C8 - Charge and Current

Question 1

Which direction do electrons move around a circuit?

Answer 1

Negative to positive

Question 2

What do electrons carry through a circuit?

Answer 2

Energy

Question 3

In which direction does current flow around a circuit?

Answer 3

Positive to negative

Question 4

What is current?

Answer 4

The rate of flow of charge, measured in amperes.

Question 5

What is electrical charge?

Answer 5

A physical property, considered to be a measure of ‘charged-ness’ which can be positive or negative.

Question 6

What is relative charge?

Answer 6

A simplified measurement of electrical charge of an object as a multiple of elementary charge.

Question 7

What is elementary charge?

Answer 7

The charge equivalent to the electric charge on a proton (1.6 * 10-19) represented by ‘e’.

Question 8

What is net charge of an object given by?

Answer 8

Q = +/- ne

n is number of electrons added or removed
e is elementary charge

Question 9

How are current, charge and time related?

Answer 9

I = ΔQ / Δt

Current = change in charge / change in time

Question 10

What is the charge of an electron?

Answer 10

-1.6 * 10 -19

Question 11

What does it mean for an object to be quantised?

Answer 11

It is charged (with specific values) with integers of multiples of ‘e’.

E.g. An object with a charge of ‘1.92 * 10 -18 C’ has a charge of +12e.

Question 12

What can cause a larger current?

Answer 12

A greater number of e- moving of a point each second.

The same number of e- moving faster through the metal.

Question 13

What’s an electrolyte?

Answer 13

A liquid that can carry electric current.

Question 14

How are ammeters and voltmeters placed in circuits?

Answer 14

Ammeters are placed in series (current is same in series) as they have very low resistance.

Voltmeters is placed in parallel (voltage is shared in series) as they have very high resistance.

Question 15

What is Kirchhoff’s first law?

Answer 15

For any point in an electrical circuit, the sum of currents into that point is equal to the sum of currents out of that point.

Question 16

What is number density?

Answer 16

The number of free electrons per cubic metre of material.
The higher the density, the greater the number of free electrons so the better the conductor.

This enables materials to be classed as conductors, semi conductors and insulators.

Question 17

What is the equation for current, linking area, number density, elementary charge and mean drift?

Answer 17

I = Anev

Current = cross sectional area * number density * elementary charge * mean drift.

Question 18

How does changing cross sectional area affect the flow of current?

Answer 18

The narrower the velocity, the greater the drift velocity must be in order for the current to be the same.

To maintain the same current, electrons must move faster through narrower wires.
Mean drift velocity is inversely proportional to C.S. area.

Question 19

Explain how you would determine experimentally the e.m.f. E and internal resistance r of a cell.

Answer 19

Draw a cell across variable resistor R ammeter in series and voltmeter in parallel across R or cell.

Take a set of readings of V and I for different positions/values of the variable resistor

Plot a graph of V against I

Find the y-intercept = E

Find the gradient of the V against I graph which equals the internal resistance in magnitude
or take one pair of values of V,I and substitute into equation E = V + Ir to find r or E