Module 4- Eletricty

Question 1

What is current

Answer 1

current is the rate of flow of charge

Question 2

What is the direction of conventional current flow? And brief explanation of what it is?

Answer 2

From positive to negative regardless of the movement of electrons (because it was made before an understanding of charge carriers ect). Therefore conventional current travels in the opposite direction to electrons

Question 3

What is the equation for current

Answer 3

I = change in Q/ change in t

Question 4

What is a coulomb?

Answer 4

A coulomb is the unit of charge

So one coulomb is defined as the amount of charge that passes in 1 second when the current is ampere

Question 5

What carry charge through a circuit and what charge do they have (what is the charges name)?

Answer 5

Electrons or sometimes ions. Electrons all carry the same charge (-e) where e is the elementary charge (e=1.6x10^-19)

Question 6

What is charge known as and why

Answer 6

Charge is said to be quantized because the net charge will always be a integer multiple of e (and therefore only take certain values)

Question 7

What charge does a Na+ ion have

Answer 7

A charge of 1.6x10-19 (protons have an charge of positive e)

Question 8

How does current flow in metals.

Answer 8

In metals the charge carriers are free (delocalized) electrons from an atoms outer shell. These charge carriers travel through a lattice positive ions. These ions cannot move but are able to vibrate more with increasing temperatures.

Question 9

How do ions in electrolytes (liquids that can carry a current) cause a current?

Answer 9

In an electrolyte (molten ionic crystals or ionic solutions) the positive and negative ions are the charge carriers. They will carry a charge when a positive and negative electrode are put in the electrolyte solution because these cause and ions to move (causing a flow of charge which is current).
This can also cause a flow of electrons in the metal between the electrodes because while the cathode gives out electrons the anode takes in electrons

Question 10

State Kirchhoffs first Law what it is the conservation of

Answer 10

The total current entering and junction equals the total current leaving it
Conservation of charge

Question 11

Conservation of charge explained and how it relates to Kirchhoffs first law

Answer 11

As charge flows through a circuit, it doesn’t get used up or lost it is conserved and therefore the amount in the universe is constant. (In a closed circuit)
So what ever flows into a junction will flow back out.
Current is rate of flow of charge it follows that whatever current flows into a junction is the same as the current flowing out.

Question 12

Do circuit symbols sheet

Answer 12

…..

Question 13

In a circuit diagram which is the positive terminal?

Answer 13

The longer line (+ requires more ink)

Question 14

What is potential difference

Answer 14

The work done (energy transferred from electrical to other sources) per unit charge moved between two points in a circuit (V=W/Q)

The potential difference across a component is one vault when you do one joule of work moving one coulomb of charge through the component

Question 15

What is the equation for potential difference

Answer 15

W=VQ

Question 16

How should voltmeters be connected

Answer 16

In parallel

Question 17

How does work done on a charge link to kinetic energy in terms of equations?

Answer 17

When a charged particle is accelerated by potential difference, the energy transferred is equal to the work done on the particle. Hence for an electron of charge size e W=Ve.
W also equals Kinetic energy gained by the electron.

Hence eV=1/2mv^2

Question 18

What is electromotive force

Answer 18

The total amount of work the battery does to each coulomb of charge is called it’s electromotive force (energy transferred from other sources to electrical) e.m.f - measured in volts

Question 19

Difference between emf and potential difference

Answer 19

One is the energy transferred to the charge carriers while the other is the energy transferred by the to charge carriers

Question 20

What is voltmeter resistance and why

Answer 20

Very high due to so no current passes through the voltmeter itself

Question 21

What is the equation for emf

Answer 21

e=w/q

Question 22

What is resistance? With and equation

Answer 22

Resistance can be thought of as a measure of how difficult it is for current to flow through a component. The resistance of a component is defined as the ratio between the potential difference across a component and the current flowing through it. It is measured in ohms.

R=V/I

Question 23

Define the ohm

Answer 23

A components has a resistance of 1 ohm if a potential difference of 1 makes a current of 1 A

Question 24

What happens to the resistivity of a metal as it temperature increases?

Answer 24

Heating up a metal makes it harder for electrons to move about. The ions
vibrate more when heated around their positions , so the electrons collide with them more often increasing the work done of the electrons and the energy lost to other forms.
The resistivity of a metal increases as the temperature increases.

Question 25

What is Kirchhoffs second law

Answer 25

The total emf around a series circuit = the sum of the p.d across each component.
(Conservation of energy)

Question 26

What is it helpful to do when you need to identify amplitudes and voltage on a circuit diagram?

