Electricity and Magnetism

Question 1

What is an electric charge? What are the properties of charge? [5]

Answer 1

• A property of matter, that can be positive or negative.
• Like charges repel, unlike charges attract.
• Charge is quantized - it only comes in discrete amounts.
• Measured in Coulombs.
• Charge is conserved.

Question 2

What is a conductor and an insulator?

Answer 2

Materials that have and do not have ‘free’ electrons respectively.

Question 3

How can you work out the force between two charges

Answer 3

Coulomb’s law shows that force is inversely proportional to the square of the separation of the charges and proportional to the product of the two charges.
F = k (q1q2)/r² DB
where k = 1/4πɛ. DB
where ɛ is the permittivity of the medium which the charges are in.

Question 4

What is an electric field?

What direction does it go?

Answer 4

• A space around a charge in which small point charges experience an electric force.
• Field lines show the direction of force on a test charge
  (‘test charges’ are positive)

Question 5

What is electric field strength?

Answer 5

It is the electric force per unit charge on a small positive test charge (at a particular point in space) (a vector quantity).

Question 6

How can you calculate the electric field strength?

Answer 6

E = F/q DB
(q is the charge of a test charge, E is in NC⁻¹)
Combine with F = k(q1q2)/r² to get E = kQ/r²
(Q is the charge creating the field) not in DB

Question 7

How can you calculate the electric field strength between two charged parallel plates?

Answer 7

The uniform electric field is
E = V/d not in DB
V is the voltage between the plates
d is the plate separation.

Question 8

What is an electric current? What is the formula for it?

Answer 8

It is the rate of flow of charge through the cross section of a conductor.
I = △q/△t DB
I (A)
△q is the net charge passing through the cross section
△t is the time interval

Question 9

What is the drift velocity? How can you calculate it?

Answer 9

The resultant velocity of charge carriers. Worked out by imagining a cylinder of charges moving through a cross section in time △t.
I = nAvq DB
n = no. of charge carriers per unit volume
A = cross sectional area
v = drift velocity
q = charge of one carrier

Question 10

What is potential difference?How can you calculate it?

Answer 10

between two points the p.d. is the work done per unit charge to move a point charge from one point to the other.
V = W/q DB
V = p.d. in volts
W = work done in J
q = charge that that is to be moved in C.

Question 11

What is the implication of a potential difference?

Answer 11

Whenever there is a p.d., there has to be an electric field.

Question 12

What is an electronvolt?

Answer 12

It is the work done when a charge equal to one electron charge is taken across a p.d. of 1 V.

Question 13

What happens when a charge is placed in a p.d.?

Answer 13

It is accelerated, and the work done on the charge = the kinetic energy transferred to the charge. Use this to calculate the speed of a charge.

Question 14

How do electric currents in conductors have heating effects?

Answer 14

The electric field within the conductor accelerates the electrons.
The electrons suffer inelastic collisions with the metal ions, causing the atoms to vibrate with increased KE, and causing the electrons to lose KE. The process repeats.

Question 15

What is electric resistance? How can you calculate it?

Answer 15

The p.d. V across the ends of a conductor divided by the current passing through it.
V = IR

Question 16

What is Ohm’s law?

Answer 16

For metallic conductors at constant temperature, I ∝ V

For an ohmic conductor, V = IR is a straight line graph passing through the origin, R is constant. /

Question 17

What is resistivity?

Answer 17

Resistivity, ρ (Ωm), is a property of material that depends on temperature. From experiment, R of a wire at constant T
R = ρ L/A DB
L = length of the conductor
A = cross-sectional area

Question 18

What are the voltage current characteristics of:

• An ohmic conductor?
• A filament lamp (non-ohmic, temp changes)
• A semiconductor diode

Answer 18

• A straight line graph passing through the origin, R is constant. /
• A drawn out S shape. R not constant.
• A flatline, with a sharp positive current when the voltage is a bit positive _/

Question 19

What is voltage?

Answer 19

It is the potential difference across the ends of a resistor.

Question 20

How can you calculate electric power?

Answer 20

P = VI = I²R = V²/R. DB

(W) of thermal energy or work performed by electrical device.

Question 21

What is electromotive force?

Answer 21

Emf is a source of p.d.,
- work done per unit charge in moving charge across the battery terminals (with no internal resistance. ɛ = W/q (V)
- Power supplied by the battery per unit current ɛ = P/I
Neither in DB

Question 22

What is a series circuit, and how can you calculate the resistance of the circuit?

