National 5 Electricity Flashcards
Label the following model of the atom, indicating the charge of each particle.
How many types of electric charge are there, and what are their names?
There are two types of electric charge - positive and negative
How do the types of electric charge interact?
Opposites attract - positive and negative
Like charges repel - positive and positive or negative and negative
What is an electric field?
An electric field is the space around a charged particle where another charged particle will experience a force.
Describe what would happen to a positive charge if it was placed in the field shown.
A positive charge would accelerate towards the negative plate.
Sketch the path of an electron at point A which moves towards the point charges shown in the diagram as shown and reaches point B
The electron is attracted by the postive charge but repelled by the negative charge, as shown.
Describe what would happen to a negative charge if it was placed in the field shown.
A negative charge would accelerate towards the positive plate.
In the diagram shown, what type of charge does the particle have?
Explain your answer.
The particle has a negative charge.
The particle is negative because it is attracted to the positive plate.
What effect does an electric field have on a conductor?
An electric field will cause the free electrons in a conductor to drift towards the positive end of the battery.
Explain in words what is meant by current.
An electric current if a flow of charge.
What is the numerical definition of current?
Current is how much electrical charge flows (past a point) in one second.
Q = It
(Define symbols and their units)
Q - charge (C)
I - current (A)
t - time (s)
Example
Calculate the current flowing during a bolt of lightning which lasts for 10 ms and transfers 50 C of charge.
Q = 50 C
I = ?
t = 10 ms = 10 x 10-3 s
Q = It
50 = I x 10 x 10-3
I x 10 x 10-3 = 50
I = 50/10 x 10-3
I = 5000 A
What does DC stand for and what does it mean?
DC stands for direct current, which means the electron current flows in one direction only (negative to positive).
What does AC stand for, and what does it mean?
AC stands for alternating current, which means that the current periodically reverses direction..
What is meant by the peak voltage of an AC supply?
The peak voltage of an AC supply is the maximum voltage it reaches in its cycle.
What is the quoted voltage of an AC supply?
The quoted voltage is an average value which enables fair comparison with a DC battery.
How do quoted and peak voltage compare?
Peak voltage is greater than quoted voltage.
What is the quoted voltage of the UK mains?
230 V
What is the frequency of the UK mains?
50 Hz
The voltage of a supply is a measure of the ………………. given to the charges in a circuit.
The voltage of a supply is a measure of the ENERGY given to the charges in a circuit.
Describe how to use an ammeter to measure the current in a component. Draw a diagram showing how an ammeter is connected.
An ammeter must be connected in series with a component.
Describe how to use a voltmeter to measure the voltage across a component. Draw a diagram to show how a voltmeter is connected.
A voltmeter must be connected in parallel with a component.
State the circuit rule for current in a series circuit.
The current in a series circuit is the same at all points OR
Is = I1 = I2 = …
State the circuit rule for voltage in a series circuit.
The voltages across the components in a series circuit adds up to the supply voltage OR
Vs = V1 + V2 + …
State the circuit rule for current in a parallel circuit.
The current entering in a parallel circuit divides into smaller currents through each component OR
Ip = I1 + I2 + …
State the circuit rule for voltage in a parallel circuit.
The voltage in a parallel circuit is the same across all components OR
Vp = V1 = V2 = …
What is the name of this component?
L.D.R. (light-dependent resistor)
What is the name of this component?
Cell
What is the name of this component?
Resistor
What is the name of this component?
Battery
What is the name of this component?
Lamp
What is the name of this component?
Motor
What is the name of this component?
Capacitor
What is the name of this component?
Current can only flow through this component in one direction - which?
Diode
Current can only flow ‘against the arrow head’ e.g. here, right to left.
What is the name of this component?
Variable resistor
What is the name of this component?
npn transistor
What is the name of this component?
Loudspeaker
What is the name of this component?
Fuse
What is the name of this component?
Switch
What is the name of this component?
Thermistor
What is the name of this component?
Photovoltaic Cell (Solar Cell)
What is the function of a cell or battery?
(including the energy change)
A cell or battery supplies the energy for a circuit, converting chemical energy to electrical energy.
Define Ohmic and Non-Ohmic conductors, giving an example of each.
Ohmic conductors have a constant resistance meaning that a V-I graph is a straight line. e.g. resistor.
The resistance of non-ohmic conductors changes as the current changes e.g. the resistance of a light bulb increases when current increases because this causes it to heat up (producing more light). This gives a curved V-I graph.
What is the name of this component?
MOSFET (transistor)
How could the resistance of a component be found from this graph?
The resistance could be found by
(a) calculating the gradient of the graph (best method)
(b) choosing a point on the graph and calculating voltage / current (approximate method).
Example
Calculate the total resistance of the circuit shown.
