10 - Electrical Circuits

Question 1

State Kirchhoff’s first law

Answer 1

At any junction in a circuit, the total current leaving the junction is equal to the total current entering the junction (ΣI(in)=ΣI(out)).

Question 2

State the physical quantity that is conserved in Kirchhoff’s first law

Answer 2

Charge

Question 3

State Kirchhoff’s Second law

Answer 3

In any circuit, the sum of the electromotive force is equal to the sum of the potential differences around a closed loop. Σε=ΣV in a closed loop.

Question 4

State the physical quantity that is conserved in Kirchhoff’s second law

Answer 4

Energy

Question 5

How is the total resistance found in a series circuit?

Answer 5

Rtotal = R1 + R2 +R3

Question 6

How is the total resistance found in a parallel circuit?

Answer 6

1/Rtotal = 1/R1 + 1/R2 + 1/R3. Then reciprocate the answer. The total resistance must be lower than the lowest resistance.

Question 7

Internal resistance equation

Answer 7

ε = Ir + V
emf = lost volts + terminal pd
ε = Ir + IR
ε = I(R+r)

Question 8

What is terminal p.d?

Answer 8

The amount of energy available to the rest of the circuit (electrical to other forms) per unit charge. The e.m.f supplied to the cell, minus the energy lost in the cell as heat.

Question 9

What is lost volts?

Answer 9

The energy lost in the cell as heat.

Question 10

What is internal resistance?

Answer 10

The internal resistor in a cell causing some energy to be transferred into thermal energy / lost as heat in when a current is drawn through.

Question 11

Interpret the y-intercept and gradient of a V-I graph in relation to internal resistance

Answer 11

V= -rI + ε
The y-intercept represents the emf and the gradient is negative internal resistance.

Question 12

Explain why replacing a car battery with many individual cells won’t be able to turn the motor of the car on

Answer 12

Car batteries have a very low internal resistance in order to provide a very high current to turn the starter motor of a car on. Using many individual cells won’t be sufficient enough to be able to turn the starter motor on as cells have a very high internal resistance therefore the current is not high enough.

Question 13

What is a potential divider?

Answer 13

A potential divider is a simple circuit that uses resistors (or thermistors/LDRs) to supply a variable output potential difference

Question 14

Potential divider equation (Vout =)

Answer 14

V.out = R2 / (R1+R2) x V.in

Question 15

How is pd shared between resistors?

Answer 15

The p.d. across each resistor in a potential divider depends on their resistances. If they have the same resistance, then the p.d. is shared equally. If one has twice the resistance of the other, then this one will receive two-thirds of the total p.d. (V1/V2=R1/R2)

Question 16

How does loading a potential divider change V.out?

Answer 16

Loading refers to connecting a component or circuit to V.out, placing a component in parallel with R2. This lowers the resistance of this part of the circuit and so lowers V.out. Adding a large load has little effect on the V.out but if the load has small resistance, V.out is significantly reduced.

Question 17

What is a sensing circuit?

Answer 17

A Sensing circuit is a potential divider circuit with a variable V.out. Replacing one of the fixed resistors with a variable resistor, thermistor or LDR, allows the V.out to be varied.

Question 18

Changing position of V.out in sensing circuits

Answer 18

The position of V.out can be changed depending on the desired outcome as a result of the changed conditions in an environment e.g. in a light-sensing circuit, putting the V.out across the LDR, means that when it is dark, the resistance of the LDR will be higher and so receives a greater proportion of the p.d., resistor receives lower proportion of pd, meaning V.out is increased which could mean a street lamp turns on when it is dark.

Question 19

Sensing circuits with variable resistors

Answer 19

Increasing the resistance of the variable resistor, increases V.out when V.out is across the variable resistor.

Question 20

light-sensing circuits (Sensing circuits with LDRs)

Answer 20

An LDR can be used in place of a fixed resistor to produce a potential divider that gives an output dependent on the light intensity. As the light intensity increases, the resistance of the LDR falls and so the p.d. across it decreases. The fixed resistor receives a greater proportion of the p.d. and so V.out increases if V.out is across the fixed resistor.

Question 21

How is a potentiometer built?

Answer 21

A Potentiometer works in the same way as a potential divider but rather than using two resistors in series, it uses a length of resistance wire with a sliding

Question 22

How does a potentiometer work?

Answer 22

When the contact is moved towards towards A, V.out increases, until at A it is equal to V.out. Wen the contact is moved towards B, V.out decreases until at B it is zero. Potentiometers can be made compact so are useful for portable electronic devices.

Question 23

Advantage of Potentiometer?

Answer 23

A potentiometer can also be constructed so that the change in resistance can be linear or logarithmic.