Ohm’s Law, DC Circuits And Kirchoff’s Law

Question 1

What is Electric Current? What is its measurement and unit?

Answer 1

The flow of electric change in a circuit measured in amperes with the symbol “I”

Question 2

What is Voltage (Electric Potential)? What is its measurement and unit?

Answer 2

The driving force that pushes electric charge through a circuit. Measured in Volts (V) with symbol V

Question 3

What is resistance? What is its measurement and unit?

Answer 3

Resistance opposes the flow f electric current in a circuit measured in ohms(Ω) with symbol R

Question 4

What is Ohms Law?

Answer 4

a law stating that electric current is proportional to voltage and inversely proportional to resistance.

Ohm’s law states that the voltage across a conductor is directly proportional to the current flowing through it, provided all physical conditions and temperatures remain constant and inversely proportional to the resistance.

Question 5

What does Ohm’s law explain?

Answer 5

The fundamental principle relating voltage current and resistance.

V = IR

Question 6

What are the two types of current?

Answer 6

Directing and Alternating

Question 7

What happens in a direct current?

Answer 7

Flow of electrons is consistently in one direction around the circuit

At any junction in the circuit the total current leaving the circuit is equal to the total current entering the junction.

Question 8

What happens in an alternating current?

Answer 8

The direction of electron flow continually reverses

Question 9

What is are electrons and what do the do?

Answer 9

Electrons are negatively charged particles and they transfer energy through wires as electricity.

Question 10

What is electrical current?

Answer 10

Electrical current is the rate of flow of electric charge. Charge is measured in coulombs (C)

Question 11

What is the equation for electrical charge?

Answer 11

Charge = current x time

when:
charge (Q) is measured in coulombs (C)
current (I) is measured in amps (A)
time (t) is measured in seconds (s)

Question 12

What is one amp?

Answer 12

One amp is the current that flows when one coulomb of charge passes a point in a circuit in one second.

Question 13

How is current measured?

Answer 13

Current is measured using an ammeter. To measure the current through a component, the ammeter must be placed in series with that component.

Question 14

How is potential difference measured?

Answer 14

To measure the potential difference across a component, a
Voltmeter must be placed in parallel with that component to measure the energy difference from one side of the component to the other.
Potential difference is also known as voltage and is measured in volts (V).

Question 15

What does a potential difference of 1 volt mean?

Answer 15

A potential difference of 1 volt means that 1 joule of work is done per coulomb of charge.

Question 16

What is potential difference and how is it calculated?

Answer 16

Potential difference (or voltage) is a measure of energy, per unit of charge, transferred between two points in a circuit.

V = E/Q

P.D(V) = Energy(J)/Charge(C)

or V = IR

P.D (V) = Current(A) x Resistance(Ω)

Question 17

What is 1 V

Answer 17

the potential difference when one coulomb of charge transfers one joule of energy.

Question 18

What is the relationship between conductors, insulators and resitance?

Answer 18

Conductors(Copper) have a low resistance. Insulators(wood) have a high resistance.

Question 19

What is an Ohmic Conductor? What is the relationship between voltage and current in an Ohmic conductor?

Answer 19

Ohmic conductors exhibit a linear relationship between the electric field (voltage) applied across them and the resulting current.
This relationship is described by Ohm’s Law: V=I×R, where V is voltage, I is current, and R is resistance.
Semiconductor:

Question 20

Explain the relationship between p.d and current in a semiconductor.

Answer 20

Semiconductors exhibit a non-linear relationship between voltage and current. Unlike ohmic conductors, their conductivity can be significantly altered by factors like temperature, impurities, and external conditions.

Question 21

What happens in a fixed resistor?

Answer 21

For a fixed resistor, the potential difference is directly proportional to the current.
Doubling the amount of energy into the resistor results in a current twice as fast running through the resistor.
This relationship is called Ohm’s Law and is true because the resistance of the resistor is fixed and does not change. A resistor is an ohmic conductor

Question 22

What is a filament bulb? Explain its working principle?

