P3.2 - Simple Circuits Flashcards
How can objects become charged?
Objects can be charged by transferring electrons to/from them, by the force of friction…simply by rubbing.
What do you need to make current flow in a circuit?
Current can only flow if there exists a difference in voltage between two points; if there exists a potential difference.
What is the result of charges being easier to detect?
- Electrons are incredibly difficult to count.
- Charge is easier to detect.
- Therefore, current and potential difference are
always defined in terms of charge flow (not
electron flow).
What is voltage?
Voltage is a measure of the energy carried by the current per coulomb of charge.
What is potential difference?
- Potential difference is the difference in voltage
between two points. - Potential difference is a measure of work done per
coulomb of charge
What is the formula for potential difference?
- V = W/Q
- V = Potential difference - V
- W = Work done - J
- Q = Charge - C
How is potential difference measured across components?
Potential difference are measured across components in a circuit using a voltmeter.
What does the p.d. across a component indicate?
The p.d. across a component indicates how much energy (per coulomb of charge) was transferred by the current as it passed through the component.
What is resistance?
Resistance is the opposition to the passage of charge.
What is the idea of resistance?
- In a wire component, ions vibrate up and down.
-These vibrating ions will get in the way of the
passing electrons, stopping them from flowing,
reducing the current. - The faster the ions vibrate (the hotter the wire) or
the bigger the ions, the greater the resistance
encountered, and the smaller the current flowing
What does a higher resistance mean?
The higher the resistance, the more energy transferred by the current as it passes through the resistance.
What is the fundamental equation of electricity?
- V=IR
- V = Potential difference - Volts - V
- I = Current - A
- R = Resistance - ohms
What is a series circuit?
- A series circuit is one in which the electrons must
travel through every component. - The electrons get ““no choice”” - Must transfer
energy to the same components - The circuit has no branches, but is just one loop.
How is the current in a series circuit?
The current through every component in a series circuit is the same.
How is the resistance in a series circuit?
The total resistance in a series circuit is the sum of all the component resistances.
How is the potential difference of a series circuit?
- The p.d. across components in a series circuit is
shared out. - The greater the resistance, the greater the share
of the total p.d. it has across it.
What is a parallel circuit?
- A parallel circuit is one in which the electrons don’t
travel through the same components. - The electrons get ““a choice”” - transfer energy to
different components. - A parallel circuit has branches.
How is the potential difference in a parallel circuit?
The p.d. across components in a parallel circuit is the same.
How is resistance in a parallel circuit and why?
- The total resistance in a parallel circuit is less than
the component with the smallest resistance in the
circuit. - The are more paths for the current to flow through.
How is current in a parallel circuit?
- The current in a parallel circuit is split between
components/branches. - The greater the resistance of the component, the
less current through it.
What do variable resistors do?
Variable resistors change the current in a circuit by varying its resistance.
Why is potential difference not a dependant variable?
The potential difference for a circuit is a constant set by the battery/cell of a circuit.
Why are you unlikely to be injured by an electric current in your house?
The p.d. of main circuits are 230V which is much lower than required to give rise to a current of 1A.
What is an I-V graph?
An I-V graph is a plot of current, I (y-axis) against p.d., V (x-axis).
What does the gradient of an I-V graph give?
- The gradient of an I-V graph gives the opposite of
resistance. - A diagonal straight line = Constant resistance.
- A steep gradient = Small resistance.
- A shallow gradient = Large resistance
Draw a simple circuit which is used to plot a characteristic curve.
- An ammeter and component connected in
series with a cell. - A voltmeter is connected in parallel to the
component.
What is Ohm’s Law?
The current through a component is directly proportional to the potential difference across it, as long as the temperature is kept constant.
What happens as the potential difference increases across a component?
The component gets hotter.
What type of circuit element is a wire or fixed resistor?
- A linear circuit element.
- An Ohmic conductor.
What does an I-V graph of a wire or fixed resistor show?