Answer 26

Helpful to draw the separate loops see figure 10 in note book
Think about a closed loop as a single possible path for the current

Question 27

What happen to current, resistance and voltage when kirchoffs laws are applied to a parallel circuit

Answer 27

Current is split at each junction I = I1+I2+I3
Total Potential difference at each loop with equal the EMF
1/R= 1/R1 + 1/R2 + 1/R3 (I/V=I1/V + I2/V + I3/V)

Question 28

What happen to current, resistance and voltage when kirchoffs laws are applied to a series circuit

Answer 28

Same current at all points in the circuit
EMF split between voltage so e=V1+V2+V3
R=R1+R2+R3 (because IR=IR1+IR2+IR3)

Question 29

What causes resistance

Answer 29

Often resistance is caused by electrons loosing energy (it being transferred to other stores) during collisions.

Question 30

What is internal resistance

Answer 30

The resistances caused by electron collisions with atoms inside the battery (this causes batteries to warm up as they are used)

Question 31

What are the equations you need to know involving internal resistance and E.M.F

Answer 31

E=V+Ir
E=V+v
V=E-v
E=I(R+r)

Question 32

What is the load/external resistance

Answer 32

It is the total amount of resistance of all the components in the external circuit

Question 33

what is Terminal PD

Answer 33

The potential difference across the load resistance and therefore It is the work done when one coulomb of charge flows through the load resistance (terminal PD) because V=W/Q.

Question 34

Is the terminal p.d the same as emf?

Answer 34

No because there is internal resistance causing energy to be lost.

Question 35

What is lost volts

Answer 35

The energy wasted per coulomb of charge

Question 36

Therefore how do we find total energy supplied by the source

Answer 36

Energy per coulomb used in external resistance + energy per column used wasted in internal resistance

(Proves conservation of energy)

Question 37

How can you work out the e.m.f of multiple cells in series

Answer 37

Total E = E1 + E2 + E3

Question 38

How can you work out the e.m.f of multiple cells in parallel

Answer 38

For Identical cells in parallel the total e.m.f is the same as each individual cell
E total = E1 = E2 = E3

Question 39

Give a brief explanation of how you would investigate internal resistance and e.m.f in a circuit?

Answer 39

Vary the current in the circuit by changing the external resistance with a variable resistor. Measure the p.d for several values of current. Record the data for V and I in a table.

Plot a graph of voltage against current and use the equation V=-Ir + e (e= y intercept and r=gradient)

Question 40

What is ohms law

Answer 40

Ohms law (obeyed by ohmic conductors) states that: provided the temperature is constant the current flowing through an ohmic conductor is directly proportional to the potential difference across it (V=IR)

Question 41

Why does voltage go down when current increases sometimes

Answer 41

If we apply the V=IR equation to internal resistance we see that lost volts = IR

So as current increases the voltage lost also increases

Question 42

Why does more current always go to the smaller resistance of two junctions in parallel?

Answer 42

The total PD across each branch is the same and because V=IR, Current splits in the inverse ratio to resistance (because a higher resistance must have a lower current )

Question 43

What happens when a charged particle is accelerated by a PD

Answer 43

When a charged particle is accelerated by a potential difference, the energy transferred to it is equal to the work done on the particle, W = VQ. For an electron with charge size e this can be written as W = Ve.

Question 44

Write an equation given that the energy transferred to an electron is equal to the Kenetic energy gained

Answer 44

eV=1/2mv^2

Question 45

What is the mean drift velocity of a charge carrier c

Answer 45

The mean drift velocity is just the average velocity and it’s much, much less than the electrons’ actual speed (because they move randomly in all directions, but tend to drift one way.)

Question 46

Equation for current involving mean drift velocity

Answer 46

I=Anev

A= cross sectional area
I = current
n = number density of electron (number per unit volume) m^-3
v = mean drift velocity
e= size of charge on one electron

(Note if using different charge carrier e can be replaced with charge and each carrier and n is number density of charge carriers)

Question 47

Explain the different numbers of charge carriers in semi conductors, insulators and metals

Answer 47

In a metal, the charge carriers are free electrons - they’re the ones from the outer shell of each atom. Thinking about the formula I = Anev, there are loads of charge carriers per unit volume, making n big. The drift velocity is small, even for a high current.

2) Semiconductors have fewer charge carriers, so the drift velocity needs to be higher to give the same current

3) A perfect insulator wouldn’t have any charge carriers, so n = 0 in the
formula and you’d get no current. Real insulators have a very small n.
You’ll be given this equation

Question 48

What are the things that determine resistance in a wire

Answer 48

Length (L). The longer the wire the more difficult it is to make a current flow.

2) Area (A). The wider the wire the easier it will be for the electrons to pass along it.