R<sub>T</sub> = ? R<sub>1</sub> = 1·5 kΩ = 1500 Ω R<sub>2</sub> = 100 Ω
R<sub>T</sub> = R<sub>1 </sub>+ R<sub>2 </sub>+ .... R<sub>T</sub> = 1500 + 100 R<sub>T</sub> = 1600 Ω
Example
Calculate the total resistance of the circuit shown.
RT = ? R1 = 5 Ω R2 = 20 Ω
1/R<sub>T</sub> = 1/R<sub>1 </sub>+ 1/R<sub>2 </sub>+ .... 1/R<sub>T</sub> = 1/5 + 1/20 1/R<sub>T</sub> = 1/4 R<sub>T</sub> = 4/1 = 4 Ω
Example
Calculate the total resistance of the circuit shown.
25 & 25 in series
RT = ? R1 = 25 Ω R2 = 25 Ω
R<sub>T</sub> = R<sub>1 </sub>+ R<sub>2 </sub>+ .... R<sub>T</sub> = 25 + 25 R<sub>T</sub> = 50 Ω
50 (above) in parallel with 100
RT = ? R1 = 50 Ω R2 = 100 Ω
1/R<sub>T</sub> = 1/R<sub>1 </sub>+ 1/R<sub>2 </sub>+ .... 1/R<sub>T</sub> = 1/50 + 1/100 1/R<sub>T</sub> = 3/100 R<sub>T</sub> = 100/3 = 33.3 Ω (to 3 sig fig)
In the circuit below calculate
a) the battery voltage
b) the voltage across the 10 Ω resistor.
a) First find the total resistance
Two 20 Ω resistors in parallel
RT = ?
R1 = 20 Ω
R2 = 20 Ω
1/R<sub>T</sub> = 1/R<sub>1</sub> + 1/R<sub>2</sub> 1/R<sub>T</sub> = 1/20 + 1/20 1/R<sub>T</sub> = 1/10 R<sub>T</sub> = 10 Ω
Now combine this 10 Ω resistor with the other one in series
RT = ?
R1 = 10 Ω
R2 = 10 Ω
R<sub>T</sub> = R<sub>1</sub> + R<sub>2</sub> R<sub>T</sub> = 10 + 10 R<sub>T</sub> = 20 Ω
Use this total resistance along with the total current to get the total voltage
V = ?
I = 450 mA = 450 x 10-3 A
R = 20 Ω
V = IR
V = 450 x 10-3 x 20
V = 9 V
The battery voltage is 9 V
b) The ammeter and 10 Ω resistor are in series so will have the same current.
V = ? I = 450 mA = 450 x 10<sup>-3</sup> A R = 10 Ω
V = IR
V = 450 x 10-3 x 10
V = 4·5 V
The voltage across the 10 Ω resistor is 4·5 V
Will the LED in this circuit switch on?
Explain your answer.
No - the LED will not switch on.
The positive terminal of the LED is not connected to the positive terminal of the battery. Current can only flow ‘against the arrow head’.
Will the LED in this circuit switch on?
Explain your answer.
Yes - the LED will switch on.
The positive terminal of the LED is connected to the positive terminal of the battery. Current flows ‘against the arrow head’.
Why does an LED always have a resistor connected in series with it?
An LED always has a resistor connected in series with it to protect the LED to safe values. Excessive currents damage the LED.
The resistor will limit the current in the LED to prevent damage to it.
Example
Calculate the value of the protective resistor for an LED rated at 4V, 20 mA being used with a 9V cell.
First calculate the voltage across the resistor using ‘voltage in series’ rule
Vs = 9 V
V1 (LED) = 4 V
V2 (R) = ?
V<sub>s</sub> = V<sub>1</sub> + V<sub>2</sub> 9 = 4 + V<sub>2</sub> V<sub>2</sub> = 9-4 = 5 V
The current is the same through LED and R using ‘current in series’ rule = 20 mA
For the resistor
V = 5 V
I = 20 mA = 20 x 10-3 A
R = ?
V = IR
5 = 20 x 10-3 x R
20 x 10-3 x R = 5
R = 5/20 x 10-3
R = 250 Ω
The protective resistor should have a resistance of 250 Ω
What is the following circuit usually called?
A potential divider or voltage divider.
Explain the function of a potential divider circuit such as the example shown.
A potential divider or voltage divider divides the battery voltage between the resistors in proportion to their resistance i.e. larger resistors will take a larger share of the voltage.
Example
Calculate V1 in this circuit.
V<sub>1</sub> = ? R<sub>1</sub> = 100 Ω (This is R<sub>1</sub> because it has been marked as V<sub>1</sub> in the question) R<sub>2</sub> = 150 Ω V<sub>s</sub> = 10 V
V<sub>1</sub> = R<sub>1</sub>/(R<sub>1</sub>+R<sub>2</sub>) x V<sub>s</sub> V<sub>1</sub> = 100/(100+150) x 10 V<sub>1</sub> = 4 V
(NB The formula sheet uses V2 = R2/(R1+R2) x Vs but this can be adapted as above as long as the voltage and resistor in bold agree with each other i.e.
