Current Electricity Flashcards
What is electric current? State it’s units
•The rate of flow of charge
•It is measured in amperes
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
• 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 physics conditions (such as temperature) remain constant
How can you measure the current in a circuit?
By connecting an ammeter in series with the component
How do you measure potential difference across a component?
By connecting a voltmeter in parallel across the component
What does the gradient of a current-potential difference graph represent?
Rate of change of current with respect to voltage.
Why does the current increasing on a filament lamp cause an increase in the resistance?
•Current flows through the lamp
•Electrical energy is converted to heat energy so the metal ions vibrate with increasing amplitude
•this obstructs the movement of electrons as they collide with the ions
What is a diode?
A diode is a appliance that only allows current to flow in one direction
Should you assume voltmeters have zero resistance or infinite resistance?
You should assume they have infinite resistance as current will not flow through it
Why should you assume that an ammeter has zero resistance unless stated otherwise?
This means that there would be 0 potential difference across the ammeter and no energy is lost across it. It does not affect the circuit.
What is a Light Dependent Resistor (LDR)?
•A semiconductor that is sensitive to light
•As the light intensity increases, it’s resistance decreases
How does a thermistor work?
•As the temperature increases, the resistance decreases.
•This means thermistors have a negative temperature coefficient
What is resistivity?
•A property that describes the extent to which a material opposes the flow of electric current through it
•p = RA / L
Describe an experiment to determine the resistivity of a metal
- Measure the diameter of the wire with a micrometer. Calculate the cross-sectional area with πd^2 / 4
- Set up the circuit
- Vary the wire length and record the voltage and current for each length
- Use R = V/I to work out the resistance
- Plot 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?
•R total = R1 + R2 + R3 + …
•Add the individual resistances of each component
How does the current vary between each component of a series circuit?
The current through all of the components is same
Is the current in parallel components the same?
No, each branch of a parallel circuit can have a different current through them according to Kirchoffs’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 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 flow current and voltage
P = IV
What is the purpose of a potential divider?
•To provide variable potential difference
•To provide a constant specific potential difference
What is emf?
•Electromotive force
•The electrical energy transferred by a power supply per unit charge
Rearrange the equation E = I(R + r) into the form y = mx + c
- E = IR + Ir
- E = V + Ir
So for a graph of V against I
V = -rl + E
Gradient = -r and y-intercept = E
What are the advantages of superconductors?
Low power dissipation, high speed operation, and high sensitivity
State Ohm’s law
Ohm’s law states that the electrical current (I) flowing in an circuit is proportional to the voltage (V) and inversely proportional to the resistance (R) when the temperature remains constant
Examples of superconductor use
•MRI scans
•Power cables - to reduce energy loss
through heating
