Section 4: Electricity

In series the current is the same everywhere; in parallel the voltage is. Find the total resistance first, then the current, then each voltage.

A lesson for this section

Section 1 style: multiple choice

Question 1Section 1 style

A current of

0.25\ \text{A}

flows in a lamp for

2.0

minutes. How much charge passes through the lamp?

Question 2Section 1 style

A charger delivers a steady current of

1.5\ \text{A}

. How long, in minutes, does it take to deliver

900\ \text{C}

of charge?

Question 3Section 1 style

The graph shows two oscilloscope traces. Trace 1 is a horizontal straight line at

3\ \text{V}

. Trace 2 is a wave that rises to

3\ \text{V}

above zero and falls to

3\ \text{V}

below zero, over and over. Which statement about the two supplies is correct?

Question 4Section 1 style

1.5\ \text{V}

cell and a

9.0\ \text{V}

battery are each used in a different circuit. Which statement is correct?

Question 5Section 1 style

Electrons, protons, neutrons and alpha particles each pass in turn between two oppositely charged parallel plates. Which one passes between the plates without changing direction?

Question 6Section 1 style

The current in a

22\ \Omega

resistor is

0.40\ \text{A}

. What is the voltage across the resistor?

Question 7Section 1 style

A student plots current (vertical axis) against voltage (horizontal axis) for a resistor. The line of best fit is straight, through the origin and the point (

12\ \text{V}

0.48\ \text{A}

). What is the resistance of the resistor?

Question 8Section 1 style

A graph of voltage against current for a filament lamp is a curve, not a straight line. Why?

Question 9Section 1 style

A student carries out an experiment to verify Ohm's law for a resistor. Which quantity must be kept constant?

Question 10Section 1 style

Which statement about the resistance of a metal wire is correct?

Question 11Section 1 style

A student wants to measure the voltage across a lamp and the current in it. How should the meters be connected?

Question 12Section 1 style

Which component changes light energy into electrical energy?

Question 13Section 1 style

What is a relay?

Question 14Section 1 style

In a transistor switching circuit, what is the function of the transistor?

Question 15Section 1 style

In a temperature alarm, a thermistor and a fixed resistor are connected in series across a

9.0\ \text{V}

supply. The gate of an n-channel enhancement mode MOSFET is connected so that its gate voltage equals the voltage across the fixed resistor. The MOSFET switches the alarm on when its gate voltage reaches

2.0\ \text{V}

. The thermistor's resistance decreases as its temperature increases. What happens as the temperature rises?

Question 16Section 1 style

12\ \Omega

resistor and a

6.0\ \Omega

resistor are connected in parallel. What is their total resistance?

Question 17Section 1 style

In the circuit shown, resistor

R_1

(

10\ \Omega

) is in series with a parallel pair,

R_2

(

20\ \Omega

) and

R_3

(

30\ \Omega

). What is the total resistance of the three resistors?

Question 18Section 1 style

Another resistor is connected in parallel with the resistors already in a circuit. The supply voltage does not change. What happens to the total resistance and to the current from the supply?

Question 19Section 1 style

4.0\ \Omega

resistor and a

12\ \Omega

resistor are connected in parallel across a

12\ \text{V}

supply. What is the current from the supply?

Question 20Section 1 style

Question 21Section 1 style

A potential divider is made from

R_1

= 2.2\ \text{k}\Omega

and

R_2

= 6.8\ \text{k}\Omega

in series across a

9.0\ \text{V}

supply. What is the voltage across

R_2

Question 22Section 1 style

An LDR and a fixed resistor are connected in series across a

6.0\ \text{V}

supply. A voltmeter is connected across the fixed resistor. The light falling on the LDR gets brighter. What happens to the voltmeter reading?

Question 23Section 1 style

A thermistor and a

1.5\ \text{k}\Omega

resistor are connected in series across a

12\ \text{V}

supply. A voltmeter across the

1.5\ \text{k}\Omega

resistor reads

4.0\ \text{V}

. What is the resistance of the thermistor?

Question 24Section 1 style

Two identical lamps, L1 and L2, are connected in parallel to a supply whose voltage stays constant. A third identical lamp is then connected in parallel with them. What happens to the brightness of L1 and to the current from the supply?

Question 25Section 1 style

Which of these defines electrical power?

Question 26Section 1 style

The current in a

150\ \Omega

resistor is

0.30\ \text{A}

. What power is developed in the resistor?

Question 27Section 1 style

6.0\ \text{V}

supply is connected across a

4.0\ \Omega

resistor. What power is developed in the resistor?

Question 28Section 1 style

A kettle rated at

2.3\ \text{kW}

is connected to the

230\ \text{V}

mains. What is the current in the kettle?

Question 29Section 1 style

A lamp is rated

6.0\ \text{V}

3.0\ \text{W}

. What is its resistance when it works at its rated voltage?

Question 30Section 1 style

The voltage across a resistor is doubled. The resistance of the resistor does not change. What happens to the power developed in the resistor?

Question 31Section 1 style

Question 32Section 1 style

A data logger records the current in a motor. The graph shows that the current is

0.65\ \text{A}

for the first

40\ \text{s}

and then

0.20\ \text{A}

for the next

80\ \text{s}

. A second circuit has a steady current that transfers the same total charge in the same

120\ \text{s}

. What is the current in the second circuit?