Answer 22

A filament bulb, also known as an incandescent bulb, is a type of electric light bulb that produces light by heating a wire filament until it emits visible light
When an electric current passes through the filament, it heats up due to its resistance to the flow of electricity.

Question 23

Explain the relationship between current and P.D in a filament bulb.

Answer 23

In a filament bulb, the current does not increase as fast as the potential difference.

Doubling the amount of energy does not cause a current twice as fast.

The more energy that is put into the bulb, the harder it is for the current to flow - the resistance of the bulb increases.

As the potential difference increases, so does the temperature of the thin wire inside the bulb, the filament.

The higher temperature increases the vibrations of the ions in the filament, which makes it harder for the electrons to get past.

Question 24

What is the graph of filament bulb?

Answer 24

A curved S shape when plotting current against voltage (P.D)

Question 25

What is the graph of an ohmic conductor when plotting Current against P.D (V)

Answer 25

Linear straight line graph

Question 26

How does a semi conductor diode behave?

Answer 26

A semiconductor diode only allows current to flow in one direction.
If the potential difference is arranged to try and push the current the wrong way (also called reverse-bias) no current will flow as the diode’s resistance remains very large.

Current will only flow if the diode is forward-biased.
The diode’s resistance is very large at low potential differences but at higher potential differences, the resistance quickly drops and current begins to flow.

Question 27

what does the current-voltage graph of a semi conductor diode look like ?

Answer 27

It remains flat on the x axis before becoming linear upwards.

Question 28

What is resistivity? Its equation?

Answer 28

Resistivity is a fundamental property of materials that describes how strongly a specific material opposes the flow of electric current.

It is denoted by the symbol ρ (rho) and is measured in ohm-meters (Ω⋅m) in the International System of Units (SI).

Resistivity is a fundamental property of a material and is independent of its shape and size.

The relationship between resistivity (ρ), resistance (R), and the dimensions of a conductor is given by the formula

p =RA/L

R is the resistance of the material (measured in ohms, Ω),
ρ is the resistivity of the material (measured in ohm-meters, Ω⋅m),
L is the length of the conductor (measured in meters, m),
A is the cross-sectional area of the conductor (measured in square meters, m^2).

Question 29

What is the equation for power loss?

Answer 29

P = I^2 x R
where:
P is the power loss,
I is the current flowing through the cable,
R is the resistance of the cable.

Question 30

How is ohm’s law useful?

Answer 30

Ohm’s Law is a fundamental tool in electrical engineering, used in the design and analysis of electronic circuits.

This formula states that the voltage across a resistor is equal to the product of the current flowing through it and its resistance. Ohm’s Law is particularly useful in understanding and analysing simple electrical circuits, especially those involving resistors.

Question 31

What is an electric circuit and what are its components?

Answer 31

An electric circuit is defined as a closed loop of conducting elements through which current can flow.

It consists of the following components:

Voltage or a Current source
Connecting wires
Elements like Resistors, Capacitors, Inductors, transistors, diodes, etc.

Question 32

What are the two types of arrangements in circuit elements?

Answer 32

Series and parallel

Question 33

When is a circuit a series circuti?

Answer 33

If all the elements of a circuit are arranged in such a way that the magnitude of current that flows through each element is equal to the total current in the circuit, then the circuit is said to be a series circuit.

Question 34

What makes a parallel circuit?

Answer 34

If the elements of a circuit are arranged in such a way that the magnitude of the current that flows through each element is not equal to the magnitude of the total current in the circuit, then the circuit is said to be a parallel circuit.

Question 35

What are the properties of a series circuit?

Answer 35

The same amount of current flows through each element.

The voltage drop across each element is not the same.

The total resistance of the circuit is the sum of the resistance of all the resistors.

If one of the elements fails, then the current will stop flowing in the whole circuit.

