Electronic circuit elements

Question 1

What is current?

Answer 1

Current (I) is the amount of charge (Q) that flows past a point in a given amount of time

I = Q/t = amperes = coulombs/sec

Current is caused by the movement of electrons between two points of significant potential difference of an electric circuit. Free electrons will accelerate towards the positive connection. As they move, they will collide with atoms in the substance, losing energy (which we observe as heat). The net effect is a drift of electrons at a roughly constant speed towards the positive connection.

Question 2

What is an electric current?

Answer 2

The motion of electrons at a roughly constant speed towards a positive connection, is an electric current.

As electrons are removed by the electric potential source at the positive connection, electrons are being injected at the negative connection. The potential can be considered as a form of electron pump.

Question 3

If the magnitude of an electric potential is increased, what happens to the electrons in a current? Give Ohm’s law

Answer 3

The electrons will accelerate faster and their mean velocity will be higher (increase in current).

This will result in more collisions, manifesting itself as heat (known as Joule heating).

This is true in ohmic conductors, which is materials that obey Ohm’s law (such as metals):

V = IR

Question 4

What is Ohm’s law?

Answer 4

V = IR

Question 5

What is the conventional direction of current flow?

Answer 5

The direction of positive charge movement

Question 6

what is electrical resistance (R) and resistivity (ρ)? Give the formula for resistance

Answer 6

The measure of opposition to the flow of electrons in a substance. Resistivity (ρ) is an inherent property of a substance.

R = ρI/A

The units of resistance are Ohms, sumbolized by Ω

1 Ω = 1 volt/ampere

Question 7

How does temperature affect resistance?

Answer 7

Resistance increases with temperature because the thermal motion of molecules increases with temperature and results in more collisions between electrons, which impede their flow.

Question 8

Give the formula for calculating power loss (P) from electrical resistance

Answer 8

power loss = Vlt/t = VI = watts
watts = volts x amperes = joules/sec

The energy loss may be used to perform work, these relations hold for power (P)

P = VI = (IR)(I)

Question 9

In a circuit, what are series elements and parallel elements?

Answer 9

Series: Components are in a series when they have only one point in common, that is the current travelling from one of them back to the emf source, must pass through the other. In a complete circuit series the current (I) is the same over each component and the total voltage drop in the circuit elements (resistors, capacitors, inductors, internal resistance of emf sources etc.) is equal to the sum V of all the emf sources.

Parallel: Two components are in parallel when they are connected to two common points in the circuit. That is, the current travelling from one such element back to the emf source need not pass through the second element, because there is an alternate path.

Question 10

What is an electromotive force (emf) source?

What is net voltage?

Answer 10

It maintains a constant potential difference between its terminal points.

It replaces lost energy with by moving electrons.

It includes batteries (chemical energy to electrical energy) and generators (mechanical energy to electrical energy)

The units of emf are volts, the actual voltage delivered to a circuit is not equal to the value of the source. This is reduced by an internal voltage lost, which represents the voltage loss by the internal resistance (r) of the source itself. THe net voltage is called the terminal voltage (terminal potential - Vt)

Question 11

What happens when two emf sources are connected in opposition (eg. positive pole to positive pole)?

Answer 11

The charge loses energy when passing in the second emf source

Therefore, if there is more than one emf source in a circuit, the total emf is the sum of the individual emf sources not in opposition reduced by the sum of individual sources in opposition in a given direction

Question 12

What are capacitors?

Answer 12

Things that can store and separate charge.

They can be filled with dielectrics, which are materials which can increase capacitance. They are made of two or more conductors with opposite but equal charges placed near each other.

Question 13

How does dielectric strength effect capacitance?

Answer 13

The higher the dielectric strength (electric field strength at which a substance ceases to be an insulator and becomes a conductor) of the medium, the greater the capacitance of the conductor.

Question 14

What is capacitance?

Answer 14

Capacitance is an inherent property of a conductor. It is the number of coulombs that must be transferred to a conductor to raise its potential by one volt. The amount of charge that can be stored depends on the shape, size, surroundings and the type of the conductor.

