Electricity

Question 1

Draw a open and close switch

Cell and battery

Dioide and LED

Resistor , variable resistor , fuse and thermistor
and LDR

Lamp

Voltmeter and ammeter

Question 2

Equation of charge flow

Answer 2

Charge flow = current X time

Q = It

Question 3

What does the current depend on

Answer 3

Resistance (R) of the component and the potential difference (V) across the component.

The greater the resistance the smaller the current

The smaller the reistance the greater the current

Question 4

How can potential difference be calculated

Answer 4

Potential difference = current X resistance

V= Ir

Question 5

Required practical: use circuit diagrams to investigate factors affecting resistance of circuits

Answer 5

1) attach a crocodile clip to the wire level with 0cm on the ruler

2) attach another one onto the wire e.g 10cm away from the first clip. Write down the length of the wire between the clips.

3) Close switch then record current through the wire and the pd

4) open the switch move the second crocodile clip and do the same as the previous step

5) repeat step 4 for a number of times

6) Use R=V divided by i and plot a graph

Current through an ohmic conductor is directly proportional to the potential difference across the resistor-constant

Lamps,diodes,thermistors and LDRs is not constant; it changes with the current through the component

Resistance of a filament lamp increases as the temp of it increases

A diode has 1 direction of flow. The dioide has a very resistant reverse direction

Resistance of a thermistor decreases as temp increases - thermostats,

The resistance of an LDR decreases as light intensity increases- switching lights, burglar detector

Question 6

What is an electric current

Answer 6

A flow of electrical charge. Requires potential difference.

Question 7

Required practical: Investigating IV characteristics of circuits

Answer 7

1) set up a circuit (dc series circuit) x draw it

2) vary the variable resistor alters the current and pd

3) take several readings from the ammeter and voltmetee to see pd and how it varie. Repeat each twice more to get an avg pd at each current

4) swap wires reverse current direction

5) plot a graph of current against voltage for the component

6) Ohmic conductor pd and current direcrly propotional

Filament lamp- current increases so does temp (Resistance). As resistance increases current decreases.

Diode- current has one direction of flow high resistance in reverse direction

Question 8

Series and parallel circuits

Answer 8

Components connected in series:

There is the same current through each component

Total pd of the power supply is shared between components

Total R of 2 components is the sum of the R of each component

Rt=R1+R2

Components connected in a parallel circuit:

Pd across each component is the same

Total current through circuit is the sum of the currents through seperate components

Total R of the 2 resistors is less than the R of the small individual resistor

Question 9

Differences of direct and alternating pd

Answer 9

When using AC it is very easy to use a transformer to increase or decrease pd (AC= 50Hz and 230V)

DC pd does not change (same across all components)

Question 10

What is the domestic energy supply UK.

Answer 10

Frequency of 50Hz and 230V

Question 11

Why is a live wire dangerous when a switch in the AC circuit is open and what are the dangers of connecting between live wire and earth

Answer 11

Body is at 0V. Live wire can send a large pd theough you and a current which can injure or killl you even when a switch is open (body makes a link with between supply and earth)

Connections with live and earth can be dnagerous if a low R path to earth is created a huge current will flow (fire).

Question 12

Different wire types and what do they do

Answer 12

Live wire-brown: carries alternating pd from the supply

Neutral wire-blue:close to or at earth wires potential

earth wire- green and yellow stripes: At 0V only carries a current if there is a fault

Question 13

Calculation for power

Answer 13

P=VI
Power = pd X current

P=I^2R
Power= Current^2 X resistance

Question 14

How do different domestic appliances transfer enegy from batteries or ac mains to kinetic energy of motors or heating

Answer 14

Amount of energy an appliance transfers depends on how long the appliance is witched on for and the power of the appliance

Kettkes transfer energy electrically from the mains ac supply to the thermal energy store of the heating element

Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the motor

Higher the current the more energy is transferred to the thermal energy store

Question 15

Energy transferred calcuation

Answer 15

Energy transferred = power X time

E=Pt

Energybtransferred= charge flow X pd

E=QV

Question 16

How is the power of a circuit relates to the pd and current and energy transferred over time

Answer 16

Total energy transferred by an appliance depends on how long the appliance is on and power

More energy transfer results in more power
Power = the product of current and pd

When there is a change in pd energy is transferred. Energy is supplied to the charge at the power source to raise potential. Charges releases energy through a pd drop in components

Bigger pd = greater supply of energy for every coulomb of charge.

Question 17

Describe with examples the relationships between power rarings for domestic electrical appliances and changes in stored energy when used.

Answer 17

Applinces have a power rating = max opertaing power

Power rating is the maximum amount of energy transferred between stores per second when in use

Lower the power the less electricity is used (cheaper)

Greater power doesn’t always mean more efficient

Question 18

What is the national grid

Answer 18

System of cables and transformers linking power stations to consumers

Question 19

Why is the national grid system an efficient way to transfer energy

Answer 19

Electrical power is transefrred from the power stations to consumers using a national grid

Step- up transformers are used to increase the pd from the power station to the transmission cable then step-down transformers are used ro decrease, to a much lower value, the pd difference for domestic use.