Module 4: Section 1 - Charge, Curent And Potential Difference

Question 1

What is current ?

Answer 1

Current is the rate of flow of charge

Think about it like this - the current in a wire is the same as the water flowing in a pipe - the amount of water that flows depends on the flow rate and the time

The equation for current is:

I = Q/t

I=current Q=charge t=time

Question 2

What is the definition of a coulomb ?

Answer 2

The coulomb is the unit for CHARGE

One coulomb (C) is defined as the amount of charge that passes in 1 second when the current is 1A

Question 3

How can you measure current in a circuit ?

Answer 3

You can use an ammeter to measure current flowing through a part of a circuit

You have to attach the ammeter in series with the component that your investigating

Question 4

In electrical circuits, what is charge carried by ?

Answer 4

In electrical circuits, charge is usually carried by electrons - or sometimes by ions

Question 5

What charge do all electrons carry ?

Answer 5

Electrons all carry the same charge, -e , where ‘e’ is elementary charge

So their charge in C is 1.60x10*-19C

Question 6

What’s the formula for elementary charge ?

Answer 6

1e = 1.6 x 10 *-19 C

Question 7

What charge do protons carry ?

Answer 7

Protons carry an opposites charge of the same magnitude of what electrons carry - protons carry e+ , where ‘e’ is elementary charge

So basically, electrons can carry -e which means that it can carry -1.60 x 10-19 C so it can carry that much charge and protons also carry charge but that’s opposite so one proton can carry - or the net charge on a proton is +1.60 x 10-19 C or one elementary charge so —> +e

Question 8

What is elementary charge ?

Answer 8

Elementary charge is the smallest unit that charge come in - the net charge of any particle or object will always be a multiple of ‘e’ - elementary charge therefore we can say that charge is quantised

Question 9

What’s the net charge of any object or particle a multiple of ?

Answer 9

The net charge of any particle or object will always be given as a multiple of ‘e’

Question 10

What is potential difference ? And Whats the equation ?

Answer 10

Potential difference or voltage is defined as the work done per unit charge
- As to make electrical charge flow through a ( conductor wire ) circuit you need to do work on it

That’s why we think of the pd as a ‘push’ that drives electrons down the circuit - the ‘push’ is the work done per unit charge which makes the electrons move down the circuit

V = W / Q

V = Voltage, W = work done (J), Q = charge (C)

Question 11

Define the Volt ?

Answer 11

If you do 1J of work - moving 1C of charge through a component - then the pd across the component is 1V

1V = 1JC * -1

Question 12

How can you measure the potential difference over a component ?

Answer 12

You can use a voltmeter - and you have to place the voltmeter in parallel with the component that you are investigating

This is because in a parallel circuit the pd is the same on each branch

Question 13

When a charged particle is accelerated by a potential difference ( because work has been done on. The particle causing it to move down the circuit ) , the energy transferred to the particle is equal to the =

Answer 13

The energy transferred to the particle is equal to the work done on the particle
—> because work done = energy transferred

Work done = Voltage x Charge —> W = VQ

For an electron this can be written as - W = Ve because the charge of the electron has a size of e

The energy transferred to the electron ( by the pd ) is equal to the kinetic energy gained by the electron - this is because when a pd acts on a electron - it does work on the electron and this makes energy get transferred to the electron and this means that the electron gains kinetic energy and can be moved down the circuit

So therefore if work done = voltage x charge (e) —> and if work done ( energy transferred ) on the electron is equal to the kinetic energy gained by the electron then:

eV = 1/2 mv*2 —> because the kinetic energy is equal to the work done and work done is eV as W=eV

Question 14

How to find the velocity of a single electron accelerated through a potential difference ?

Answer 14

eV = 1/2 mv*2

Question 15

What is the mean drift velocity ?

Answer 15

Mean drift velocity is the average velocity of the charge carriers

When current is flowing thorough a wire, you would think that the electrons all move uniformly in the same direction however they all move randomly in all directions - but tend to drift one way - so the mean drift velocity is just the average velocity of the electrons - and its also much less than the electrons actual speed

Question 16

What is the actual speed of an electron ?

Answer 16

106 ms-1

Question 17

What is the equation for mean drift velocity and explain how it works

Answer 17

Current depends on mean drift velocity

I = Anev

I=current, A=cross sectional area, n=number density of electrons ( no. Of electrons ), e=size of charge on one electron, v=m,ean drift velocity

So:
- if you double the amount of electrons, the current doubles
- If you double the cross sectional area then the current doubles - the wire gets a bigger radius
- If the electrons move twice as fast, the current doubles as twice as many electrons move past a certain point in the same amount of time

Question 18

How do different materials determine the mean drift velocity ?

Answer 18

If you have a metal wire - then the charge carries are free, delocalised electrons ( from the outer shell of each atom ) - therefore using I = Anev , we can see that the ‘n’ is going to be very big as there is a lot of free electrons so therefore if there is a lot of electrons then the current gets very big but the mean drift velocity is small this is because there is a lot of electrons squished in one tube then they will move slower as they have to move past each other so therefore the man drift velocity is lower

Semi conductors - they have fewer charge carriers so therefore the mean drift velocity has to be high to give the same current as the material above

A perfect insulator - this wouldn’t have any charge carriers so n=0 so therefore you would have no current as there is no charge carriers to carry around the charge

Question 19

What are charge carriers in liquids and gases called ?

Answer 19

Ions

Question 20

How come ionic crystals can conduct electricity even though they are insulators ?

Answer 20

This is because once the ionic crystal is molten - the liquid conducts electricity - the positive and negative ions become the charge carriers.

The same thing happens in ionic solutions like copper sulphate solution

Question 21

What is an electrolyte ?

Answer 21

It is a substance that contains ions which conduct electricity - this could be a molten solution like the ionic crystals or a ionic solution like copper sulphate solution

Question 22

How can you make gases cause a spark ?

Answer 22

Gases are insulators meaning they don’t conduct electricity however if you apply a high enough voltage to the gas - electrons get ripped out of the atoms in the gas - this gives you ions along a path - this causes a spark