Question 36

What is a voltage drop?

Answer 36

Voltage drop refers to the reduction in voltage that occurs as electrical current flows through a conductor (such as a wire or a circuit).

It is a natural phenomenon that happens due to the inherent electrical resistance of the conductor.

Question 37

What are the properties of a parallel ciruit?

Answer 37

The total current gets divided in different proportions in all the branches of the circuit. The proportion of current in each branch is given by the ‘current divider rule’.

The voltage drop across all the parallel elements is same.

The total resistance of the circuit if given by the formula: 1/Total Resistance = 1/RBranch1 + 1/RBranch2 + …1/RBranchN

If any one element fails, the current stops flowing only in that particular branch of the circuit where the faulty element exists.

To say in simple words, if the current in a circuit is divided into branches then it is a parallel circuit otherwise a series circuit.

Question 38

What happens when an element stops working in a series circuit vs a parallel circuit?

Answer 38

In a series circuit if ant element stops working, then it breaks the entire circuit, whereas in case of parallel circuit if any element stops working then current won’t flow only through that particular branch. This is the reason why all the appliances on our homes are connected in parallel combination.

Question 39

What happens in a junction in a circuit?

Answer 39

The total current leaving the junction is equal to the total current entering the junction.

Question 40

What are the potential difference rules

Answer 40

The potential difference (pd), or voltage, between any two points in a circuit is
defined as the energy transfer per coulomb of charge that flows from one point to
the other.

• If the charge carriers lose energy. the potential difference is a potential drop.
• If the charge carriers gain energy, which happens when they pass through a battery or cell, the potential difference is a potential rise equal to the pd across
  the battery or cell’s terminals

Question 41

What is the relationship of potential difference across components in series?

Answer 41

For two or more components in series, the total pd across all the components is equal to the sum of the
potential differences across each component

the pd across the battery terminals is equal to the sum of the potential differences across the three
resistors. This is because the pd across each resistor is the energy delivered per coulomb of charge
to that resistor.

V0 = V1 + V2 + V3

Question 42

What is the relationship of potential difference across components in parallel?

Answer 42

total pd is equal to the pd in any branch at a given time.

Vt =V1 =V2= V3

Charge carriers can pass through either of the two resistors in parallel.
The same amount of energy is delivered by a charge carrier regardless of
which of the two resistors it passes through.

The variable resistor is adjusted so the pd across it is 4 V. If the battery pd
is 12 V, what the pd across each of the two resistors in parallel is?
8 V (= 12 V - 4 V)
This is because each coulomb of charge leaves the battery with 12J of
electrical energy and uses 4J on passing through the variable resistor.
Therefore, each coulomb of charge has 8J of electrical energy to deliver
to either of the two parallel resistors.

Question 43

How does conservation of energy relate to an electrical circuit?

Answer 43

For any complete loop of a circuit, the sum of the emfs around the
loop is equal to the sum of the potential drops around the loop.

The battery has an emf of 9 V. If the variable resistor is adjusted
so that the pd across the light bulb is 6 V, the pd across the
variable resistor is 3 V (= 9 V - 6 V).

Question 44

What is the relationship of resistors across components in series?

Answer 44

R = RI + R2 + R3 + …

Resistors in series pass the same current. The total pd is equal to the sum of the individual pds.

For two or more resistors R1, R2, R3, etc. in series, the theory can easily be extended to show that the total resistance is equal to the sum of the individual resistances.

Question 45

What is the relationship of resistors across components in parallel?

Answer 45

1/R = 1/R1 + 1/R2 + 1/R3….

Resistors in parallel have the same pd. The current through a parallel combination of
resistors is equal to the sum of the in dividual currents.