Resistance is a constant and does not change with potential difference.
What type of circuit element is a filament light bulb?
A non-linear circuit element.
What does the I-V graph of a filament light bulb show?
- Resistance increases with the increasing p.d.
- Resistance increases as the filament gets hotter
What is a diode?
- An electrical component that only allows current
to only flow through one way. - Arrow inside points in the direction you want the
current to flow. - Smaller line is connected to the negative terminal
of the cell/battery.
What does an I-V for a diode/LED show?
- Forward resistance is low
- Reverse resistance is high
- As you apply a potential difference in the ‘forward’
direction very little current flows. Then the current
suddenly increases.”
What are thermistors?
- Thermistors are components (resistors) whose
resistance depends upon temperature. - Resistance decreases as the temperature
increases.
Why do thermistors not behave like regular components?
Thermistors do not behave like regular components because they are made of a special type of material called semiconductors such as silicon.
What happens as you heat up a thermistor/semiconductor or give more light energy to a LDR?
- Ions vibrate faster and fewer electrons move
through the wire per unit time. - More electrons gain enough energy to become
liberated from the semiconductor which results in
more electrons moving through the wire per unit
time. - Current increases - Resistance decreases
What are LDRs?
- Light dependent resistors are components whose
resistance depends upon light intensity. - Resistance decreases as the light intensity
increases.
What is net resistance and how is it calculated in a series circuit?
- Net resistance is the total resistance of a circuit.
- Net resistance in a series circuit is calculated by
summing the individual resistances of all
components in the circuit.
What happens to net resistance in a series and parallel circuit?
- In a series circuit: As you add more resistors,
the net resistance increases. - In a parallel circuit: As you add more resistors,
net resistance decreases (will always be smaller
than the smallest resistance of a single branch)
(current also increases).
What is a sensor circuit?
A sensor circuit is one which turns on a component on/off automatically, in response to external stimuli.
Give an example of thermistors and LDRs in sensor circuits.
- Thermistors are used in sensor circuits in
thermostats to turn central heating systems
on/off at desired temperature. - LDRs are used in sensor circuits for street
lamps to turn the lights on/off with changing
light levels.
Describe was happens in a sensor circuit including a thermistor.
- A fixed resistor of resistance R1 is connected in
series with a thermistor of resistance R2. - The p.d. of the battery is shared out amongst the
fixed resistor and the thermistor - (most p.d. is
dropped across the resistor with the highest
resistance). - The thermistor is connected in parallel with a
component like a heater, the potential difference
across both is Vout. - As the temperature in house decreases,
resistance of thermistor, R2 increases - The p.d. dropped across the thermistor Vout
increases - Once Vout increases to a high enough level,
the “working potential” of the heater is reached
and it switches on.
What is the “ratio trick”?
The ratio of resistance of two components is the same as the ratio of the potential difference between the two components.
How do you work out the p.d across each component is a series circuit?
- Work out net resistance.
- Calculate the current in the circuit.
- Apply V=IR to work out the p.d.
What two factors increase the resistance of a wire?
- Increasing the length of the wire.
- Decreasing the thickness of the wire.
What is the equation for electrical power in electrical circuits?
- P=IV
- P=I(Squared)R
What is electrical power?
Electrical power is the rate of electrical energy transfer.
Give two properties of a.c. circuits.
- In an a.c. circuit the direction of the current
changes regularly. - In an a.c. circuit the size of the current varies with
time.
What circuit symbol is this?
open switch
What circuit symbol is this?
closed switch
What circuit symbol is this?
cell
What circuit symbol is this?
battery
What circuit symbol is this?
diode
What circuit symbol is this?
resistor
What circuit symbol is this?
variable resistor
What circuit symbol is this?
LED
What circuit symbol is this?
lamp
What circuit symbol is this?
fuse
What circuit symbol is this?
voltmeter
What circuit symbol is this?
ammeter
What circuit symbol is this?
thermistor
What circuit symbol is this?
LDR
