Topic 2- Electricity

Question 1

Equation for electric charge flow

Answer 1

charge flow= current x time

Question 2

What can be said about the current in a single closed loop?

Answer 2

current is the same at all points

Question 3

what two factors does a current in a circuit depend on?

Answer 3

resistance (R)
potential difference (V)

Question 4

Explain why the resistance stays the same when the current changes:

Answer 4

The current through an ohmic conductor is directly proportional to the potential difference across the resistor. (meaning the resistance stays the same at the current changes)

Question 5

Explain the changes in resistance through lamps, diodes, thermistors

Answer 5

filament lamp: as temperature of the filament increases, so does the resistance.
diodes: current through a diode flows in one direction only, diode has a very high resistance in the reverse direction.

Question 6

State the conditions required for an ohmic conductor

Answer 6

• a conductor where the current is directly proportional to the potential difference.
• resistance remains constant as the current changes
• temperature must be constant.

Question 7

Ohms law:

Answer 7

ohms law states that the current through a conductor between two points is directly proportional to the voltage across two points.
formula: voltage= resistance x current

Question 8

Examples of ohmic conductors

Answer 8

Copper or constantan wire

Question 9

examples of non-ohmic conductors (4)

Answer 9

-Filament lamp/bulb
-diode
-LDR
-Thermistors
(resistance changes when the current changes)

Question 10

How is resistance caused?

Answer 10

When electrons collide with ions

Question 11

Explain how the resistance changes when the current changes in a filament lamp

Answer 11

The resistance increases as the temperature increases. resistance is caused when electrons collide with the ions, as the filament gets hotter the ions move and vibrate more or faster. This causes more collisions with the flowing electrons, hence an increased resistance.

Question 12

Explain how the resistance changes when the current changes in a diode

Answer 12

The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.

Question 13

Explain how the resistance changes when the current changes in an LDR

Answer 13

You can see from the graph that the resistance falls as the light intensity increases.
LDRs can be used for switching lights on automatically when it gets dark.
There’s also a symbol, remember

Question 14

Explain how the resistance changes when the current changes in a thermistor

Answer 14

You can see from the graph that the resistance falls as the temperature increases.
Thermistors can be used in the home for thermostats to control the central heating.
There’s also a symbol, remember it.

Question 15

When can a thermistor be used

Answer 15

• in a thermostat, to turn a heater on

- in a freezer, to lower temperature

Question 16

Give an application for an LDR

Answer 16

• street lights turn on when it gets dark

Question 17

What two main factors does the amount of energy transferred by an appliance depend on?

Answer 17

• how long the appliance is being used for

- power of the appliance

Question 18

describe the energy transfers in a battery powered torch

Answer 18

• battery converts chemical energy into electrical energy

- bulb converts electrical energy into light as well as waste energy in the form of heat

Question 19

Describe the energy transfer in a battery powered motor

Answer 19

• battery converts chemical energy into electrical energy

- motor coverts electric energy into kinetic energy, forming waste energy in the form of heat due to friction.

Question 20

what determines the power of a circuit device?

Answer 20

• potential difference across the circuit
• the current through the circuit
• amount of energy transferred in a specific amount of time.

Question 21

What is the national grid?

Answer 21

Electrical power transferred from power stations to consumers using the national grid

Question 22

Two types of transformers used in the national grid:

Answer 22

• step-up transformers

- step-down transformers

Question 23

Where are step-up transformers found in the national grid?

what do they do?

Answer 23

• they are used when connecting power stations to transmission cables
• they increase the potential difference

Question 24

Where are step-down transformers found in the national grid?
what do they do?

Answer 24

• used to connect transmission cables to domestic buildings

- they decrease the potential difference

Question 25

Why do transmission lines transfer electricity at high potentials?

Answer 25

• a high potential, means low current
• the lower the current, less energy wasted as heat
• therefore its more efficient

Question 26

why does the potential need to be decreased between transmission lines and domestic buildings?

Answer 26

• lower potentials are safer, reduces the likely hood for electrocution
• appliances are designed for 230V