For two or more resistors 𝑅1, 𝑅2,𝑅3, etc. in parallel. the theory can easily be extended
to show that the total resistance R is given by

Question 46

Explain resistance heating and its equation

Answer 46

The heating effect of an electric current in any component is due to the resistance of
the component.
For a component of resistance R, when current I passes through it, the pd across the
component, V = IR.
Therefore, the power supplied to the component,

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

Hence the energy per second transferred to the component as thermal energy= 𝐼^2𝑅.

The rate of heat transfer = 𝑰^2 x R

Question 47

What is internal resistance?

Answer 47

The internal resistance of a source is the loss of potential difference per unit current in
the source when current passes through the source.
The internal resistance of a source may be shown as a resistor (labelled ‘internal
resistance’) in series with the usual symbol for a cell or battery.

Question 48

What is emf? How is it calculated?

Answer 48

The electromotive force (emf, symbol 𝜺) of the source is the electrical energy per unit charge
produced by the source. If electrical energy E is given to a charge Q in the source,

𝜺 =𝑬/𝑸

Question 49

What causes internal resistance?

Answer 49

The internal resistance of a source of electricity is due to opposition to the flow of charge
through the source. This causes electrical energy produced by the source to be dissipated
inside the source when charge flows through it.

Question 50

What is the relationship between emf, internal resistance and external resistor?

Answer 50

When a cell of emf 𝜀 and internal resistance r is connected to an external resistor of resistance R, all the current through the cell passes through its internal resistance and external resistor.

So, the two resistors are in series, which means that the total
resistance of the circuit is r + R. Therefore, the current through the cell, 𝐼 =𝜀/𝑅+𝑟

So, the emf 𝜀 =𝐼𝑅 + 𝐼𝑟

The lost pd inside the cell is equal to the difference between the cell emf and the pd across its terminals.

ln energy terms, the lost pd is the energy per coulomb dissipated or wasted inside the cell due to its internal resistance.

Question 51

How is power delivered to a resistor calculated?

Answer 51

Power supplied by the cell, 𝐼𝜀 =𝐼^2𝑅 + I^2𝑟

In other words, the power supplied by the cell = the power delivered to R + the power wasted in the cell due to its internal resistance.

The power delivered to R = 𝐼^2𝑅 =𝜀^2/(𝑅+𝑟)^2 𝑅

since 𝐼 =𝜀/𝑅+𝑟
The power delivered to R varies with the value of R

The peak of this power curve is at R = r.

maximum power is delivered to the load when the load resistance is equal to theinternal resistance of the source.

Question 52

How is internal resistance measured?

Answer 52

The pd across the terminals of a cell, can be measured by connecting a high-resistance voltmeter directly across the
terminals of the cell.

The measurements of terminal pd and current for a given cell can be plotted on a graph, The terminal pd decreases as the current increases. This is
because the lost pd increases as the current increases.

The terminal pd is equal to the cell emf at zero current

Question 53

What does a terminal pd- current graph look like?

Answer 53

The graph is a straight line with a negative gradient.

𝜀 = 𝐼𝑅 + 𝐼𝑟 by rearranging it 𝐼𝑅 = 𝜀 − 𝐼𝑟
then 𝑉 = 𝜀 − 𝐼𝑟

The equation for a straight line is 𝑦 = 𝑚𝑥 + 𝑐, a graph of 𝑉 on the y-axis against 𝐼 on the x-axis gives a straight line with a
gradient –𝑟 and a 𝑦 -intercept 𝜀.

Question 54

How is internal resistance calculated when the terminal pd is known for two different values of currents?

Answer 54

𝑟 = (𝑉1−𝑉2 )/ (𝐼2−𝐼1)

Question 55

What does kirchoff’s law state?

Answer 55

At any junction in a circuit, the total current entering the junction is equal to the total current leaving the junction.

For any complete loop in a circuit, the sum of the emfs around the loop is equal to the sum of the potential drops around the loo

Question 56

What is kirchoff’s law used for?

Answer 56

analyse any dc circuit, regardless of how many loops and cells are in the circuit.