Current

A charge of 12C passes through a wire in 600 seconds. Calculate the current that flows through the wire.

Input the correct answer in mA.

\(I=\Delta Q/{\Delta t}=12/600 = 20\space mA\)

Charge

There is a current of 8A in a wire. What charge passes through this wire in 40 seconds?

Select the correct answer.

\(\Delta Q = I\Delta t=8\times40=320\space C\)

Current - plot

There is a current in a wire. The plot below shows the charge that has passed over time. Calculate the current flowing through the wire.

Input the correct answer in A.

Using the definition of current, \(I =\Delta Q/ {\Delta t}\), and measuring the increments of charge and time between two different points, \(I = (4-2)/(1-0.5)=4\space A\)

Charge - plot

The plot below shows the time dependence of a current in a wire. What charge passes through the wire in 4 seconds?

Input the correct answer in C.

. Charge can be found as an area under the line: \(\Delta Q = {12\times 4\over{2}}=24\space C\).

Voltmeter

Select a circuit in which voltmeter is connected correctly to measure the voltage of the lamp.

Select the correct answer.

To measure the voltage of the lamp correctly, voltmeter has to be connected parallel to the lamp. 

Ammeter

Choose a circuit where ammeter is connected correctly to measure the current across the lamp.

Select the correct option.

Ammeter must be connected in series with the lamp.

Superconductivity

Select the range of temperatures at which the phenomenon of superconductivity is typically observed in certain metals and alloys.

Select the correct option.

Most superconductors exhibit their superconductive properties under temperatures below 30K. Some of the recently discovered alloys show almost no resistance at temperatures lower than 100K.

Resistance

Calculate the resistance of a silver wire with a length of 4m and with a cross-sectional area of \(2 \space mm^2\). The resistivity of silver is known (at T=300K) and equals \(0.016 \space {Ohm\times mm^2\over m}\).

Input the correct answer in mOhms.

Using the formula for the resistance of a conductor with resistivity \(\rho\), length l and cross-section s,\(R = {\rho\times l\over S} = 0.016\times 4/2=32 \space mOhm\)

Validity of Ohm's law

At which operating temperatures is Ohm's law valid for resistors?

Select the correct option.

Ohm’s law is an empirical linear relation between voltage and current of a conductor. It can be observed under room conditions in most conductors (but not semiconductors). At other temperatures, V/I relation becomes non-linear. 

Power dissipation

A circuit is shown on a picture below. Calculate the power dissipation on the lamp.

Input the correct answer in W.

Power dissipation is the rate at with energy is transferred to an element of circuit. It is measured in watts. \(P = U\times I= 6\times 2 = 12W\)