Unit 2 Electricity

Question 1

What is a conductor and how does it conduct electricity?

Answer 1

A material which allows electric current to pass through it.

In a conductor some electrons can move freely from atom to atom. These electrons are called free or delocalised electrons.

Metals and graphite are good conductors as they contain a large number of delocalised electrons

Question 2

What is an insulator and why doesn’t it conduct electricity?

Answer 2

A material which does not allow electric current to pass through it.

In an insulator all the electrons are tightly bound to their atoms.

Question 3

What is electric current?

Answer 3

An electric current is a flow of electric charge or charge carriers. In electrical circuits, these charge carriers are electrons.

Question 4

What is conventional current?

Answer 4

The flow of current from the positive to negative terminal of the power source.

Question 5

What is electron flow?

Answer 5

Electrons flow in the circuit from the negative to positive terminal of the power source.

Question 6

What is the unit of charge and equation for charge?

Answer 6

Unit: Coulomb
1 coulomb of charge contains 6.75x10 18 electrons

Q (C) = I (A) x t (s)
I (A) = Q (C)/ t (s)
t (s)= Q (C)/ I (A)

Question 7

What is another name for voltage?

Answer 7

Potential difference (p.d.)

Question 8

Why do electrons flow from the negative to positive terminal?

Answer 8

Because they have a negative charge and are attracted to the positive terminal.

Question 9

What is voltage measured with and how is thispiece of equipment connected to the battery?

Answer 9

Voltage is measured with a voltmeter which is connected in parallel with an electrical device

The positive terminal of the battery/power supply is connected to the positive/ red terminal of the voltmeter.

Question 10

How do current and voltage behave in series and parallel circuits?

Answer 10

Series-
Current is the same everywhere
Voltage splits between components

Parallel-
Current splits between branches
Voltage is the same across each branch

Question 11

What is resistance?

Answer 11

Resistance is the opposition to the flow of current in a circuit

It is defined as the ratio of voltage to current (i.e. R= V/I)

Question 12

State Ohm’s law

Answer 12

The current through a conductor, which obeys Ohm’s law, is directly proportional to the voltage across it provided the temperature remains constant.

Question 13

What is the equation for voltage?

Answer 13

V (V) = I (A) x R (ohms)
I (A) = V (V) / R (ohms)
R (ohms) = V (V)/ I (A)

Question 14

What is the conclusion to the metal wire investigation (current and voltage at a higher temperature)?

Answer 14

1. We can use the results to prove that voltage and current are directly proportional.
2. The graph drawn with voltage on the y-axis and current on the x-axis produced a straight line which passes through (0,0)
   This means that current and voltage are directly proportional

Question 15

What value does the gradient of the voltage (y-axis) and current (x-axis) give?

Answer 15

Resistance (ohms)

Question 16

What is the conclusion of the filament bulb experiment?

Answer 16

From the table of results, we can see that as current increases, resistance increases

R = V / I, the gradient of the graph represents the resistance of the filament.
As the gradient increases the resistance increases

As the current through the filament bulb increases the temperature of the bulb also increases. This means that as the temperature of the bulb increases so too does the resistance. The increased temperature of the light bulb causes heat energy to be transferred to the atoms (or ions) of the filament. The ions gain kinetic energy and vibrate more about their fixed positions.
These increased vibrations make it more difficult for the electrons to travel through the filament.
There are a greater number of collisions between the travelling free electrons and the vibrating ions of the filament wire.

The filament bulb does not obey Ohms Law, it is known as a non-ohmic conductor.

Question 17

What is the general rule for resistors in parallel?

Answer 17

1/Rt = 1/R1 + 1/R2 + 1/R3 …

Question 18

What happens to the electrons when potential difference is applied to the circuit?

Answer 18

The electrons will drift towards the positive terminal of the battery.

Question 19

What is the conclusion to the length of wire investigation?

Answer 19

The resistance of the wire is directly proportional to its length - straight line that passes through the origin (0,0).

Question 20

What piece of equipment is used to measure resistance directly?

