Mod 4 : Electrical Circuits

Question 1

State Kirchhoff 2nd law

Answer 1

The sum of emf is equal to the sum of p.d in a closed loop

Question 2

Define internal resistance

Answer 2

The resistance of a cell due to the chemicals within it

Question 3

Define emf

Answer 3

The energy transferred per unit charge from the power supply to the charge carriers

Question 4

Define terminal pd

Answer 4

The p.d measured across the terminals of the power supply

Question 5

Define a potential divider

Answer 5

A circuit that gives an output pd that is some fraction of the fixed input emf

Question 6

What is a potentiometer

Answer 6

A variable resistor with an adjustable slider, the output p.d across this slider will be some fraction of the total p.d access the ends of the variable resistor

Question 7

Why does internal resistance occur

Answer 7

Due to the chemical energy used to make electrons move inside the battery, however these electrons collide with the atoms inside the battery and lose energy.

Question 8

What causes a battery to warm up when in use

Answer 8

It’s internal resistance

Question 9

What is the little v and the little r in internal resistance stand for

Answer 9

Lost volts - v
Internal resistance- r

Question 10

Why is there lost volts

Answer 10

Due to the internal resistance of the battery the terminal p.d will be less than the emf

Question 11

Why may the terminal p.d decrease when current is increased in a circuit

Answer 11

When current is increased, charge flowing through the cell each second is also increased therefore more work is done by the charge carriers increasing the number of lost volts. This lowers the terminal p.d

Question 12

Compare the emf across connecting cells in series vs parallels

Answer 12

Connecting in series will increase emf however in parallel the emf will be the same as 1 cell as p.d is the same when injected in parallel

Question 13

A battery is connected to an NTC thermistor which is connected 1 resistor in series and 2 resistors in parallel .What happens to the p.d accross an NTC thermistor when it’s temp is decreased

Answer 13

The temp of the NTC is decreased therefore its resistance is increased due to its negative coefficient behaviour. As this is a parallel circuit, increasing the resistance would decrease the overall resistance in the circuit. Therefore current increases and due to ohms law p.d would also increase

Question 14

How would you connect 3 resistors in a circuit so that the circuit dissipates the least power and why

Answer 14

Connect them. In series because this means there is a decrease in current therefore less power can be dissipated

Question 15

What happens to the p.d across the internal resistance of a power supply when the e.m.f is constant, and a variable resistor in the circuit is at max resistance

Answer 15

The increase of resistance leads to a decrease in current through he variable resistor. Therefore the internal resistor will also experience the same current, this means less current is drawn from the power supply therefore the p.d across the internal resistance decreases as less voltage is dropped across the internal resistance.

Explanation- increasing resistance makes it harder for electricity to flow therefore the power supply does less work to push electricity, the internal resistance also experiences this.decreased push as a result it gets less of the power supplies voltage.

Question 16

Why may terminal p.d increase when an external resistance is increased

Answer 16

Because there is a larger portion of emf that is available as less current flows through the internal resistance therefore less voltage is lost

Question 17

I1 and I2 are entering a junction. The current flowing out of the junction is I3
State the relationship between the three currents in terms of charge

Answer 17

I3= I2+ I1
A charge is conserved over the same time period

Question 18

A potential difference is applied across the metal filament of a light bulb and charge flows. By referring to mean drift velocity of the electrons, explain what happen to the current in the metal filament if the p.d is unchanged and the temp of the metal increases.

Answer 18

The ions in the metal filament vibrate more resulting in more frequent collisions between electrons and lattice ions. Therefore mean drift velocity decreases and current also decreases

Question 19

2 conductors of the same material and length carry the same current. Conductor X has twice the cross sectional area of conductor Y. The drift velocity in X is half the drift velocity in Y. Explain the difference in resistance of conductor x and Y in terms of the difference in drift velocity

Answer 19

the resistance in Y would be grater because the current is the same yet the velocity is greater in Y. Therefore thee would be more collisions between electrons and ions meaning a greater p.d must be applied across Y to get the same current as X

Question 20

A voltmeter with a resistance of 5R i connected in parallel with a resistor of resistance R. The voltmeter reading is lees than the actual value of the p.d accross the resistor. Explain why and what device can be used instead

Answer 20

The total resistance of the voltmeter is less than R as the resistance in parallel circuits is less than the smallest value of resistance. This means there is volts that are lost. Use a datalogger instead

Question 21

A student connects a calibrated 10 ohm resistor accros a chemical cell of emf 150V. The student expects the current in the resistor to be 0.15A. Why is it not

Answer 21

The current will be less as the cell has internal resistance

Question 22

A voltmeter and lamp are connected in series to a cell. A resistor is then connected in parallel. Explain why the lamp will not turn on even though it is not faulty and how can you rectify this

Answer 22

The voltmeter has infinite R therefore the p.d accross the lamp would be 0 so it will not carry charge and light up. To fix this add the voltmeter in parallel to the lamp

Question 23

A student conduct an experiment using two identical filament lamps and a variable power supply of negiligabele resistance. The lamps are connected in series o the supply. the emf is then doubled, explain why the current in the lamps do not double

Answer 23

Lamps are non ohmic conductors therefore the resistance increases with current and temperature so the current is slowed down

Question 24

A potential divider circuit is set up so that a fixed resistor and an LDR are connected in series. A voltmeter is connected in parallel to the LDR. What will happen to the voltmeter reading when the light intensity decreasest

Answer 24

The resistance of the LDR will increases, therefore the resistance in the circuit will decrease.
This means the current in the circuit will also decrease.
Therefore as the fixed resistor has a constant resistance, the p.d accross the fixed resistor will decrease as V=IR. This means the LDR will get a larger portion of the p.d as both p.ds will have to add up to make up the e.m.f (kirchioff’s 2nd rule

Question 25

A student connects a variable resistor in series with a lamp and a cell. A voltmeter is added in parallel to the lamp. The maximum resistance of the variable resistor is 60 ohms. Explain why this circuit will only provide large values for the voltage accross the lamp

Answer 25

When the variable resistor (VR) is close to zero, the p.d accross the VR is very small there’re the lamp will receive a larger portion of the p.d (close to the total e.m.f).
When the vR is at its maximum, the current in the circuit will decrease and as V=IR the p.d accross the VR will also decrease, therefore once again the lamp will have a larger portion of the e.m.f

Question 26

A student is given a chemical cell, an ammeter, a voltmeter, a variable resistor and connecting wires. What experiment can be used to determine the internal;l resistance of the cell using a graph

Answer 26

Connect a variable resistor in series with an ammeter and a cell. Connect a voltmeter in parallel to the VR. Measure the p.d accross the VR and the current in the VR for various different resistances. Then plot a graph of V against I. Using the equation : E= V + Ir, rearrange to V= -Ir + E, therefore the gradient is -r