Answer 22

• Where all of the components are in a single loop.
• Rtotal = R1 + R2 + R3 … DB
  found by adding different Vs together, sub IR and factorise I.

Question 23

What is a parallel circuit?

How can you calculate the resistance of the circuit?

Answer 23

• Where the components are connected by multiple loops.
• 1/Rtotal = 1/R1 + 1/R2 + 1/R3 … DB
  Found by adding different Is together, sub V/R, factorise V.

Question 24

What is Kirchhoff’s 1st Law?

Answer 24

ΣIin = ΣIout not in DB

At any junction the vector sum of the currents must be zero, because of the conservation of charge.

Question 25

What is Kirchhoff's 2nd Law?

Answer 25

In any complete loop in a circuit, the sum of the emfs is equal to the summer of the potential differences. So, ΣV = 0. DB For any loop, Σɛ = ΣIR not in DB

Question 26

How can you work out the energy gained and lost by charges as they flow through a load or an emf?

Answer 26

Energy lost per unit charge, W = qV Energy gained per unit charge, W = qɛ

Question 27

How can you use Kirchhoff's laws to work out current and resistance in a circuit?

Answer 27

Draw the circuit. Label all known and unknown ɛ, I, R, V in different loops of the circuit. Form equations for I at junctions using K1 Choose a loop (any loop) in the circuit. Choose a direction. Use K2 to equate all sources of ɛ and IR in the loop, noting that if the current is in the opposite direction of the loop, the value will be -ve. Repeat for other loops. Use siml eqns to solve for unknowns.

Question 28

What is an ideal ammeter and ideal voltmeter?

Answer 28

- Ammeter has no resistance | - Voltmeter has infinite resistance.

Question 29

What is a potential divider circuit? | What is the advantage

Answer 29

- One where the supply voltage, V, is divided into parts, where the p.d. across each resistor depends o the resistance of the resistor/total resistance. - The p.d. across the resistor can be varied from 0 to ɛ, compared to 0 up to something less than ɛ.

Question 30

What is internal resistance? | How can you work it out?

Answer 30

Batteries have internal resistance, r. The terminal p.d. is the p.d. across the terminals of the battery, V. V = ɛ - Ir not in DB or, ɛ = I(R + r) where R is the total resistance of the circuit.

Question 31

What is the difference between a primary and secondary cell?

Answer 31

Secondary cells are rechargeable.

Question 32

What does the magnetic field look like around a: Wire Solenoid Bar magnet?

Answer 32

- Concentric rings, pointing according to the RH grip rule. The thumb is the direction of current. - Loops traveling through the solenoid, pointing according to the RH grip rule. - Loops going from N to S

Question 33

What is magnetic flux density? | How can it be represented on a diagram?

Answer 33

The magnitude of a magnetic field, B. | Crosses or dots in circles to show it entering or leaving the page.

Question 34

How can you work out the force on a moving charge in a magnetic field? What happens if it moves in the direction of the field?

Answer 34

F = Bqvsinθ. DB B = magnetic field strength, T q = charge on particle v = velocity of particle θ = angle between field lines and velocity vector - It will experience no force. θ = 0 Use Fleming's LH rule to determine direction of F

Question 35

How can you work out the force on a current carrying wire in a magnetic field?

Answer 35

``` F = BILsinθ B = magnetic field strength, T I = Current L = length of wire in field region θ = angle between current and direction of magnetic field. RH rule to find direction of F ```

Question 36

What does the path of a moving charge in a MAGNETIC field look like?

Answer 36

Circular - velocity constantly changing, and F always perpendicular to v Entering at an angle, the path will be helical.

Question 37

Why do current carrying wires in a magnetic field experience a force?

Answer 37

Moving electrons experience a [downwards/upwards] as a result of the magnetic field, and accumulate on one side of the wire. The electric field created between the different sides of the wire exerts an electric force on the fixed positive charges in the middle of the wire.

Question 38

What is the work done by magnetic forces?

Answer 38

0, because the magnetic force is always normal to the velocity of the charge.

Question 39

How can you work out the force between two current carrying wires?

Answer 39

- Current in one of the wires produces a magnetic field around the wire - The second wire is in the magnetic field. - Use F LH rule to find direction of force on second wire. - N's 3rd law shows that the force on both wires is equal and opposite - Parallel wires attract, anti-parallel repel

Question 40

What is the definition of the ampere?

Answer 40

The current flowing in two parallel wires of length 1 m which are 1m apart when the force between the wires is 2 x 10^-7 N