Electricity

Question 1

What is electric current ?

Answer 1

The amount of charge flowing in the circuit per second / the rate of flow of charge

Question 2

What is the current measured using ?

Answer 2

An ammeter , a device with zero resistance connected in series to allow all the charge to flow through accurately reading the current

Question 3

What is a coulomb ?

Answer 3

The amount of charge that passes a fixed point in one second with a current of 1 ampere.

Question 4

What is the charge of one electron ?

Answer 4

• 1.6 * 10 ^-19

Question 5

What is potential difference ?

Answer 5

Potential difference is the amount of work done or energy transferred per coulomb of charge that passes between two points

Question 6

How Is the potential difference measured ?

Answer 6

A voltmeter in parallel across the component in a circuit with almost infinite resistance to have 0 current flowing through it to accurately measure the work done

Question 7

What is resistance ?

Answer 7

The ratio of potential difference to current and tells us how easy or difficult it is for current to flow through it , measured in ohms

Question 8

What does ohms law state ?

Answer 8

The potential difference is directly proportional to the current , provided that the temperature is constant

Question 9

What is an ohmic conductor ?

Answer 9

A conductor that follows ohms law and has a constant resistor

Question 10

What are the factors that the resistive depends on ?

Answer 10

The length - the greater the length the higher the resistance
Area of cross section - the greater the area, the lower the resistance
Material - constant for restivity

Question 11

What is a ntc thermistor?

Answer 11

A negative temperature coefficient thermistor is a resistor where the resistance decreases as the temperature increases. They absorb the energy and release free electrons increasing the current flow.

Question 12

What is an ldr ?

Answer 12

A light dependent resistor is a resistor where the resistance decreases as the light intensity increases . They are used as light sensors to switch on devices such as street lamps when it gets dark

Question 13

What is superconductivity ?

Answer 13

It is a property of certain materials which have zero resistivity at and below a critical temperature which depends on the material.

Question 14

What is meant by critical temperature?

Answer 14

The temperature at which a material becomes a superconductor

Question 15

What are benefits of using superconductors -

Answer 15

• as there is no resistance there is no energy and power loss
• when the current is high due to low resistance they generate strong magnetic fields

Question 16

Uses of superconductors:

Answer 16

• mei scanners , transformers and generators, motors , particle accelerators

Question 17

If resistors are places in series what is the total resistance ?

Answer 17

The sun of each resistance so he total resistance would be greater than the each individual resistance

Question 18

What is the total resistance of resistors in parallel ?

Answer 18

1/rt = 1/r1 + 1/r2 … so the total resistance would be smaller than each individual resistance

Question 19

When cells are placed in series what would the total potential difference be ?

Answer 19

The sum of the potential differences

Question 20

When cells are placed in parallel , the total potential difference supplied is ?

Answer 20

The total potential difference supplied is equal to the potential difference of one of the cells

Question 21

What does Kirchhoff first law state ?

Answer 21

The total current entering a junction is equal to the current leaving a junction , due to the conservation of charge

Question 22

What does Kirchoffs second law state ?

Answer 22

In any complete loop within a circuit , the total voltage supplied must be equal to the sum of potential differences across all the components, due to the conservation of energy

Question 23

Define electrical power :

Answer 23

The rate at which work is done or the rate at which energy is transferred

Question 24

Equation for the current ?

Answer 24

I=Q/t

Question 25

How do you work out the amount of electrons if you have the charge ?

Answer 25

Charge / 1.6*10^-19

Question 26

What is the equation for the potential difference ?

Answer 26

W/ Q or E/ Q

Question 27

Equation for the resistance ?

Answer 27

R= V/I

Question 28

Why is a variable used when measuring the resistance of a component in a circuit ?

Answer 28

To adjust the voltage and the current

Question 29

What causes resistance ?

Answer 29

Collisions between the free electrons and the metal ions causing resistance

Question 30

Why is the resistance increased when the temperature increases ?

Answer 30

As the temp increases the metal ions gain more kinetic energy - vibrating resulting in more frequent collisions

Question 31

If the resistance is high what is the current flow like ?

