Electricity

Question 1

What is a coulomb?

Answer 1

One coulomb is the amount of charge which flows past a point when a current of one amp flows for one second

Question 2

What is Kirchhoff’s 1st law?

Answer 2

It states that the sum of the currents flowing into any point in on a circuit is equal to the sum of the currents flowing out of that point

Question 3

What is a volt?

Answer 3

One volt is one joule per coulomb

Question 4

What is electromotive force (emf)?

Answer 4

The energy transferred per unit charge when one other type of energy is converted into electrical energy

Question 5

What is potential difference?

Answer 5

The electrical energy transferred per unit charge when electrical energy is converted into another form of energy

Question 6

What is resistance?

Answer 6

The resistance of a component is the ratio of potential difference across the component to the current flowing through it

Question 7

What is resistance caused by?

Answer 7

Resistance is caused by collisions between free electrons and the metal ions

Question 8

What factors cause resistance to increase?

Answer 8

If the length increases, temperature increases, or cross sectional area decreases

Question 9

What is Ohm’s law?

Answer 9

For a conductor at constant temperature, the current in the conductor is proportional to the potential difference across it

Question 10

Why does an increase in length of a wire cause increased resistance?

Answer 10

It increases the chance of collisions

Question 11

Why does an increase in temperature cause increased resistance?

Answer 11

Higher temperature means the ions move more, increasing the chance of collisions

Question 12

Why does increased cross-sectional area cause decreased resistance?

Answer 12

There are a greater number of possible routes for electrons to take and so there is a lower chance of collisions (cross sectional area is inversely proportional to resistance)

Question 13

What is conventional current?

Answer 13

Flow of charge, opposite to electron flow

Question 14

What is current?

Answer 14

Electric current is the flow of charged particles per unit time

Question 15

What are the benefits of diodes?

Answer 15

• Efficient, they operate on low pds
• They switch on instantly
• They are robust and have a long working life

Question 16

What are the uses of diodes?

Answer 16

• Converting AC to DC
• Generating radio frequency oscillations
• Electronically tuning radio and TV receivers
• Producing light (LEDs)

Question 17

What is resistivity?

Answer 17

The resistance per unit length of a piece of material of cross-sectional area 1m^2
The units are Ωm

Question 18

What is the difference between resistance and resistivity?

Answer 18

Resistance is the ratio of current and voltage, and resistivity is the resistance per unit volume of a material

Question 19

What are each of the terms in I = nqvA?

Answer 19

n = number density (m^-3) (number of charge carriers per m^3) 
v = drift velocity (ms^-1) 
A = cross sectional area (m^2) 
q = electric charge (C)

Question 20

What are semi-conductors?

Answer 20

Materials that sometimes conduct, depending on the environment

Question 21

What materials are semiconductors often made of?

Answer 21

Silicon

Question 22

How do semiconductors work?

Answer 22

At higher temperatures, extra energy moves electrons to the “conduction band” (from the valence band, more energy in the system allows electrons to move out of the orbital and become delocalised, which only happens in semiconductors, not metals), meaning there are more conduction electrons, so resistance decreases

Question 23

What components can be made from semiconductors?

Answer 23

• Thermistors (resistance decreases as temperature increases)
• Light dependent resistors (resistance decreases as light intensity increases)

Question 24

What is meant by superconductivity?

Answer 24

If we cool a material to less than a critical temperature (-243°C), the resistance drops to zero. This is useful when a large current is needed

Question 25

How is the voltage in series relationship derived?

Answer 25

``` Etotal = E1 + E2 + E3 (conservation of energy) VQtotal = VQ1 + VQ2 + VQ3 Vtotal = V1 + V2 + V3 (Q is the same throughout) ```

Question 26

How is the relationship for resistors in series derived?

Answer 26

``` Vtotal = V1 + V2 + V3 (voltage in series relationship) IRtotal = IR1 + IR2 + IR3 Rtotal = R1 + R2 + R3 ```

Question 27

How is the relationship for resistors in parallel derived?

Answer 27

``` Qtotal = Q1 + Q2 + Q3 (conservation of charge) Ittotal = It1 + It2 + It3 (cancel t) Itotal = I1 + I2 + I3 (V/R)total = (V/R)1 + (V/R)2 + (V/R)3 (1/R)total = 1/R1 + 1/R2 + 1/R3 (voltage is the same in each loop) ```

Question 28

What are the best ways to measure the voltage and current characteristics of a component?

Answer 28

It is best done using a variable power supply, or it can be done using a fixed power supply and variable resistor

Question 29

To investigate the voltage and current characteristics of a component, is it better to use a circuit with all components in series, or one with the power supply and variable resistor in one loop, and fixed resistor, ammeter and voltmeter in the other?

Answer 29

The second, with the parallel circuit. This allows the full range from 0-6V, whereas, in series, the range is much more limited

Question 30

What is internal resistance?

Answer 30

The resistance in the power supply that comes as a result of current flowing through chemicals and electrodes in the power supply.

Question 31

What are the lost volts?

Answer 31

The voltage which is needed to push current through the internal resistance

Question 32

What is emf?

Answer 32

The maximum theoretical voltage that the power supply can deliver (which cannot be measured directly). The actual voltage available is the terminal/circuit pd

Question 33

Potential dividers

Answer 33

- A potential divider allows the voltage to be shared between two components - It is usually constructed of 2 resistors in series - The same current passes through both resistors, so the pd across each resistor is proportional to its resistance, expressed by V1 = (R1/ (R1 +R2)) x Vsupply

Question 34

N-type thermistors

Answer 34

- Resistance of an n-type thermistor decreases as temperature increases - Made from a semiconductor (e.g. sintered metal oxide) - Raising the temperature increases the number of electrons able to move about. The more charge carriers available, the more current

Question 35

What is a use for thermistors?

Answer 35

In an electrical circuit to monitor ambient temperature and control heating/air conditioning system

Question 36

Light dependent resistors

Answer 36

- The resistance of an LDR decreases exponentially as light intensity increases - Made from a semiconductor

Question 37

What is a use for an LDR?

Answer 37

In an electrical circuit to monitor ambient light conditions, so can control lighting systems like street lights

Question 38

How can an LDR be used in nightlights?

Answer 38

- Lightbulb is connected across the LDR (in parallel) - As light intensity drops, resistance of LDR increases, so voltage share across the LDR increases - Therefore the voltage in the bulb increases because it is connected in parallel - So when light intensity drops to a certain level, there will be a high enough voltage and the bulb will turn on

Question 39

Method to investigate efficiency of an electric motor

Answer 39

- Set up a circuit with a battery, ammeter connected in series and voltmeter connected in parallel across the motor - Secure the motor and pulley apparatus - Measure the height through which the mass will be lifted (top of the hanger to the cut-off switch) - Turn on the motor and time how long it takes to raise the mass, recording time, current and voltage - Calculate energy in, energy out and then efficiency - Repeat for more masses

Question 40

How does a fuse work?

Answer 40

When the current in a circuit gets too high, it will be above the fuse rating of the fuse fitted in a circuit. This means that the small wire in the fuse will melt if the current is high, breaking the circuit and stopping current flow. This stops the circuit being damaged or catching fire, as well as stopping the danger of electrocution

Question 41

Explanation of the graph relationship of voltage and current when investigating the internal resistance of a cell (voltmeter across the power supply, variable resistor, ammeter in series)

Answer 41

- As circuit resistance is reduced, the current increases (for the same emf) - This means that the lost volts increases - Therefore less useful voltage is available in the circuit - So the the terminal pd decreases