Electricity Flashcards
Define Energy Efficiency
A measure of how usefully energy is used by a device
Percentage efficiency = ?
Efficiency x 100
What does the voltmeter measure in a closed circuit?
Potential difference
A potential difference of 1 volt transfers what?
1 Joule per coulomb of charge
Describe the symbol for an electric motor
A circle with an M in it
What does the voltmeter measure in an open circuit?
Emf
Describe the symbol for a cell
A big line and a small line with a small gap between them, with the wire passing perpendicularly through their centres
Describe the symbol for a resistor
A rectangle with its long edges parallel to the wires, which are entering through the centres of the short sides
What happens when a potential difference is applied across a metal?
The electrons are attracted towards the positive terminal and repelled by the negative terminal
What is the maximum efficiency possible for an object and why?
1.0 because the useful energy can never be greater than the energy supplier
Which way does conventional current flow?
From positive to negative
Which way is electron flow?
Negative to positive
What is electric current?
The flow of electric charge
What are the charge carriers in metals?
Negatively charged conduction electrons
What happens when the temperature of a semi-conductor is increased?
More charge carriers are produced and the semi conductor turns from an insulator to a conductor
Describe the symbol for a light dependent resistor (LDR)
A circle with a rectangle in it, with two arrows pointing from the top left of the circle to the rectangle.
What do charge carriers do inside a metal?
They move about, repeatedly colliding with each other and the fixed positive ions of the metal
What do charge carriers consist of in other conducting substances such as acids, low pressure gases and molten salt?
Both positive and negative ions
Good conductors have many what?
Free to move charge carriers
Insulators have few what?
Free to move charge carriers
Describe the symbol for a diode
A circle with an isosceles triangle and a straight line. The triangle’s longest edge vertical and parallel to the straight line. The straight line is at the point where the two short sides of the triangle meet.
Describe the symbol for a thermistor
A rectangle (with its longest sides horizontal) with a diagonal line going through it -and out either side- from bottom left to top right. There is a small horizontal line going right from the top end of the diagonal line.
Define resistance
Opposition to the flow of charge in a circuit
Define emf
The electrical energy produced per unit charge from the power supply
What are the two equations for efficiency?
Useful power output/power input
Useful work output/energy supplied
What is charge?
The product of the current flowing through a component and the time taken
What are charge carriers?
Charged particles that move through a substance when a pd is applied across it.
What is Kirchoff’s First Law?
- The current passing through two or more components in series is the same through each component.
- At a junction, the total current in = total current out
What is potential difference?
The work done per unit charge across a component.
What is Ohm’s Law?
V = I R
What is the equation for resistors in series?
R(total) = R1 + R2 + R3+…
Define current?
The rate of flow of charge around a circuit due to the movement of electrons.
What is one of Kirchoff’s Laws relating emf and p.d?
The total emf = the total potential difference in a closed loop.
What is the equation for resistors in parallel?
1/R(total) = 1/R1 + 1/R2 + 1/R3 +…
What is the potential divider equation?
V2= V0 x (R2/(R1 + R2))
What does the I/V graph look like for a diode in forward bias?
Current starts at 0 or just above 0 until a threshold voltage is reached.
Then there is a rapid increase of I for a small increase in V.
What does the I/V graph look like for a diode in reverse bias?
The current starts at zero or just below 0 until voltage reaches a particular value (between -50 and -500V).
Then there is a sharp downward curve.
What is the threshold voltage for a silicon diode?
About 0.7V
Explain the I/V curve of a filament lamp.
At low voltages, current increases proportionally- Ohm’s Law is obeyed.
As voltage increases, greater currents heat the filament wire.
This causes resistance to increase.
The rate of increase of I with V therefore decreases and the graph becomes shallower. (The gradient is I/R so it decreases).
The same shape happens but in the negative quadrant.
What is an equation for potential difference involving energy or work done?
V = W / Q
or
V= E / Q
When solving equations to do with readings on voltmeters, what is usually the assumption made?
That there is no current through the voltmeter and that it has a very high/infinite resistance.
What graph should be plotted when determining the resistivity of a wire (if A is known and L and R are measured)?
R on the y axis and l on the x axis.
What is the unit for resistivity?
Ohm meter
When a battery is connected to a component, such as a bulb, and the potential difference is measured across the component, why is this measurement lower than the emf of the battery?
Because the battery has internal resistance and part of the emf is the voltage across the internal resistance.
When components are in a series circuit what is the relationship between emf of the battery and potential difference across each component? (Ignoring internal resistance)
Emf = sum of potential differences across each component
When components are in parallel what is the relationship between the potential differences across them?
Potential difference is the same for all of them
What is a potential divider?
A combination of resistors in series connected across a voltage source to produce a required pd.
What happens when a cell is placed in reverse when in series with other cells?
It’s voltage has to be taken away rather than added.
What happens when current enters a junction where there are two resistors in parallel?
The current is split in the ratio of the resistors, but with the highest resistor getting the least amount of current.
State what is meant by superconductivity.
A material has zero resistivity
What is required for a material to become a superconductor?
It must be cooled down enough to get below the transition temperature.
Define internal resistance of a battery.
The resistance of the materials within the battery
How does increasing the temperature of a thermistor affect the resistance?
Decreases resistance.