Question 33Section 1 style

The graph shows voltage against current for two resistors, X and Y. Line X is straight, through the origin and the point (

0.20\ \text{A}

6.0\ \text{V}

). Line Y is straight, through the origin and the point (

0.30\ \text{A}

3.0\ \text{V}

). X and Y are connected in parallel across a

6.0\ \text{V}

supply. What is the current from the supply?

Question 34Section 1 style

Question 35Section 1 style

A table: at 8, 6, 4, 2 and 0 degrees Celsius the thermistor's resistance is 1.5, 3.0, 4.5, 6.0 and 9.0 kilohms.
Temperature (°C)	Resistance of thermistor (kΩ)
8	1.5
6	3.0
4	4.5
2	6.0
0	9.0

A frost alarm uses a thermistor in series with a

3.0\ \text{k}\Omega

resistor across a

9.0\ \text{V}

supply. The gate voltage of a MOSFET equals the voltage across the thermistor, and the alarm is on when this voltage is

6.0\ \text{V}

or more. The table gives the thermistor's resistance:

1.5

3.0

4.5

6.0

and

9.0\ \text{k}\Omega

8

6

4

2

and

0\ ^{\circ}\text{C}

. Of the temperatures in the table, which is the highest at which the alarm is on?

Question 36Section 1 style

An LDR and a fixed resistor are connected in series across a supply to make a potential divider. An npn transistor switches a lamp on when the voltage across the component connected between its base and the

0\ \text{V}

line rises above a set value. The lamp must come on when it gets dark. Which arrangement works, and why?

Question 37Section 1 style

A heater on the

230\ \text{V}

mains has two identical elements whose resistance does not change. On setting 1 the two elements are connected in series and the heater's power is

500\ \text{W}

. On setting 2 the two elements are connected in parallel. What is the power on setting 2, and which fuse, 3 A or 13 A, should the plug have?

Question 38Section 1 style

Lamp L1 is rated

12\ \text{V}

24\ \text{W}

and lamp L2 is rated

12\ \text{V}

6.0\ \text{W}

. The two lamps are connected in series across a

12\ \text{V}

supply. Assume the resistance of each lamp does not change. Which lamp is brighter, and why?

Section 2 style questions

Verifying Ohm's lawSection 2 style · 7 marks

A student investigates how the current in a resistor depends on the voltage across it. Her results are shown in the table: voltage 1.0, 2.0, 3.0, 4.0 and 5.0 V; current 0.045, 0.091, 0.136, 0.182 and 0.227 A.

A table of results: voltage 1.0, 2.0, 3.0, 4.0 and 5.0 V; current 0.045, 0.091, 0.136, 0.182 and 0.227 A.
Voltage (V)	Current (A)
1.0	0.045
2.0	0.091
3.0	0.136
4.0	0.182
5.0	0.227

(a) Describe how the student could carry out this experiment. Your answer should say which meters she uses and how they are connected. [3]

0 words · two or three sentences

(b) Use the results to determine the resistance of the resistor, in Ω, to two significant figures. [2]

Type fractions as 3/8, powers as x^2 and roots as sqrt(5).

(c) The student repeats the experiment with a filament lamp in place of the resistor. Her graph of voltage against current is now a curve. Explain why. [2]

0 words · a sentence or two

A fan controllerSection 2 style · 10 marks

A circuit switches on a cooling fan when a greenhouse gets warm. A thermistor and a

1.2\ \text{k}\Omega

resistor are connected in series across a

6.0\ \text{V}

supply, with the thermistor nearer the positive terminal. The gate of an n-channel enhancement mode MOSFET is connected to the point between them, so the gate voltage equals the voltage across the

1.2\ \text{k}\Omega

resistor. The MOSFET switches the fan on when the gate voltage reaches

2.4\ \text{V}

. The thermistor's resistance decreases as its temperature increases; at

20\ ^{\circ}\text{C}

it is

3.6\ \text{k}\Omega

(a) Calculate the gate voltage at 20 °C, in V. [3]

Type fractions as 3/8, powers as x^2 and roots as sqrt(5).

(b) Explain why the fan switches on when the temperature in the greenhouse rises enough. [3]

0 words · two or three sentences

Type fractions as 3/8, powers as x^2 and roots as sqrt(5).

(d) State the function of the MOSFET in this circuit. [1]

0 words · a sentence

An electric heaterSection 2 style · 11 marks

An electric heater has two heating elements, each of resistance

46\ \Omega

. When the heater is on full, both elements are connected in parallel across the

230\ \text{V}

mains supply.

(a) Calculate the power of one element, in W. [3]

Type fractions as 3/8, powers as x^2 and roots as sqrt(5).

(b) Calculate the current from the mains supply when the heater is on full, in A. [3]

Type fractions as 3/8, powers as x^2 and roots as sqrt(5).

0 words · a sentence or two

(d) Calculate the energy transferred by the heater on full in 40 minutes, in J. [3]

Type fractions as 3/8, powers as x^2 and roots as sqrt(5).