Answer 20

Ohmmeter or Multimeter

Question 21

What is the relationship between resistance and CSA?

Answer 21

Resistance decreases as CSA increases.

Resistance is directly proportional to 1/CSA (inverse of CSA) - meaning that RESISTANCE and CSA are INVERSELY PROPORTIONAL.

Question 22

What are the equations linking constants to length and CSA?

Answer 22

R/L = constant

R x CSA = constant

Question 23

Why does a wire heat up when electric current flows through it (heating effect of electricity)?

Answer 23

A metallic conductor has a large number of free electrons in it. When a potential difference is applied across the ends of a metallic wire, the free electrons begin to drift from the low potential to high potential region. These free electrons collide with the positive ions (the atoms have lost their electrons). In these collisions, the energy of the electrons is transferred to the positive ions and they begin to vibrate more violently. As a result, heat is produced. The greater number of electrons flowing per second, the greater will be the rate of collisions and hence more heat is produced.

Question 24

What is the equation linking energy, power and time?

Answer 24

P (W) = E (J) / t (s)
E (J) = P (W) x t (s)
t (s) = E (J) / P (W)

Question 25

What is the equation linking power, current and voltage?

Answer 25

P (W) = I (A) x V (V)
I (A) = P (W) / V (V)
V (V) = P (W) / I (A)

E = VIt

Question 26

What is the equation for the number of units used?

Answer 26

No. of units used (kWh) = Power (kW) x time (hours)

Question 27

What is the equation for total cost?

Answer 27

Total cost = No. of kilowatt hour units x price for 1 unit

Question 28

Describe a one-way switch

Answer 28

It is basically a make or break switch, because:
-when it is turned on, the two terminals are connected, and when it is turned off, the contact between the two terminals is broken.
-the switch is always placed on the positive or live side of the circuit.

Question 29

Describe a two-way switch and where would you find them?

Answer 29

There is no defined position for the off and on states when used in this way, as flipping either switch can turn the lights on or off.

You would find them at:
-Staircases
-Long corridors

Question 30

Where must all switches me placed in a circuit?

Answer 30

All switched must be placed in the live wire. If they were in the neutral wire the appliance and power sockets would still be live when the switched are turned off.

Question 31

What are wires made of and why?

Answer 31

Each wire as a copper core because copper is a good conductor of electricity. The outer layers are flexible plastic, because plastic is a good electrical insulator.

Question 32

What are the colours and functions of the 3 wires in a three pin plug?

Answer 32

Live - brown - it is held at a voltage of 230V and provides the current. The voltage changes in size and direction in a cyclic manner (it is the dangerous wire).

Neutral - blue - completes the circuit (usually held at 0V).

Earth - green and yellow stripes - a safety wire to stop the appliance becoming live. This wire connects the metal frame of an electrical appliance to earth.

Question 33

What is the definition of a fuse and where are they placed in the circuit?

Answer 33

A fuse is a short length of thin wire made from a material which has a low melting point. When the fuse overheats, due to a fault in the circuit, it melts and the circuit is broken.

Fuses are always placed in the live wire.

Question 34

What are 3 common standard values for fuses?

Answer 34

3A, 5A and 13A

Question 35

How do the earth wire and fuse work together to earth an electrical appliance?

Answer 35

-If a fault develops in an electrical appliance the live wire is able to come into contact with the metal case, then anyone touching that metal case would possibly receive a fatal electric shock as current will pass through their body to earth.
-The earth wire and the fuse work together to prevent this from happening.
-The earth wire provides a low resistance route allowing the current to reach earth through the wire rather than the person.
-As a result of the low resistance, current increases and blows the fuse in the appliance which will then need to be fixed before reusing.

Question 36

What is double insulation?

Answer 36

-Appliances such as hairdryers and vacuum cleaners are normally double insulated.
-These appliances only have a live and neutral wire connecting them to the mains supply.
-They DO NOT have an earth wire.
-To protect the user from electric shock these appliances are completely encased in an insulated plastic case so that people cannot come into contact with any live components.