Chapter 5: Electricity Flashcards
What is electric current? State its units.
The rate of flow of charge. It is measured in amperes (amps)
What is potential difference?
The work done moving a unit charge between 2 points in a circuit.
V=W/Q
What is resistance?
How difficult it is for current to flow through an appliance.
A component has a resistance of 1Ω if 1A flows through it when p.d. of 1V is applied across it.
R=V/I
What is meant by an ohmic conductor?
A conductor that obeys Ohm’s law, meaning that current is directly proportional to potential difference providing physical conditions (such as temperature) remain constant.
How can you measure the current in a circuit?
You can measure the current in an ammeter connected in series with the component.
How do you measure potential difference across a component.
Using a voltmeter, connected in parallel across the component being measured.
What does the gradient of a current-potential difference graph represent?
Rate of charge of current with respect to voltage.
This is not the same as 1/R
Why does the current increasing on a filament lamp cause an increase in the resistance?
As current flows through the lamp, electrical energy is converted to heat energy so the metal ions vibrate increased amplitude. This impedes the movement of electrons through the lamp as they collide with the ions (resistance has increased).
What is a diode?
A diode is an appliance that only allows current to flow in one direction.
What is a Light Dependent Resistor (LDR)?
A semiconductor that is sensitive to light.
As the light intensity increases, its resistance decreases.
How does a thermistor work?
Similar to an LDR but, as the temperature increases, the resistance decreases (This means thermistors have a negative temperature coefficient).
What is resistivity?
The resistance of a 1m cylinder with a cross sectional area of 1m^2. Resistivity is an inherent property of a material.
ρ=RA/L
Describe an experiment to determine the resistivity of a metal.
- Measure the diameter of the wire with a micrometer. Then calculate the cross-sectional area
- Set up the circuit as shown.
- Vary the wire length and record the voltage and current for each length.
- Use the R=V/I to work out the resistance
- Plot a graph of resistance against wire length
- The gradient = resistivity / cross-sectional area
- So resistivity = gradient x cross-sectional area
What is a superconductor?
A material that has a resistivity of zero at or below a critical temperature. The critical temperature is an inherent property of the material.
How do you find the total resistance in a series circuit?
Rtotal = R1 + R2 + R3 + …
If 6 cells, each of voltage 5V, are arranged in parallel what is the voltage in the circuit?
5V
If 6 cells, each of 5V, are arranged in series what voltage is provided to the circuit?
30V
How does the current vary between each component of a series circuit?
The current through all of the component is the same so the current does not vary.
Is the current in parallel components the same?
No, each branch of a parallel circuit can have different currents through them according to Kirchhoff’s first law.
What is Kirchhoff’s first law?
All of the current going into a junction is equal to the current leaving the junction.
What is Kirchhoff’s second law?
For any path (loop) of a circuit, the sum of all of the potential differences must be equal to total emf of the circuit.
What is power?
The rate of energy transfer.
Give an equation for power in terms of current and voltage.
P=IV
What is the purpose of a potential divider?
- to provide variable p.d., or
- to provide a constant specific potential difference
A circuit is set up with a cell providing a voltage of 12V to 2 resistors of 6 Ω and 7Ω respectively, in series. What is the voltage across the 7Ω resistor?
Total resistance = 6+7 = 13Ω
(7/13) x 12 = 6.5V
What is emf?
Electromotive force: the electrical energy transferred by a power supply per unit charge.
Rearrange the equation ε = I(R+r) into the form y=mx+c
ε = IR + Ir
ε = V + Ir
So for a graph of V against I
V = -rI + ε
Gradients = -r and y-intercept = ε
What are two applications of superconductors?
- power cables which would reduce energy loss through heating to zero during transmission.
- strong magnetic fields, which would not require a constant power source. These could be used in maglev trains, where there would be no friction between the train and rail and in certain medical applications.
What is the total resistance of a parallel arrangement of 3Ω, 7Ω and 9Ω resistors?
1/Rt = 1/3 + 1/7 + 1/9
1/Rt = 37/63
Rt = 63/37 = 1.7Ω
