CH9 Flashcards
What is potential difference (or voltage)
It is the measure of the transfer of energy by charge carriers. It is defined as the energy transferred from electrical energy to other forms (heat, light etc)
What is the relationship between voltage and electric charges?
Voltage is the potential energy difference that causes electric charges to move, while electric current is the flow of electric charges themselves. Voltage provides the push for the current to flow, similar to a difference in water pressure causes water to flow through a pipe
How is potential difference measured
In Volts
How is 1 Volt defined?
1 volt is the PD across a component when 1J of energy is transferred per unit charge passing through the component.
1 V = 1 J C-1
What does a PD of 1000 V mean?
That 1000 J of energy is transferred per coloumb of charge
What is the equation for PD?
V = W/Q
where V is p.d. measured in Volts, Q is charge is coloumbs and W is the energy transferred by charge Q
What is used to measure p.d.?
A voltmeter
How is a voltmeter connected across a particular component?
In parallel
What is meant by Electromotive Force?
It is the energy transferred from chemical energy (or another form) to electrical energy per unit charge.
Despite the name it is not a force. It is used to describe when work is done on the charge carriers. Essentially the charges are gaining energy as they pass through a component like a cell, battery etc
What is the equation for Electromotive Force?
E = W/Q
where e is the e.m.f. measured in volts, Q is the charge in coulombs and W is the energy transferred by charge Q.
What is the key difference between e.m.f. and voltage?
e.m.f is the total energy provided by a source, whereas voltage is the energy difference between two specific points in a circuit
Formula to calculate energy transfer
W = VQ or W = EQ
What is resistance?
It is opposition to the flow of electric current. The higher the resistance of a component the more energy is needed to push electrons though it.
What is the formula of Resistance?
R = V/I
where V is the pd. across the component and I is the current in the component
What is the Unit of measure of resistance?
Ohm (symbol is Greek letter Omega)
How is Ohm defined?
Ohm = 1VA-1
A component with resistance of 1 Ohm will have a p.d. of 1 V per ampere of current in it
What does Ohm Law say?
For a metallic conductor kept at constant temperature the current in the wire is directly proportional to the p.d. across its ends.
So when p.d. doubles he current also doubles.
Ohm law mentions that temperature in the wire must stay constant. What happens when temperature increases?
When temperature in the wire increases the positive ions inside the wire have more internal energy and vibrate with greater amplitude about their mean positions. The frequency of the collisions between the charge carriers (free electrons in the metal) and the positive ions increases and so the charge carriers do more work, i.e. transfer more energy as they move through the wire
How does emf differ across components in a parallel and series circuit?
A series circuit has only one path for the current. From Kirchoff First Law we know that the rate of flow of charge is the same at all points in the circuit. The charge is not used up.
A parallel circuit provides more than one path for charge. K. first law tells us that the current into each junction must be equal to the current out of the junction.
What are the I-V characteristics for a diode?
- The p.d. across a diode (or LED) is not directly proportional to the current through it. This means that:
1. a diode does not obey Ohm’s Law
2. the resistance in the diode is not constant - the diode’s behaviour depends on the polarity
What is the relationship between the resistance R of a wire and its lenght?
R is directly proportional to L
What is the relationship between the resistance R of a wire cross section?
Inversely proportional
What happens to the Resistance in a thermistor when the temperature increases and why?
If T increases R decreases. This is because in this semiconductor an increase in T causes an increase in the number density of charge carriers
Two different components made of for eg copper have the same resistance and resistivity?
Same resistivity but not resistance. The last one varies based on other factors as well.
In what region in the resistivity of a conductor, semiconductor and insulator?
Conductor (10)-8
Semiconductor (10)-5
Insulator (10)16
How is the resistance in a resistor?
Constant
In a resistor what is the relation between pd and current?
Directly proportional
What happens to the resistance in a filament when pd increases? Why?
The R increases as well. This is because:
As the current increases also the rate of flow of charges increases. So more collisions happen between electrons and ions. When this collision happens, energy is transferred to the ion, which makes them vibrate more
In a filament lamp what is the relation between pd and current?
Not directly proportional
The behaviour of which component depends on polarity: resistor, filament lamp, diode or thermistor?
Diode
What is the distinctive feature of a diode?
The diode’s distinctive feature is that it conducts current in one direction, but not the other.
What is meant by threshold pd in a diode?
With small positive voltage, almost no current flows. The resistance in the diode is infinite. Then, as the p.d. increases, R starts to drop. This point is called threshold pd. above the threshold pd the R drops sharply for every small increase in pd
What is and LDR?
Light dependent resistor
In a LDR what is the relation between light and Resistance
Dark: Resistance is very high
Light: the number density increases and R decreases
What is the definition of electrical power?
Rate of energy transfer by each electrical component
Equation for electrical power - list all 3 equations
- P = V * I
where V is p.d. in Volts and I is current in Ampere - P = (I)2 * R
- P = (V)2/R
Unit of measure of Electrical Power
Watts
How can you determine the energy transferred in a given time?
W=VIt
where
W=energy transferred in J
V=pd in Volts
I= current in A
t=time in seconds
What is an electricity meter?
It is a device that every house needs to have by law and it records the transfer of energy from the National Grid to the house
What is the general equation for power?
P=energy transferred/ time taken
P=W/t
What is the equation for the energy transferred by an electrical device?
W=P*t
What is the SI for power?
Joule
Why do energy bills use a derived unit for Power and what is it?
Because Joule is too small. They use Kilowatthour (kWh)
Define kWh
energy transferred by a device with a power on 1kW operating for one hour
To how many J is 1kWh equal to?
3.6 MJ
What is the tipical cost of energy?
6-15 pence per kWh
