Electricity

Question 1

What direction does conventional current go in?

Answer 1

+ to -.

Question 2

What direction do electrons travel round a circuit in?

Answer 2

• to +.

Question 3

What is the formula for charge involving time?

Answer 3

Charge Q (C) = Current I (A) x Time t (s)

Question 4

What is the formula for charge involving the charge of an electron?

Answer 4

Charge Q (C) = Number of electrons n x Charge of an electron e (1.6 x 10^-19)

Question 5

Explain Q = ne

Answer 5

Charge is the charge of an electron times the number of electrons, so charge per electron x electrons simplifies to charge.

Question 6

Explain Q = It

Answer 6

Sum of the charges of all the charged particles through a point is the charge that passes through a point in 1 second. So in 3 seconds, the charge at that point is the charge in 1 second x 3, so Q = It.

Question 7

What is current (in words) based on the Charge formula?

Answer 7

The rate of flow of charged particles.

Question 8

What is Kirchhoff’s First Law?

Answer 8

The sum of the current into a junction = the sum of the current out of a junction

Question 9

Why is Kirchhoff’s First Law what it is?

Answer 9

Current can’t leave or join the circuit, so all of the current going in leaves through one of the exit branches.

Question 10

Define potential difference.

Answer 10

The difference in electric potential energy charge carriers have between 2 points, or the energy given away as electrical energy between those two points.

Question 11

What happens to charged particles as they go through a power source?

Answer 11

They are given chemical energy from the cell or battery, all of which is given away as they go round the circuit.

Question 12

What is the electromotive force?

Answer 12

The energy given to charged particles for every coulomb of charge (or 6.25 x 10^18 electrons).

Question 13

What is the formula for electromotive force?

Answer 13

Electromotive force ℰ (V) = Energy E (J) / Charge Q (C)

Question 14

What is the potential difference across a component?

Answer 14

The energy given away as electric potential energy across the component per unit of charge.

Question 15

What is the formula for potential difference in terms of Work done?

Answer 15

Potential Difference V (V) = Work done W (J) / Charge Q (C)

Question 16

What is the potential difference at a point?

Answer 16

The energy per coulomb of charge that still needs to be given away at that point.

Question 17

What is the potential difference between 2 points?

Answer 17

The energy given away between those 2 points.

Question 18

Why is the potential difference equal to the emf?

Answer 18

The energy given has to be the same as the energy given away.

Question 19

What is Kirchhoff’s Second Law?

Answer 19

The voltage supplied equals the voltage given away in a closed loop in a circuit.

Question 20

Why can voltmeters measure voltage well?

Answer 20

They have almost no current so they need an high resistance so that the resistance multiplied by the current is equal to the voltage, so they don’t take current away from the other path.

Question 21

Why will something in a circuit have kinetic energy?

Answer 21

Work is done to it, so it accelerates, so it has kinetic energy.

Question 22

Derive a formula for the kinetic energy of each electron (not 1/2 mv^2)

Answer 22

W = VQ
Kinetic energy = 1/2 mv^2
For one electron, eV = 1/2 mv^2

Question 23

What is the definition of power?

Answer 23

The energy transferred in a given time.

Question 24

Derive the formula linking power and voltage.

Answer 24

W = Pt
Q = It
V = W/Q
V= Pt/It = P/I
P = IV