Section 5 - Electricity and Currents Flashcards
What is current?
The flow of electrical charge around the circuit / the rate of flow of charge
Unit: Ampere, A
When will a current only flow/be produced?
If there is a potential difference across that circuit, and if the circuit is complete.
The current then depends/changes depending on the potential difference and resistance
What is potential difference (voltage)
The driving force that pushes the charge round / the energy transferred per coulomb of charge (between 2 points)
Unit: Volt, V
Higher Potential difference = higher current
What is resistance?
Anything that slows the current down
Unit: Ohm, Ω
Higher resistance = Lower current
What is the current caused by in metals?
Flow of electrons
What is the equation for current (amps) ?
Charge (coulombs / C) / Time
Q / t
What is the equation for potential difference?
For general
Potential difference = energy transferred
/charge moved
V = E / Q
1V = 1J/C
What is the equation for potential difference, current and resistance?
Potential difference = Current x Resistance
V (volts) = I (amps/A) x R (ohms/mikewasouski)
How does resistance increase with temperature?
(Most of time)
1) When an electrical charge flows through a component, it has to do work against resistance
2) This causes an electrical transfer of energy - some of this useful, but some dissipated to the thermal energy stores of the component and its surroundings
3) When a current flows through a resistor, the resistor heats up – cont
Why does a resistor heat up as a current passes through it?
Electrons collide with the ions in the lattice that make up the resistor as they flow through it. This gives the ions energy, which causes them to vibrate and heat up.
-> The more the ions heat up, the harder it is for electrons to get through the resistor (+collisions) - for a given P.d, current will decrease as the resistor heats up
->If the resistor gets too hot, no current will be able to flow
The only exeption to this is a thermistor which decreases res as temp +
What does an ammeter do?
Mesures the current (in amps) flowing through the component - Has to be put in series with component, not parralel
What does a voltmeter do?
Mesures the p.d. across the component. It must be placed in parralel to the component under testing
what does changing the output of a p.d. supply do? ( when testing a component)
Alters the current flowing through the circuit and the potential difference across the component
How to stay safe when investagating circuits?
Dont let it get too hot - unplug for a bit to let it cool down
What is a diode?
a circuit component that only allows current to flow in one direction
What does a thermistor do?
their resistance decreases as the temperature increases
- Make temp detectors e.g. car engine temp sensors and electronic thermostats
What is an LDR?
LDRs (light-dependent resistors) are used to detect light levels, eg in automatic security lights. Their resistance decreases as the light intensity increases.
How could you repeat the standard test circuit for a DIODE test?
After you’ve finished taking mesurements for a range of currents, remove the diode and swap its direction. You should find that the current cannot flow through the diode anymore
How could you repeat the standard test circuit for a Thermistor test?
Keeping the supply P.d constant, gradually heat the thermistor. (E.g. put it in a hot beaker of warm water)
-> You should find that as temp increases, the current through the thermistor increases as the resistance decreases
How could you repeat the standard test circuit for a LDR test?
Conduct your experiment in a dim room - keep supply of p.d. constant and slowly adjust the light level near the LDR (e.g. by using a lamp connected to a dimmer switch).
-> You should find as the light level gets brighter, the current through the LDR increases as the resistance decreases
What are series circuits?
where the different components are connected in a line, end to end (except voltmeters, which are always parralel but dont count as part of the circuit)
What happens if you remove one component in a series circuit?
The circuit is broken and they all stop working
-> not good, not used for stuff cus of this
In a series circuit
When more cells are in series what does this do?
Basic math - more cell = add them all = bigger p.d
In a series circuit
What is the rule for current?
The same everywhere, I1 = I2 = I3
-> Depends on the total p.d. and total resistance