Electrical Current Flashcards
Describe Current, Voltage and Resistance in a Series Circuit
- Current - same everywhere in the circuit
- Voltage - shared across all components in the circuit
- Resistance - shared across all components in the circuit
Describe Current, Voltage and Resistance in a Parallel Circuit
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 …
What is Power
The energy transferred per second
What are the Power Equations
- 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)
What is the Energy Equation
E = I x V x t
Energy (J) = Current(A) x Voltage(V) x Time (s)
What is always conserved in a DC (Direct Current) circuit
Charge and energy are always conserved
What are Kirchoff’s Laws and how do they show that charge and energy are always conserved
- 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
What is Electromotive Force (e.m.f)
The amount of energy supplied by the source per unit charge
Electromotive Force Equations
- 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)
What is load resistance
The total resistance of all the components in a circuit except the battery
What is internal resistance
The resistance of the materials within the battery or the limit of the flow of charge in the battery
What causes internal resistance
Internal Resistance caused by electrons colliding with atoms inside the battery, therefore some energy is lost before electrons even leave the battery
How is internal resistance represented in a circuit diagram
A small resistor inside the battery
What are the benefit of having a battery with low internal resistance
- 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
What is Terminal Potential Difference
The potential difference across the load resistance (R).
i.e. The energy transferred per unit charge flowing through the load resistance
What are Lost Volts
The potential difference across the internal resistance
i.e. the energy wasted by the cell per coulomb of charge
What happens in a electromotive circuit when the load resistor decreases
The current will increase and the terminal potential difference will decrease
Describe an experiment to calculate the e.m.f. and internal resistance of a cell
- 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
Describe the Electromotive Force Circuit
Describe and Explain the Terminal Potential Difference Graph
- Y-intercept - Electromotive Force
- X-intercept - The current when the cell is short circuited
- Gradient - Internal Resistance
Explain why terminal voltage decreases as current increase
- 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
When connecting a high-resistance voltmeter across a cell to find its electromotive what is the error in the results
- 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
Explain what is meant by a potential divider
- A combination of resistors in series connected across a voltage source
- To produce a required pd
What is the purpose of a potential divider
To provide a constant specific potential difference
How does the resistance of a light-dependent resistor vary
Resistance decreases as light intensity increases
How does the resistance of a NTC thermistor vary
Resistance decreases as temperature increases
Give a use of LDRs and thermistors
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
Describe a method to see how the resistance across a thermistor varies with temperature
- 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