C = charge/electric potential = Q/V = farad = coulomb/volt

Question 15

Suppose that 2 identical photoelectric devices are connected to the same electrical circuit, and the devices are connected in parallel with each other. Compared to the voltage generated by a single device, the total voltage supplied to the circuit by the parallel devices will be how many times as large/small?

Answer 15

The total voltage supplied to the circuit by the parallel devices will be the same.

Identical voltage sources connected in parallel produce the same output voltage as a single source. (whereas, if they were connected in series, the source voltages would be added).

Question 16

At a given temperature, the resistance of a wire to direct current depends only on the ___?

Answer 16

Resistivity, length and cross-sectional area. The resistivity relation at a fixed temperature is:

Resistance = Resistivity x (length/area)

Question 17

A battery in a circuit has an electromotive force given ξ and an internal resistance of r. The battery provides a current i to the circuit. What is the terminal voltage of the battery?

Answer 17

ξ - ir = terminal voltage (Ohm’s law)

The terminal voltage is the voltage provided to the external components of the circuit. The battery voltage 1/4 will be reduced by the voltage required to overcome the internal resistance, so:

V = 1/4 - ir

Question 18

Why are capacitors able to store more charge than the terminals of cells connected by a wire?

Answer 18

The cell is able to transport more electrons at a lower energy cost with capacitors, because the additional electrons are able to spread out across the plate. Eventually the cell tries to push one more electron onto the plate but cannot, because the energy cost is too great. In this case, the potential between the plates is the same as before, but the difference is the more charge is ablet be contained on the plates than on the terminals of the cell.

Question 19

What determines the capacitance of a device?

Answer 19

How it is built, the charge on it is proportional to the voltage applied to it

Q = CΔV

or

C = Q/ΔV

Where Q is the charge on one plate and ΔV is the potential difference across the plates. The units of capacitance are Coulombs/volt (aka. the Farad)

Question 20

Why does reducing electric field between two plates increases the capacitance of a capacitor?

Answer 20

Capacitance = Q/ΔV

ΔV = EΔx

Larger electric field and distance between plates reduces capacitance by increasing the voltage potential

Question 21

Incandescent lamps produce a lot of infrared (heat) which allows the filament to glow and produce light. Fluorescent lights produce an electric between two electrodes in a tube of vaporized mercury. The mercury produces UV light and this is then intercepted by a phosphor that coats the inside of the glass tube. The phosphor absorbs the ultraviolet and emits visible radiation of a longer wavelength.

Which type of light is more efficient?

Answer 21

The incandescent light loses almost all its power through heat, the fluorescent light only loses a little power at the phosphor coat (remember: longer wavelength, UV to visible light).

Fluorescent lights are much more efficient.

Question 22

You are discharging a capacitor and you want to keep current constant. How do you do this?

How do you use a light bulb to discharge a capacitor and what does this look like?

Answer 22

Decrease R continually as V decreases with the discharge.

Discharging a capacitor continuously reduces the potential across it.

You can connect the two plates of a capacitor to +/- end of a light bulb filament. The bulb will dim as the capacitor is discharged.

Question 23

Electric power for transmission over long distances is “stepped up” to a very high voltage in order to ?

Answer 23

To cut down the heat loss in the transmission wires.

P = IV
V = P/I
I = V/R
P = V^2/R

Power lost: I^2R

Using a high voltage at a lower current maximizes the power to the user and minimizes the heat loss in the transmission line due (V = P/I)

Question 24

Define the polarity of electrodes in:

• Galvanic cells

- Electrolytic (voltaic) cells

Answer 24

Galvanic

• Anode (-)
• Cathode (+)

Electrolytic

• Anode (+)
• Cathode (-)

Question 25

Give the two renditions of the right hand rule.

Answer 25

Current in wire: Right hand twists around wire, fingertips point in direction of magnetic field (B)

Current travelling through space: Open hand, fingers together. Used for a electric field. Thumb points in direction of current’s motion. FORCE ON CURRENT from electric field comes out of palm. Direction of B comes out of fingertips.

Question 26

In circuits, capacitors in series add up how? In parallel?

Answer 26

Series: 1/C + 1/C

Parallel: C + C

REMEMBER: This is opposite to the case of resistance!