Answer 31

Low current flow as the resistance is inversely propoproportional to the current

Question 32

What causes resistance ?

Answer 32

Collisions between metal ions and the free electrons. As the temperature increases the metal ions gain more Ek - vibrating results in more frequent collisions

Question 33

How do you convert from charge to the number of electrons ?

Answer 33

Charge / charge of an electron

Question 34

Describe the I-V characteristics for a resistor at a constant temperature

Answer 34

A straight line passing through the origin - as it is an ohmic conductor when the temperature is kept constant ( I is proportional to v )

Question 35

Describe the I-V characteristics for a filament lamp ?

Answer 35

Initially the voltage would be directly proportional to the current , but starts to tail as the voltage is increase - not proportional anymore

Question 36

Why does the current tail as the voltage is increased for an I-V graph of a filament lamp ?

Answer 36

Some of the electrical energy is conveyed into heat energy within the lamp - as the energy increases the resistance increases ( more collisions between the electrons and metal ions ) - harder for the electrons to flow. This decreases the current

Question 37

Describe a I-V graph for a diode.

Answer 37

- low current ( almost zero ) when the potential difference is negative (reverse bias ) - rapid increase in current when the potential difference is positive and exceeds a threshold voltage - sharp transition

Question 38

Why is the hardy any flow of current in the reverse biased position of a diode ?

Answer 38

The resistance is very high , minimal current

Question 39

Explain why the potentiometer circuit is more suitable for obtaining the I-V characteristics of a diode that a variable resistor.

Answer 39

- using a potential divider allows you to reduce the current and the pd across the component to be reduced to zero - not possible with a variable resistor

Question 40

What are saftey precautions to take when using a diode ?

Answer 40

Limiting the current to avoid damage to the diode - through the use of a protective resistor

Question 41

What are the advantages of using a data logger ?

Answer 41

- more frequent reading can be taken - obtain a computer display of the results

Question 42

How must the diode be connected when investigating the i-v characteristics of a diode ?

Answer 42

In the forward biased

Question 43

What happens to the resistance of a ntc thermistor when the temperature increases ?

Answer 43

- resistance decreases allowing the current to increase

Question 44

What equation should you use for power loss ?

Answer 44

I^2*R

Question 45

What is a potential divider circuit ?

Answer 45

A circuit with two or more resistors in series with each other and with a source of a fixed potential difference

Question 46

How are voltages in series circuits shared ?

Answer 46

The voltage is shared in the ratio of the resistances in series

Question 47

What is emf ?

Answer 47

The energy supplied per unit charge by the battery - the amount of chemical energy converted to electrical energy per coulomb of charge when it passes through the battery

Question 48

How would you work out the emf ?

Answer 48

Connect a voltmeter across the cell when no current is flowing ( open circuit )

Question 49

What is internal resistance ?

Answer 49

The resistance within a battery (r)

Question 50

What is lost volts ?

Answer 50

The Voltage that is lost when the switch is closed and a current flows through the battery due to the internal resistance of the battery - the energy transferred per unit charge across the internal resistance

Question 51

What is the terminal pd ?

Answer 51

The energy transferred per unit charge to the load resistance

Question 52

What will the terminal pd be equal to if there hadn’t been any internal resistance ?

Answer 52

Emf

Question 53

How would you inverstogate the emf and the internal resistance ?

Answer 53

- a circuit with an ammeter and a load resistor ( variable resistor ) connected in series with the power supply - have a volt metre across the load resistor - vary the resistance of the load resistor and measure the current - plot a graph v against I and the gradient givers us -r - to work out the emf extrapolate the graph backwards to find the y intercept

Question 54

An ideal voltmeter connected directly across a cell reads V . What is the end of the cell and explain your answer.

Answer 54

Emf is V , ideal voltmeters have infinite resistance so the current in the circuit is zero . No lost pd - measured value is equal to the emf

Question 55

Why does the terminal velocity decrease as the current increases ?

Answer 55

As the current increases , more energy lost inside the cell due to internal resistance increases - as the lost pd is greater the terminal voltage decreases

Question 56

When is the power delivered to load a maximum ?

Answer 56

When the resistance of the load is equal to the internal resistance of the power supply