Electricity Flashcards
From which terminal to which terminal do electrons flow?
From the negative terminal to the positive terminal.
What is Kirchoff’s first law?
The sum of the currents entering a point equals the sun of the currents leaving that point.
From which terminal to which terminal does conventional current flow?
From the positive terminal to the negative terminal.
Define electrical resistance.
Potential difference/current - measured in ohms.
A thermistor and a switch are connected in series.
Explain why the current increases when the switch is closed.
When the switch is closed, current flows through the circuit.
Higher current = higher temperature
higher temp of thermistor = lower resistance
lower resistance = increase in current
State ohms law
The current through a device is directly proportional to the potential difference across its two ends.
What would the I-V graph of a metallic conductor look like?
A straight line through the origin. I is directly proportional to V.
What would the I-V graph of a semiconductor look line?
A smooth curve (divergent) starting at the origin.
What would the R-V graph of a metallic conductor look like?
A smooth curve (divergent) starting at the origin.
What is the difference between e.m.f. and p.d.?
E.m.f. is energy transferred into all forms of energy, whereas p.d. is energy transferred into electrical energy only.
Define current
The rate of flow of charge
What does ‘number density’ mean? Give the unit
The number density of charge carriers is a property of a material that shows how many electrons that pass through the material per m3. Unit: m-3
Define the coulomb.
When a current of 1A flows for 1s, 1C of charge is said to be passed.
What four factors does the resistance depend on?
1) Material
2) Length
3) Cross-sectional area
4) Temperature
Define the ohm
A device has a resistance of 1 ohm if a current of 1A flows through it when a p.d. of 1V is applied across its end.
State and explain how temperature affects the resistance of ohmic conductors.
The greater the temperature, the greater the resistance. As the temperature increases, more kinetic energy is supplied to the atoms of the conductor and the delocalised electrons. This makes both the particles and electrons move faster and this causes more collisions. The collisions impede the movement of the electrons and this causes an increase in resistance.
Define resistivity. Give the unit.
The property of the material that affects its resistance. Unit Ohm-metre
Define the watt.
1J of energy is transferred in 1s.
State Kirchoffs 2nd law.
In any closed loop the sum of the e.m.f. is equal to the sum of the p.d.
How can you determine the e.m.f and Internal resistance from a V-I graph?
V=-Ir+E
the gradient is -r (the negative internal resistance)
The y intercept is the e.m.f.
Define the ‘lost volts’
The difference between the e.m.f. and the terminal p.d. is the lost volts.
What is the main idea behind the potential divider?
V1:V2=R1:R2
Define the ‘terminal p.d.’
The p.d. across the terminals of a cell.
How do you find the total resistance of two resistors connected in series?
Total resistance=R1+R2
How do you find the total resistance of two resistors connected in parallel?
1/Total resistance=1/R1+1/R2
True or false: The potential difference across the cell is equal to the potential difference across each parallel branch.
True
Define potential difference
Energy per unit charge
The energy that is converted from electrical energy to other forms of energy.
Explain what is meant by internal resistance
The resistance within the cell which causes energy to be lost to the surroundings as heat.
Explain why the time for an electron to pass an electrical signal through a material is different from the time for an electromagnetic wave to transmit a signal.
Electron moved at drift velocity. Em waves travel at the speed of light.
Describe what is meant by the term ‘mean drift velocity’ of the electrons in the wire
- average displacement travelled by electrons along the wire per second.
- the electrons move and collide with the lattice ions. Under the influence of the applied potential difference the electrons move towards the positive terminal of the power supply.
Name the charge carriers responsible for electric current in an electrolyte.
Ions
What is the physical quantity conserved in Kirchoff’s second law?
Energy
