in Joules/Coulomb or Volts

Question 2

Define Power!

Answer 2

The energy transferred per second and measured in watts

electrical power (watt, W) = current (ampere, A) x potential difference (volt, V)
P = I x V

The units of power are Joules/s or Watts

Question 3

How can a magnet and a coil of wire be used to produce electricity?

Answer 3

When a copper wire or any other conductor is moved in or out of a magnet a current will be induced. (i.e. charges begin to move) aka electromagnetic conduction.

Question 4

How can the direction of the current be changed?

Answer 4

If the wire moves in a different direction or if the magnetic field is switched

Question 5

What is a dynamo and how does it work?

Answer 5

• In a dynamo motion is used to turn a magnet within a coil of wire in order to generate an electric current (e.g. to power a bicycle lamp, radio or torch)
• Since the magnet is always moving in the same direction relative to the wire, it generates direct current (DC).

Question 6

What factors affect the strength of electromagnetic induction?

Answer 6

1. Using a coil rather than a straight wire
2. Using more turns in the coil
3. Using a stronger magnet
4. Moving the wire faster
5. Wraping the wire in an iron core

Question 7

How does a generator work?

Answer 7

1. A generator consists of a coil of wire (wrapped aroung an iron core) rotating within powerful electromagnets which generates a voltage.
2. The moving coil (within which electricity is generated) must connect to the stationary circuit without getting damaged. This is done using slip rings and carbon brushes.

Question 8

Why does a generator make alternating current (AC)?

Answer 8

The coil of wire cuts through the magnetic field in one direction as it moves up, and in the opposite direction as it moves down. Therefore the direction of the current alternates.

Question 9

Power stations change from 25 kV to 400 kV before sending it to the national grid. Why?

Answer 9

The higher the current in the wire the more it heats up and the more energy that is lost as it is transported around the country. Therefore, efficiency is increased if the current can be reduced. This is done by increasing the voltage using a transformer.

Question 10

How does a transformer work?

Answer 10

• A transformer consists of a primary coil and a secondary coil wrapped around an iron core.
• An AC supply is connected to the primary core which therefore generates and alternating magnetic field.
• The alternating magnetic field induces a current is the secondary coil just like a wire moving within a magnet.
• The secondary coil has more coils than the primary coil, and therefore generates a larger voltage. This is called a step up transformer

Question 11

What is the equation used for calculating the voltage generated by a transformer?

Answer 11

Voltage primary / Voltage secondary = turns primary / turns secondary

Question 12

Why are step down transformers often found in electrical devices?

Answer 12

Because lots of devices such as phones run on a voltage much <230. (e.g. a phone is 9v) a step down transformer is needed.

Question 13

Why does a transformer not work with a DC supply?

Answer 13

The reason is because the magnetic field generated by a DC supply doesn’t alternate hence it wouldn’t induce a secondary current in the coil.

Question 14

Describe the hazards associated with electricity transmission

Answer 14

If a person touches an elecrticity pylon (or train track) they will get shocked because the voltage in the pylon/track in the hand that touches the pylon, is much greater than the voltage in the rest of their body. Therefore, electricity will flow from the hand that touches through the rest of the body.

Question 15

How do electricity companies work out the bills?

Answer 15

Power is Joules of energy transferred per second (W)

kWh = 1000 W running for 1 hour

The bill is given in terms of kWh.

Cost = cost of 1 kWh x power x time

Question 16

How can an ammeter and voltmeter be used to measure the power in an appliane?

Answer 16

Power = I x V

Question 17

What is the law of the conservation of energy?

Answer 17

Energy Input = Energy Output

Question 18

What is energy efficiency?

Answer 18

Every appliance (i.e system that changes in terms of energy) will transfer the energy supplied into a mixture of useful and unuseful energy forms (e.g. A light bulb transfers electrical energy into light and heat).

A more efficient device is a device that ‘wastes’ less energy. (i.e less energy is transferred wastefully)

Efficiency = (Useful Energy / total energy) x 100

Question 19

What is the difference between active and potential energy?

Answer 19

Active energy can be sensed.

Potential is stored and not experienced

Question 20

What happens to the suns radiation once it reaches the earth.

Answer 20

Some is reflected and some is absorbed.

30% relected. 20% clouds, 6% atmosphere, 4% surface

70% absorbed. 51% land, 3% clouds, 16% atmosphere

The 70% is reradiated from earth (6%) and from clouds + atmosphere to spce(64%)

Question 21

What is the greenhouse effect?

Answer 21

Gasses in the atmosphere absorb radiation and re-emit keeping the earth at a warmer temp than it would be otherwise.

Question 22

What is earth’s ‘energy balance’ and how might it be disrupted by extra greenhouse gasses!

Answer 22

If heat enter and exists the earth at the same rate the temperature is constant and we say that there is an energy balance.

Extra greenhouse gasses will cause heat to leave earth at a slower rate therefore temp will rise.