Electricity and Magnetism

Question 1

What is the definition of an Electric Field?

Answer 1

“A region of space where a charged object experiences a force due to its charge”

Question 2

What is the definition of an Electric Field Strength (E)?

Answer 2

“Force per unit charge experienced by a small positive test charge placed in the field.” E = F/q (if field is uniform then: E=V/d)

Question 3

What is the potential difference between two points defined as?

Answer 3

“The work done (or energy transferred)(W) when one unit of charge (q) moves between two points” V=W/q

Question 4

Electronvolt defined as?

Answer 4

“One electronvolt is the energy an electron would gain by being accelerated by a potential difference of 1 volt.”

Question 5

I = ∆q/∆t

Answer 5

I = Current (A)
∆q = amount of charge (C)
∆t = time (s)

Question 6

Elementary Charge (e)

Answer 6

1.6 X 10^-19 C

Question 7

F = kq1q2/r^2

Answer 7

F = Force (N)
k = coulomb constant (8.99x10^9 Nm^2C^-2)
q1q2 = charges (C).
r = distance between charges (m)

Question 8

k = 1/4π Eo

Answer 8

Eo =8.85 × 10-12 C2 N-1 m-2

Question 9

E = F/q

Answer 9

E = Electric field strength (NC-1 or Vm-1).
F = force exerted on a charge (N).
q = charge placed in the field (C)

Question 10

E = V/d

Answer 10

E = Electric field strength (NC-1 or Vm-1)
V = voltage-producing field (V).
d = distance between plates (m)

Question 11

V = W/q

Answer 11

V = potential difference (V or JC-1).
W = work done (energy gained/lost) (J).
q = amount of charge (C)

Question 12

E = kq1/r2

Answer 12

The field strength at a specific distance from the charge that is creating the field

Question 13

I = nAvq

Answer 13

A = πr^2 (if circular)
q = 1.6 x 10^-19 (C)
I = current (A).
v = drift velocity (ms-1).
n = number of charge carriers per m^3

Question 14

Series Circuits

Answer 14

• Same current around circuit
• Sum of the voltages (potential differences) across components adds to supply voltage (V= V1 + V2)

Question 15

Parallel Circuits

Answer 15

• The current splits up between branches ( I = I1 + I2)
• The voltage (potential difference) across each branch is the same

Question 16

R = V/I

Answer 16

V = voltage (potential difference) (V)
I = Current (A)
R = Resistance (Ω)

Question 17

P = IV

Answer 17

P = power (W)(Js-1)
I = Current (A)
V = voltage (potential difference) (V)

Question 18

P = E/t

Answer 18

P = power (W)
E = Energy (J)
t = time (s)

*not given

Question 19

3 effects of current

Answer 19

Heating, Magnetic & Chemical

Question 20

Total resistance (R) for resistors connected in series?

Answer 20

Rtotal = R1 + R2 + …

Question 21

Total resistance (R) for resistors connected in parallel?

Answer 21

1/Rtotal = 1/R1 + 1/R2 + …

Question 22

ρ = RA/L

Answer 22

ρ = resistivity (Ωm) (property of a substance)
R = resistance (Ω)
A = cross-sectional area (m^2) (is circular A = πr^2)
L = length (m)

Question 23

Ohm’s Law?

Answer 23

“The current flowing through a piece of metal is proportional to the potential difference across it providing the temperature remains constant”

Question 24

P = I^2R

Answer 24

P = power (W)(Js-1)
I = Current (A)
R = resistance (Ω)

Question 25

P = V^2/R

Answer 25

P = power (W)(Js-1)
R = resistance (Ω)
V = voltage (potential difference) (V)

Question 26

I1 = I2 + I3

Answer 26

Kirchhoff’s Laws: At any junction in a circuit, the total current entering the junction equals the total current leaving (conservation of charge)

Question 27

+V1 - V2 - V3 = 0

Answer 27

Kirchhoff’s Laws: Around any closed path of a circuit, the total of all the potential differences (∆V) is zero. (conservation of energy)

Question 28

ϵ = I(R + r)

Answer 28

ϵ = EMF (V)
I = Current (A)
R = circuit resistance (Ω)
r = internal resistance (Ω)

Question 29

EMF (ϵ) Definition?

Answer 29

“Energy supplied per unit charge by the power supply to the circuit”

Question 30

Thermistors

Answer 30

Temperature increase - Resistance of thermistor decreases

Question 31

Light Dependent Resistors (LDR)

Answer 31

Light intensity decreases - Resistance of LDR increases

Question 32

Stain Gauges

Answer 32

When stretched cross-sectional area gets smaller - resistance increases

Question 33

Primary Cells

Answer 33

• Cannot be recharged
• Irreversible chemical reaction

Question 34

Secondary Cells

Answer 34

• Designed to be recharged
• Reversible chemical reaction

Question 35

F = qvB sinθ

Answer 35

q = charge (C)
v = velocity of charge (ms-1)
B = magnetic field strength (T)
θ = angle between velocity & magnetic field (usually 90° so F = qvB) (sin 90 = 1)

*Force is always right angles to the velocity of the charge so the resultant motion is circular

Question 36

F = BIL sinθ

The motor effect

Answer 36

B = magnetic field strength (T)
I = current in wire (A)
L = length of wire WITHIN the magnetic field (m)
θ = angle between current & magnetic field (usually 90° so F = BIL)

Question 37

Coulombs Law

Answer 37

Force between two point charges is:
- Proportional to the product of their charges
- Inversely proportional to the square of their separation

Question 38

Current

Answer 38

rate of flow of electric charge

Question 39

Drift Velocity

Answer 39

the speed at which electrons move due to the current

Question 40

emf

Answer 40

energy supplied per unit charge

Question 41

Internal resistance

Answer 41

the resistance of the components of the cell itself that leads to energy loss in the cell

Question 42

Kirchhoff’s Current Law

Answer 42

at any junction in a circuit, the total current entering the
junction equals the total current leaving

Question 43

Kirchhoff’s Voltage Law

Answer 43

around any closed path of a circuit, the total of all the potential differences (ΔV) is zero.

Question 44

Resistance (R)

Answer 44

the ratio of potential difference across the component to current flowing through the component

Question 45

Resistivity

Answer 45

a measure of how the substance opposes the flow of electric current