physics p5

Question 1

what is alternating current

Answer 1

alternating current is current which repeatedly changes its direction.

Question 2

What is direct current

Answer 2

Direct current is current which only flows in one direction(the frequency of an ac is the number of cycles it passes through each second)

Question 3

what is frequency

Answer 3

frequency is the rate at which alternating current changes direction

Question 4

What is the P.D + frequency of the UK mains supply

Answer 4

The UK mains frequency = 50hz and the P>D is 230v

The 50hz means the current reverses its direction of flow 50 times per second(mains supply is ac)

Question 5

live wire of a mains supply and neutral wire

Answer 5

The Neutral wire of a mains supply stays at 0volts

the Live Wire of the mains supply alternates between Positive and Negative P.D

Question 6

What is the national grid?

Answer 6

THe national grid is : A system of CABLES and TRANSFORMERS linking power stations to consumers

Question 7

What is the role of STEP - UP transformers?

Answer 7

STEP - UP transformers are USED AT POWER STATIONS TO TRANSFER ELECTRICITY THE NATIONAL GRID

Question 8

What is the role of STEP - DOWN TRANFORMERS

Answer 8

step - down transformers are used to SUPPLY ELECTRICITY FROM THE NATIONAL GRID TO CONSUMERS

Question 9

what are the materials of a plug and why are they used?

Answer 9

The body == stiff plastic = good insulator
Wire = copper = good conductor + bends easily
Pins = BRASS = hard material that doesn’t rust or oxidise +it is a good conductor

Cable insulation == plastic(flexible plastic) = good insulator

Question 10

Label the main components in a plug and what they do:

Answer 10

Fuse = melts therefore “breaking the current”
Live wire = provides high potential to drive the current
Neutral wire = provides zero potential to return a current
Earth wire = Allows dangerous charge to be removed from an appliance
Cable grip = keeps the wires safely contained

Question 11

what is a mains cable?

Answer 11

A mains cable is a cable made up of 2 or 3 individual insulated copper wires surrounded by an outer layer of flexible plastic case

Question 12

what are the colours of the 3 main wires in a plug

Answer 12

earth wire = green+yello
live wire = brown
neutral wire = blue

Question 13

Why must metal appliances be earthed

Answer 13

Metal appliances must be earthed because they have a metal exterior cover and if the live wire becomes loose, and touches the case,there is a risk of someone being electrocuted(because the case will then also become live)
Metal appliances must be earthed - the case is attached to the earth wire cable, which stops the metal case becoming live if the live wire touches the case.

Question 14

Why do plastic appliances not need to be earthed?

Answer 14

Plastic appliances do not need to be earthed because:THEY ARE SAID TO BE DOUBLE INSULATED -
AND ARE CONNECTED TO THE SUPPLY WITH A 2 CORE CABLE CONTAINING JUST A LIVE AND NEUTRAL WIRE.

Question 15

are cables of different thicknesses used for different purposes ?

Answer 15

YES - the larger a current to be carried, the thicker the cable needs to be.

Question 16

where/what is the earth wire connected to ?

Answer 16

The earth wire is connected to the longest pin in the plug and is used to earth metal appliances

Question 17

give 3 ways to calculate power

Answer 17

Ways to calculate power :
1. power = current(2)*resistance
2.power = P.d * current
3.power = energy transferred
                  ----------------------------
                    time taken

Question 18

What is power

Answer 18

power = rate of energy transfer

Question 19

Explain why a 2 core cable can be used for a hairdryer

Answer 19

A 2 core cable is used for a hairdryer because it does not need to be earthed + if the live wire comes loose and touches the casing, the casing will not become live so an earth cable is not needed.

Question 20

When choosing a fuse, its current rating should be slightly higher than the current through the appliance

Answer 20

wow

Question 21

what is the national grid?

Answer 21

A system of cables and transformers linking power stations to consumers

Question 22

Why is the national grid efficient?

Answer 22

The national grid is efficient because it transports electrical power at low current and high voltage so less
energy is dissipated into the surroundings.

If the electrical power was to be transported around the national grid at a high current more energy is dissipated into the surroundings through heating

Question 23

what does pd do

Answer 23

Potential difference forces the charges around a circuit causing current

Question 24

Explain what the LDR and THERMISTOR DO

Answer 24

The ldr is a Light Dependant Resistor which changes resistances according to the light intensity on it.The higher the light intensity on it, the lower its resistance.
At a high light intensity, its resistance will decrease.

Uses of Ldrs :
phones(auto brightness)
automatic lights

The thermistor is a component that changes its resistance according to its temp.For ezample if its temp is high, the resistance is low and if its temp is low than its resistance is high

Uses of thermistors:
Used in rechargeable batteries
Used in heat - activated - firealarms