3) Resistivity (p). This depends on the material the wire’s made from, as the structure of the material may make it easy or difficult for charge to flow. In general, resistivity depends on environmental factors as well, like temperature.
The resistivity of a material is defined as the resistance of a 1 m length with a 1 m^2 cross-sectional area, so p= RA/L. Resistivity is measured in ohm metres.

You’ll be given equation in exam

Question 49

EXPLAIN HOW and electron gun works and what equation it helps to explain

Answer 49

A small metal filament is heated by an electric current. The electrons in this piece of wire gain kinetic energy. Some of them gain enough kinetic energy to
escape from the surface of the metal. This process is called thermionic
emission

If the heated filament is placed in a vacuum and a high p.d. applied
between the filament and an anode, the filament acts as a cathode,
and the freed electrons accelerate towards the anode, gaining kinetic energy. If the anode has a small hole in it, then electrons in line with this hole can pass through it, creating a beam of electrons with a specific kinetic energy

As the electrons accelerate towards the anode they gain kinetic energy.
From the definition for p.d, the work done on a single electron travelling
from the cathode to the anode is equal to eV, where e is the elementary
charge, the charge on each electron, and V is the accelerating p.d.

eV = 1/2 mv^2

This assumes the electrons have negligible kinetic energy at the cathode.

Question 50

Draw IV characteristic (current y against PD x) for filament lamp, ohmic conductor, thermoister, LDR, diode

Answer 50

…

Question 51

Explain the Iv characteristic of a ohmic/ metallic conductor

Answer 51

At constant temperature, the current through a metallic conductor, e.g. a wire or a resistor, is directly proportional to the potential difference.
2) The fact that the characteristic graph is a straight line through the origin tells you that the resistance doesn’t change - it’s equal to 1 / gradient.
3) Metallic conductors are ohmic - they have constant resistance provided their temperature doesn’t change (see below).

Question 52

Explain if characteristic for a filament lamp

Answer 52

The characteristic graph for a filament lamp is a curve, which starts steep but gets shallower as the potential difference rises.
3) However, current flowing through the lamp increases its temperature, so its resistance increases (see below).

Question 53

Why are semi conductors used in diodes, LDR and thermistors

Answer 53

Semiconductors have a higher resistivity than metals because there are fewer charge carriers available. However if energy is supplied to some types of semiconductor (eg. by increasing temperature, more charge carriers are released, so the resistivity decrease). this means that they make excellent sensors for detecting, changes in the environment.

Question 54

Explain the IV characteristic of a thermistor

Answer 54

A thermistor is a resistor with a resistance that depends on its temperature. You only need to know about NTC thermistors - NTC stands for ‘Negative Temperature Coefficient’. This means that the resistance decreases as the temperature goes up.
The characteristic I-V graph for an NTC thermistor curves upwards.
This is because as a thermistor warms more electrons have enough energy to escape their atoms meaning more charge carriers available and thus a lower resistance

Question 55

Explain the IV characteristic for an LDR

Answer 55

The greater the intensity of light shining on an LDR, the lower its resistance.
The explanation for this is similar to that for the thermistor. In this case, light provides the energy that releases more electrons.
More charge carriers means a lower resistance.

Question 56

Explain the IV characteristic of a diode

Answer 56

Diodes (including light emitting diodes (LEDs)) are designed to let current flow in one
Current / A direction only. You don’t need to be able to explain how they work, just what they do.

Forward bias is the direction in which the current is allowed to flow - it’s the direction the triangle points in the circuit symbols on the right.
Most diodes require a threshold voltage of about 0.6 V in the forward direction before they will conduct.
In reverse bias, the resistance of the diode is very high and the current that flows is very tiny.

Question 57

What is power defined as with and equation

Answer 57

Power (P is defined as the rate of doing work. Its measured in watts (W, where 1 watt is equivalent to Joule of work done per second.

P=W/t

Question 58

What is an equation for power involving PD and current

Answer 58

P=VI (given in formula book)

1) Potential difference (V is defined as the work done per coulomb.
2) Current (1) is defined as the number of coulombs transferred per second.
3) So p.d. x current is work done per second, i.e. power.

Question 59

Other equation invoking power

Answer 59

P=V^2/ R and P=I^2/R since P=VI (also given in formula book)

Question 60

What unit do electricity companies charge their users with?

Answer 60

Electricity companies charge their customers for units’ of electricity. Another
name for a unit is a kilowatt hour. If you know the power of an appliance and the length of time is it’s used for you can calculate the work it does in kWh.

Question 61

Last two bits of page 68 -69

Answer 61

.

Question 62

Make flash cards on potential divided

Answer 62

.

Question 63

Draw an LED and LDR