V1 = R1/(R1+R2) x Vs
or
V2 = R2/(R1+R2) x Vs
The choice of which resistor is labelled 1 or 2 doesn’t matter unless it has already been marked in the Question).
Example
Calculate V1 in this circuit.
V1 = ?
V2 = 3 V
R1 = 12 kΩ
R2 = 6 kΩ
(Can leave R1 and R2 in kΩ since the formula is a ratio.)
V<sub>1</sub>/V<sub>2</sub> = R<sub>1</sub>/R<sub>2</sub> V<sub>1</sub>/3 = 12/6 V<sub>1</sub> = 3x12/6 V<sub>1</sub> = 6 V
Explain the function of the following voltage divider circuit when temperature increases.
- Temperature increases,
- –> Resistance of thermistor decreases
- –> Voltage across thermistor decreases.
- –> Voltage across fixed resistor increases
(this is the output voltage).
The circuit acts as a high temperature sensor i.e. its output voltage increases as temperature increases.
TURD
Temperature Up Resistance Down
Explain the function of the following voltage divider circuit when temperature decreases.
- As temperature decreases, the resistance of the thermistor increases.
- This means the voltage across the thermistor, which is the output voltage, increases.
The circuit acts as a low temperature sensor i.e. its output voltage increases as temperature decreases.
Explain the function of the following voltage divider circuit when light levels increase.
- As light levels rise, the resistance of the LDR decreases.
- This means the voltage across the LDR decreases.
- This means the voltage across the resistor, which is the output voltage, increases.
The circuit acts as a light sensor i.e. its output voltage increases as light levels increase.
Explain the function of the following voltage divider circuit when light levels fall.
- As light levels fall, the resistance of the LDR increases.
- This means the voltage across the LDR, which is the output voltage, increases.
The circuit acts as a low light (darkness) sensor i.e. its output voltage increases as light levels fall.
Explain the function of the following voltage divider circuit when the switch is open.
- An open switch has a very high resistance.
- This means the voltage across the switch, which is the output voltage, will be very high.
The output voltage of this circuit is high when the switch is open. This could be used in a burglar alarm to detect a door or window being opened.
Explain how the time delay in following voltage divider circuit could be increased.
- A capacitor with larger capacitance takes longer to fill with charge, so the time delay would be longer.
- A larger resistance reduces the current flowing so that the capacitor takes longer to fill with charge, so the time delay would be longer.
Explain how the following voltage divider circuit acts as a time delay.
- The voltage across a capacitor builds up slowly when it is switched on.
- This means the output voltage, will build up slowly, taking time to reach Vs or a specific value.
This circuit acts as a time delay.
Describe what happens to the output voltage when the potentiometer is varied from position B to A.
- When the potentiometer is close to B the resistance below the contact is low, meaning the output voltage is low.
- When the potentiometer is close to A, the resistance below the contact is high, meaning the output voltage is high.
A potentiometer allows the voltage to be varied continuously between 0V (at B) and Vs (at A).
Label the connections of an NPN transistor.
Label the connections of a MOSFET transistor.
Explain how the circuit shown switches on the motor when light levels increase.
• As light levels rise, the resistance of the LDR decreases.
• This means the voltage across the LDR decreases.
• This means the voltage across the variable resistor, which is the base voltage, increases.
• When this voltage reaches high enough, the transistor switches ON
and current can flow to the motor.
Explain how the circuit shown switches on the light when it gets dark.
• When it gets dark, the resistance of the LDR increases.
• This means the voltage across the LDR, which is the base voltage, increases.
• When this voltage reaches high enough, the transistor switches ON
and current can flow to the lamp.
Explain how the circuit shown switches on the heater when the temperature falls.
• As temperature falls, the resistance of the thermistor increases.
• This means the voltage across the thermistor, which is the gate voltage, increases.
• When this voltage reaches high enough, the transistor switches ON
and current can flow to the heater.
Explain how the circuit shown switches on the motor (air conditioning pump) when the temperature rises.
• As temperature rises, the resistance of the thermistor decreases.
• This means the voltage across the thermistor decreases.
• This means the voltage across the variable resistor, which is the base voltage, increases.
• When this voltage reaches high enough, the transistor switches ON
and current can flow to the relay.
• This switches on the circuit with the motor (air conditioning pump)
How could the air-conditioning circuit (shown) be changed so that it performs the opposite job (i.e. switch on at low temperatures instead of high temperatures)?
The circuit can be changed to perform the opposite job by swapping the thermistor with the variable resistor.
How could the air-conditioning circuit (shown) be changed so that it switches on at a different temperature?
To switch the circuit on at a different temperature, the variable resistor could be adjusted. (This is called changing the sensitivity of the circuit),
