Section 6 - Electricity Flashcards
Define current
Rate of flow of charge
What is the unit of current?
A - Ampere
What is the unit of charge?
C - Coulomb
Define coulomb
The amount of charge that passes in 1 second of the current is 1 ampere
How do you measure current, and how do you connect the component?
With an ammeter in series
Define potential difference
Work done per unit charge moved
How do you measure voltage, and how do you connect the component?
With a voltmeter in parallel
Define a volt
When one joule of energy is moving 1 coulomb of charge through a component
Define resistance
A potential difference of 1 volt making a current of 1 amp generates 1 Ohm, R = V / I
State Ohm’s law
Provided the physical conditions are kept constant, the current through an ohmic conductor is directly proportional to the p.d across it
What do I/V graphs show in terms of resistance?
How it varies
On an I/V graph for a resistor, what does a steep and shallow gradient represent?
Steep means low resistance
Shallow means steep resistance
On an V/I graph for a resistor, what does a steep and shallow gradient represent?
Steep means high resistance
Shallow means low resistance
Describe the curve for a filament lamp on an I/V and V/I graph
I/V - Starts shallow but gets steeper curve. Only touches the origin type cubic
V/I - Starts steep and gets shallower. Point of inflexion type cubic as stays at origin for a bit
What is a filament?
A coiled up length of wire
State an application for semi conductors
Sensors, diodes and thermistors
Why are semiconductors good sensors?
When energy is supplied, more charge carriers are released
What does the resistance of a thermistor depend on?
Temperature
What happens as the temperature of a thermistor increases?
Resistance decreases
Describe the resistance-temperature graph for a thermistor
Top left to bottom right inward curve
Describe the I/V and V/I graph for a thermistor
I/V - Starts steep and gets shallower
V/I - Starts shallow and gets steeper
OPPOSITE TO FILAMENT LAMP
What are diodes designed to do?
Let current flow in 1 direction
Describe the I/V and V/I graph for a diode
I/V - At 0 until about 0.6V then sharp spike
V/I - Sharp increase and then steady upward sloping line
What 3 things determine resistance?
Length - Longer the wire, the lower the current
Area - The wider the wire, the larger the current
Resistivity - Depends on material
Define resistivity
The resistance of a 1m wire with a 1m^2 of cross sectional area
What is the unit of resistivity?
Ohm meters
Describe the process to find the resistivity of a wire
- Use micrometer to measure diameter in multiple places
- Take an average and find area of wire
- Test wire should be clamped where the ruler reads 0
- Attach the flying lead to the clamped wire and record the length of the wire
- Record the voltmeter and ammeter reading and calculate resistance
- Repeat and draw a graph. Multiply the area by the gradient = resistivity
State uses of superconductors
- Power cables transmit electricity with no loss of power
- Strong electromagnets don’t need a constant power source
- Electronic circuits work really fast
Define power
The rate of transfer of energy
State the extra power equations you need to know
P = I^2R P = V^2 / R
State the energy equation you need to know
E = IVt
Where does resistance come from?
Electrons colliding with atoms and losing energy to other forms
What makes electrons move in a battery?
Chemical energy
What is emf?
The amount of electrical energy the battery produces for each coulomb of charge
What is the terminal p.d?
The energy transferred when 1 coulomb of charge flows through the load resistance
What are lost volts?
Energy wasted per coulomb overcoming the internal resistance
What is the terminal p.d if there is no internal resistance?
The same as the emf
How can you calculate the total emf in a series circuit?
By adding the their individual emfs
How can you calculate the total emf in a parallel circuit?
The total emf of the cells is the same size as the emf of each of the individual cells
Does charge get used up in a circuit?
No
State Kirchhoff’s first law
Total current entering a junction = total current leaving it
State Kirchhoff’s second law
Total emf around a series circuit = sum of pds across each component
What does a potential divider do?
It splits the pd across the voltage source into a ratio of resistances
What is the use for a potential divider?
Supply a pd between 0 and the pd across the voltage source
What is a use for a variable resistor?
To vary voltage
What is the use of a potentiometer?
Using a variable resistor to give a variable voltage
