Electrical Current

Question 1

Describe Current, Voltage and Resistance in a Series Circuit

Answer 1

• Current - same everywhere in the circuit
• Voltage - shared across all components in the circuit
• Resistance - shared across all components in the circuit

Question 2

Describe Current, Voltage and Resistance in a Parallel Circuit

Answer 2

Current - shared across each branch in the circuit

Voltage - same across each branch in the circuit

Resistance - = 1 / RT = 1 / R + 1 / R + 1 / R …

Question 3

What is Power

Answer 3

The energy transferred per second

Question 4

What are the Power Equations

Answer 4

• P = IV

Power (W) = Current(A) x Voltage(V)

• P = I² x R

Power (W)= Current² x Resistance (Ω)

• P = V² / R

Voltage² (V) / Resistance(Ω)

Question 5

What is the Energy Equation

Answer 5

E = I x V x t

Energy (J) = Current(A) x Voltage(V) x Time (s)

Question 6

What is always conserved in a DC (Direct Current) circuit

Answer 6

Charge and energy are always conserved

Question 7

What are Kirchoff’s Laws and how do they show that charge and energy are always conserved

Answer 7

• Kirchoff’s first law - the total current flowing into a junction is equal to the current flowing out of that junction. This shows that no charge is lost at any point in the circuit
• Kirchoff’s second law - the sum of all the voltages in a series circuit is equal to the battery voltage. This shows that no energy is lost at any point in a circuit

Question 8

What is Electromotive Force (e.m.f)

Answer 8

The amount of energy supplied by the source per unit charge

Question 9

Electromotive Force Equations

Answer 9

• E = E / Q

Electromotive Force (V) = Energy Supplied (J) / Charge (C)

• E = I R + I r

Electromotive Force(V) = Current(A) x Resistance(Ω) + Current(A) x Internal Resistance(Ω)

Question 10

What is load resistance

Answer 10

The total resistance of all the components in a circuit except the battery

Question 11

What is internal resistance

Answer 11

The resistance of the materials within the battery or the limit of the flow of charge in the battery

Question 12

What causes internal resistance

Answer 12

Internal Resistance caused by electrons colliding with atoms inside the battery, therefore some energy is lost before electrons even leave the battery

Question 13

How is internal resistance represented in a circuit diagram

Answer 13

A small resistor inside the battery

Question 14

What are the benefit of having a battery with low internal resistance

Answer 14

• The battery can provide a higher currnet because internal resistance limits current
• Less energy wasted in battery
• Charges quicker as current is higher and less energy wasted

Question 15

What is Terminal Potential Difference

Answer 15

The potential difference across the load resistance (R).

i.e. The energy transferred per unit charge flowing through the load resistance

Question 16

What are Lost Volts

Answer 16

The potential difference across the internal resistance

i.e. the energy wasted by the cell per coulomb of charge

Question 17

What happens in a electromotive circuit when the load resistor decreases

Answer 17

The current will increase and the terminal potential difference will decrease

Question 18

Describe an experiment to calculate the e.m.f. and internal resistance of a cell

Answer 18

• Connect the cell in series with an ammeter and variable resistor + connect a voltmeter across the cell
• Vary the current using the variable resistor.
• Record the voltage at each current
• Plot a graph of voltage against current
• y-intercept = electromotive force
• Gradient = internal resistance

Question 19

Describe the Electromotive Force Circuit

Answer 19

Question 20

Describe and Explain the Terminal Potential Difference Graph

Answer 20

• Y-intercept - Electromotive Force
• X-intercept - The current when the cell is short circuited
• Gradient - Internal Resistance

Question 21

Explain why terminal voltage decreases as current increase

Answer 21

• As current increases the energy lost due to internal resistance would increase
• and as current increases the lost volts increase
• Therefore decreasing the terminal voltage

Question 22

When connecting a high-resistance voltmeter across a cell to find its electromotive what is the error in the results

Answer 22

• Small amount of current flows through the voltmeter
• So there are some lost volts
• Measured value is very slightly less than the e.m.f.
• But this is negligible

Question 23

Explain what is meant by a potential divider

Answer 23

• A combination of resistors in series connected across a voltage source
• To produce a required pd

Question 24

What is the purpose of a potential divider

Answer 24

To provide a constant specific potential difference

Question 25

How does the resistance of a light-dependent resistor vary

Answer 25

Resistance decreases as light intensity increases

Question 26

How does the resistance of a NTC thermistor vary

Answer 26

Resistance decreases as temperature increases

Question 27

Give a use of LDRs and thermistors

Answer 27

A light sensor or temperature sensor, so they can be included in circuits that control switches, e.g. to turn on a light or heating system

Question 28

Describe a method to see how the resistance across a thermistor varies with temperature

Answer 28

• Place thermistor into water bath
• Use a thermomter to measure the inital temperature
• Measure the voltage and current at 10 degrees intervals from 0 - 100 degress using voltmeter and ammeter
• Calculate resistance using R = V/I
• Plot graph of Resistance against Temperature