Electricity

Question 1

Define CHARGE

Answer 1

Measured in COULOMBS
The measure of the amount of ELECTRICAL CHARGE an OBJECT has

Question 2

What is an ELECTRICAL CURRENT?

Answer 2

The rate of flow of charge
(OR:
The flow of free electrons around an electrical circuit)

Question 3

What is CONVENTION CURRENT?

Answer 3

When the current flows from positive to negative

Question 4

What is ELECTRON FLOW?

Answer 4

Electrons flow from positive to negative

Question 4

I = ΔQ/Δt

Answer 4

I = Current [A]
Q = Charge [C]
t = Time [S]

Question 5

How can the total charge in a current-time graph be found?

Answer 5

Area under graph

Question 5

Total charge =

Answer 5

Number of electrons * Charge per electron

Question 6

Define DRIFT VELOCITY

Answer 6

The speed at which ‘free’ electrons flow from negative to positive, transferring energy.

Question 7

What are the factors that AFFECT CURRENT?

Answer 7

Drift velocity
Number of free electrons per m³
Cross-sectional area
Length

Question 8

Derive the TRANSPORT EQUATION

Answer 8

A = Cross-sectional area of wire
L = Lenth of wire
Diagram of wire, with c-s area, length, and electrons

Volume of wire = AL

Number of free electrons / m³ = n
Drift velocity per free electron = v
Charge per electron = e

Number of free electrons in volume = nAL

Total charge flowing in volume = nALe

I = Q/t = (nALe)/t

L/t = v (as s = D/t)

I = nAve

Question 9

Define potential difference

Answer 9

Measured in volts
The amount of electrical energy transferred into other forms of energy per unit of charge between two points

Question 10

V = (The equation with work done)

Answer 10

W/Q

Question 11

Define resistance

Answer 11

The opposition of the flow of a current

Question 12

R = V/I

Answer 12

R = Resistance
V = Potential difference
A = Current

Question 13

What is Ohm’s law?

Answer 13

V ∝ I when the temperature is constant

Question 14

Derive V = RI

Answer 14

V ∝ I
V = kI, where k is a constant
V = RI

Question 15

Why does resistance occur?

Answer 15

Free electrons collide with the ions of the conductor, losing energy

Question 16

What affects resistance?

Answer 16

Length of wire
Temperature
Cross-sectional area
Type of material OR number of electrons

Question 17

Define power

Answer 17

The rate of energy transfer

Question 18

P = W/t

Answer 18

P = Power
W = Work done OR energy
t = Time

Question 19

Derive P = IV (I = Qt, V = W/Q)

Answer 19

P = Wt

I = Qt
t = Q/I

P = WI/Q

V = W/Q
W = VQ

P = VQI/Q

P = IV

Question 20

Derive P = I²R (P = IV, V = IR)

Answer 20

P = IV

V = IR
P = IIR

P = I²R

Question 21

Derive P = V²/R (P = IV, V = IR)

Answer 21

P = IV

V = IR
I = V/R

P = V/R * V

P = V²/R

Question 22

What is the purpose of resistivity?

Answer 22

To compare the resistance of different materials

Question 23

Define resistivity

Answer 23

Measured in Ωm The resistance of a material that is 1m long, and has a cross-sectional area of 1m², at a specific temperature

Question 24

R = (ρL) / A

Answer 24

R = Resistance ρ = Resistivity L = Length A = Cross-sectional area

Question 25

When ρ and A in the equation R = (ρL)/A are constant, what is the relationship between R and L?

Answer 25

R ∝ L

Question 26

When ρ and L in the equation R = (ρL)/A are constant, what is the relationship between R and A?

Answer 26

R ∝ 1/A

Question 27

What happens when metals are cooled to near 0 Kelvin?

Answer 27

They become superconductive They have no resistance

Question 28

What is a critical temperature?

Answer 28

The temperature at which resistance disappears

Question 29

What does superconducting mean?

Answer 29

The range of temperatures at which a material has no resistance

Question 30

What is used to cool materials to their critical temperatures?

Answer 30

Liquid nitrogen

Question 31

What is a high temperature superconductor?

Answer 31

A material who's transition temperature is above the boiling point of nitrogen

Question 32

Explain P.D. in a superconductive circuit

Answer 32

Needed to start the current flowing, does not need to be maintained

Question 33

What is the purpose of using superconductive wires?

Answer 33

No resistance means no heat loss Large currents with 0V P.D.

Question 34

What are some uses of superconductivity?

Answer 34

Nuclear fusion Large Hadron Colliders MRI scanners More efficient and smaller generators

Question 35

What are the negatives of superconductive wires?

Answer 35

Hard to maintain temperature Brittle and non-ductile material Hard to manufacture

Question 36

What is the rule for current in a series circuit?

Answer 36

I_1 = I_2 = I_3

Question 37

What is the rule for P.D. in a series circuit?

Answer 37

V = V_1 + V_2

Question 38

What is the rule for resistance in a series circuit?

Answer 38

R_T = R_1 + R_2

Question 39

What is the rule for current in a parallel circuit?

Answer 39

I_1 = I_2 + I_3

Question 40

What is the rule for P.D. in a parallel circuit?

Answer 40

V_T = V_1 = V_2

Question 41

What is the rule for resistance in a parallel circuit?

Answer 41

1/R_T = 1/R_1 + 1/R_2

Question 42

Define electromotive force (Units)

Answer 42

The amount of chemical energy transferred into electrical energy per coulomb as it passes through the battery, measured in V or JC^-1

Question 43

Derive the EMF equation

Answer 43

E = V + v [V = IR] E = IR + Ir E = I(R+r) or E = V + v E = V + Ir V = E - Ir V = External P.D. v = 'Volts lost' within cell E = EMF