UE 04: Electricity

Question 1

True or false?

• The total voltage (U_total) is always predetermined
• The total current (I_total) is always determined by the total resistance (R_total) as well as the total voltage (U_total) of the connected circuit

Answer 1

True!

Question 2

True or false?

When a current (I) flows through multiple resistors (e.g. a consumer, 1km of pure cable) which are connected in series the voltage (U) drops which causes a power loss (P_loss).

(Kirchhoffs mech law)

At the same time the current (I) remains constant across all resistors.

(Kirchhoffs junction law)

Answer 2

True!

Question 3

Electric charge (Q) and electrical current (I)

• Is measured in [C] = [A * s]
• Is calculated: Q = I * t
  (current (I) * charge time (t))
• Explanation: “…”

Answer 3

Electric charge Q

• Explanation:
  “–> When a current (I) of constant strength flows for a time (t), it transports charge (Q)
  –> Movement of electric charges (Q) results in electric current (I)”

Question 4

Electric voltage (U)

• Is measured in [V] = [W/A] = [ohm * A]
• Is calculated:
  –> W_el = U * Q
  –> U = R * I
• Explanation:
  “…”

Answer 4

Electric voltage U

Explanation:

• Voltage indicates the amount of work or energy (W_el) needed to separate electric charges (Q). After charge separation, energy (W_el) is stored in the charges (Q).
• Voltage (U) behaves analog to the water-pressure which exists between two masses of water which are connected to each other via a pipeline. In case of voltage (U) two electrical charges (Q) are connected to each other via a conductor or wire.

Question 5

Electric energy (W_el) and power (P)

• Electric energy (W_el) is measured in [J]
• Electric power (P) is measured in [W] = [J/s]
• Are calculated:
  –> W_el = U * Q
  –> W_el = P * t
  –> P = U * I
• Explanation:
  “…”

Answer 5

Electric energy (W_el) and power (P)

Explanation:

• Electric energy (W_el), is transported through electric current (I) and converted into various forms in consumers.
• Electrical power (P) is the rate at which electrical energy (W_el) is converted into work/or another form of energy or is transmitted from one place to another.

Question 6

On the battery of a cordless screwdriver, the following information are provided:

12 V / 1,4 Ah / 16,8 Wh / NiMH 1,2 V

Explain each term.

Answer 6

Voltage (U)

• Total battery: 12 V
• Single battery cell: 1,2 V
• Therefore the total battery consists of 10 individual cells

Charge (Q)

• [Ah] = 1/3600 [As] = 1/3600 [C]
• With a current of 1 A it takes 1,4 h to charge the battery completely. The total charge then amounts to 1,4 Ah.
• With a current of 1,4 A it would only take 1 h to charge the battery completely. The total charge then amounts to 1,4 Ah.

Stored energy (W_el)

• [Wh] = [J/s * h] = 3600 [J/s * s] = 3600 [J]
• The battery can deliver up to 16,8 W for 1 h.
• If fully charged the battery can perform 60480 J (= 16,8 * 3600) work.

Materials used in the battery

• NiMH = Nickel-Metal-Hybride

Question 7

True or false?

The larger the cross sectional area of an electric wire the lower is the power loss which occurs across this wire.

This is why e. g. the cross sectional area of wire connecting the stove is larger compared to the cross sectional area of wires of usual power outlets.

Answer 7

True!

Larger A_wire –> smaller R_wire –> smaller U_loss_wire –> smaller P_loss_wire

This is why e. g. the cross sectional area of wire connecting the stove is larger compared to the cross sectional area of wires of usual power outlets.

Question 8

True or false?

Requirements of the material used in transmission lines

• Low specific resistance –> low resistance of entire line –> low power losses across entire line
• Strength, stability and durability
• Low costs

Answer 8

True!

Question 9

What is the voltage of a usual power outlet and the stove in a household in Germany?

Answer 9

Power outlet: 230 V

Stove: 400 V

Question 10

What are High voltage lines made of?

Answer 10

Steel Core with aluminum sheath