Topic 5: Electric potential difference, current and resistance Flashcards
Define electric potential difference.
The energy that a charge has as a result of its position in an electric field.
What happens when a charge is placed in an electric field?
The charge feels a force.
What happens when a charge moves around in an electric field?
Work is done and the charge will either gain or lose electric potential energy.
Determine the change in potential energy when a charge moves between two points at different potentials.
- When a charge moves between two points at different potentials, there is an increase in electric potential energy
- If the electric potential energy at B is greater than the electric potential at A, work has to be done to push the charge from A to B
- However, the charge at B would be pushed by the electric field to A
- This push would accelerate its movement so that the loss in electrical potential energy would be the same as the gain in kinetic energy
What is the equation used to determine the change in potential energy when a charge moves between two points at different potentials that involves:
- electric potential energy change
- force
- distance?
electric potential energy change = F d
Where:
electric potential energy change in J
F is force in N
d is distance in m
What is the equation used to determine the change in potential energy when a charge moves between two points at different potentials that involves:
- electric potential energy change
- energy of the charge
- charge
- distance?
electric potential energy change = E q d
Where:
electric potential energy change in J
E is difference in potential energy in J
q is charge in C
d is distance in m
Derive an equation for the velocity of a charge
- kinetic energy gained = potential energy lost
- 1/2 m v^2 = E q d
- m v^2 = 2 E q d
- v^2 = (2 E q d) / m
- v = sqrt ( (2 E q d) / (m) )
Where:
m is the mass of the charge in kg
v is the velocity of the charge in m s^-1
E is difference in potential energy in J
q is the charge in C
d is the distance in m
Define the electronvolt.
The energy that would be gained by an electron moving through a potential difference of 1 volt.
What is the charge of an electron?
1.6 X 10^-19 C
Derive an equation for the charge of an electronvolt.
energy gained = potential difference X charge
1 electronvolt = 1 volt X 1.6 X 10^-19 C
1 electronvolt = 1.6 X 10^-19 C
What does the actual energy difference between two points, A and B, depend on?
- The charge that is moved
- If the charge is doubled, then the energy distance would also double
- The quantity that remains fixed between A and B is the energy difference per unit charge
- This is called the potential energy difference between the points
What is the equation used to determine the potential difference between two points that involves:
- energy difference
- charge?
V = E / q
Where:
V is potential difference between two points in V
E is energy difference in J
q is charge in C
Define electric current.
The force per unit length between parallel current-carrying conductors. It is the rate of flow of electrical charge.
What is the equation used to determine current that involves:
- charge
- time?
I = q / t
Where:
I is current in A
q is charge flowed in C
t is time taken in s
When is there a direct current?
If a current flows in one direction.
When is there an alternating current?
If a current constantly changes direction.
When does a current occur?
When charges move.
What is a circuit?
The path that currents follow.
Why does a current move across a body?
As a result of a potential difference across a body.
What creates a potential difference?
A power supply.
Why does work occur when charges move?
As a result of interactions between the conduction electrons and the lattice ions.
Define drift velocity.
The speed of the electrons due to the current.
Define resistance.
The mathematical ratio between potential difference and current.
What is the equation that defines resistance?
R = V / I
Where:
R is resistance in Ω
V is potential difference in V
I is current in A
What must happen for current to flow if something has a high resistance?
If something has a high resistance, it means that a large potential difference across it is needed in order for current to flow.
Define resistor.
A device with constant resistance; an ohmic device.
What is the equation used to determine resistance that involves:
- resistivity
- length
- cross-sectional area?
Where:
R is resistance in Ω
ρ is resistivity in Ω m
L is length in m
A is cross-sectional area in m s^-2
State Ohm’s Law.
The current flowing through a piece of metal is proportional to the potential difference across it, providing that the temperature remains constant.
V ∝ R (providing temperature is constant)
When is a device said to be ohmic?
If current and potential difference are proportional.
What is an example of an ohmic device?
Metal at constant temperature.
When is a device said to be non-ohmic?
If current and potential difference are not proportional.
What is an example of a non-ohmic device?
- Filament lamp
- Diode
Draw the I-V characteristics of an ohmic resistor.
Draw the I-V characteristics of a non-ohmic resistor.
Derive an expression for electrical power dissipation in resistors.
- potential difference = (energy difference) / (charge flowed)
- current = (charge flowed) / (time taken)
- P = VI
- power dissipation = ( (energy difference) / (charge flowed) ) X ( (charge flowed) / (time taken) )
- P = (energy difference) / (time taken)
Define electromotive force (emf).
The total energy difference per unit charge around a circuit. It is not a force measured in Newtons, but an energy difference per charge, measured in volts. It is the same as potential difference if no current flows.
Describe the concept of internal resistance.
Energy is used up inside the battery itself as a result of the battery’s internal resistance.
Apply the equations for resistors in series.
Apply the equations for resistors in parallel.
What happens to current and potential difference in a series circuit?
Current is constant
Potential difference is split up
What happens to current and potential difference in a parallel circuit?
Current is split up
Potential difference is constant
Describe the use of ideal ammeters.
An ideal ammeter has zero resistance.
Describe the use of ideal voltmeters.
An ideal voltmeter has infinite resistance.
Describe a potential divider.
Potential dividers divide up the potential difference within a circuit, so that parts of a circuit only receive the potential difference that they require. Potential dividers usually consist of two or more resistors arranged in series across a power supply.
Explain the use of light dependent resistors in potential divider circuits.
A device whose resistance depends on the amount of light shining on its surface. An increase in light causes a decrease in resistance.
Explain the use of thermistors in potential divider circuits.
A resistor whose value of resistance depends on its temperature. Most are semi-conducting devices that have a negative temperature coefficient (NTC). This means that an increase in temperature causes a decrease in resistance.
Explain the use of strain gauges in potential divider circuits.
A sensor whose output potential difference depends on any a small extension or compression that occurs, which results in a change of length.
What are sensors used for?
These devices can be used in potential divider circuits to create sensor circuits. The output potential difference of a sensor circuit depends on an external factor.
